A Tour of Go on a single web page to easily find what you want to recall.
This adaptation of A Tour of Go is in compliance with the BSD license under which the original is distributed (LICENSE).
Note: if you want to run the code, you can click on a section heading, which will redirect you to the corresponding part of A Tour of Go where you can run the code.
View the page as a Markdown file.
Learn how to use this tour: including how to navigate the different lessons and how to run code.
Welcome to a tour of the Go programming language.
The tour is divided into a list of modules that you can access by clicking on A Tour of Go on the top left of the page.
You can also view the table of contents at any time by clicking on the menu on the top right of the page.
Throughout the tour you will find a series of slides and exercises for you to complete.
You can navigate through them using
“previous” or PageUp to go to the previous page,
“next” or PageDown to go to the next page.
The tour is interactive. Click the
Run button now
(or press Shift + Enter) to compile and run the program on
a remote server.
The result is displayed below the code.
These example programs demonstrate different aspects of Go. The programs in the tour are meant to be starting points for your own experimentation.
Edit the program and run it again.
When you click on Format
(shortcut: Ctrl + Enter), the text in the editor is formatted using the
gofmt tool. You can switch syntax highlighting on and off
by clicking on the syntax button.
When you’re ready to move on, click the right arrow below or type the PageDown key.
/* hello.go */
package main
import "fmt"
func main() {
fmt.Println("Hello, 世界")
}
The tour is available in other languages:
Click the “next” button or type PageDown to continue.
This tour is also available as a stand-alone program that you can use without access to the internet. It builds and runs the code samples on your own machine.
To run the tour locally, you’ll need to first install Go and then run:
go install golang.org/x/website/tour@latest
This will place a tour binary in your
GOPATH’s bin directory.
When you run the tour program, it will open a web browser displaying
your local version of the tour.
Of course, you can continue to take the tour through this web site.
This tour is built atop the Go Playground, a web service that runs on golang.org’s servers.
The service receives a Go program, compiles, links, and runs the program inside a sandbox, then returns the output.
There are limitations to the programs that can be run in the playground:
In the playground the time begins at 2009-11-10 23:00:00 UTC (determining the significance of this date is an exercise for the reader). This makes it easier to cache programs by giving them deterministic output.
There are also limits on execution time and on CPU and memory usage, and the program cannot access external network hosts.
The playground uses the latest stable release of Go.
Read “Inside the Go Playground” to learn more.
/* sandbox.go */
package main
import (
"fmt"
"time"
)
func main() {
fmt.Println("Welcome to the playground!")
fmt.Println("The time is", time.Now())
}
You’ve finished the first module of the tour!
Now click on A Tour of Go to find out what else you can learn about Go, or go directly to the next lesson.
Learn the basic components of any Go program.
Every Go program is made up of packages.
Programs start running in package main.
This program is using the packages with import paths "fmt" and "math/rand".
By convention, the package name is the same as the last element of the import path. For instance, the "math/rand" package comprises files that begin with the statement package rand.
/* packages.go */
package main
import (
"fmt"
"math/rand"
)
func main() {
fmt.Println("My favorite number is", rand.Intn(10))
}
This code groups the imports into a parenthesized, “factored” import statement.
You can also write multiple import statements, like:
import "fmt"
import "math"
But it is good style to use the factored import statement.
/* imports.go */
package main
import (
"fmt"
"math"
)
func main() {
fmt.Printf("Now you have %g problems.\n", math.Sqrt(7))
}
In Go, a name is exported if it begins with a capital letter.
For example, Pizza is an exported name, as is Pi, which is exported from
the math package.
pizza and pi do not start with a capital letter, so they are not exported.
When importing a package, you can refer only to its exported names. Any “unexported” names are not accessible from outside the package.
Run the code. Notice the error message.
To fix the error, rename math.pi to math.Pi and try it again.
/* exported-names.go */
package main
import (
"fmt"
"math"
)
func main() {
fmt.Println(math.pi)
}
A function can take zero or more arguments.
In this example, add takes two parameters of type int.
Notice that the type comes after the variable name.
(For more about why types look the way they do, see the article on Go’s declaration syntax.)
/* functions.go */
package main
import "fmt"
func add(x int, y int) int {
return x + y
}
func main() {
fmt.Println(add(42, 13))
}
When two or more consecutive named function parameters share a type, you can omit the type from all but the last.
In this example, we shortened
x int, y int
to
x, y int
/* functions-continued.go */
package main
import "fmt"
func add(x, y int) int {
return x + y
}
func main() {
fmt.Println(add(42, 13))
}
A function can return any number of results.
The swap function returns two strings.
/* multiple-results.go */
package main
import "fmt"
func swap(x, y string) (string, string) {
return y, x
}
func main() {
a, b := swap("hello", "world")
fmt.Println(a, b)
}
Go’s return values may be named. If so, they are treated as variables defined at the top of the function.
These names should be used to document the meaning of the return values.
A return statement without arguments returns the named return values. This is known as a “naked” return.
Naked return statements should be used only in short functions, as with the example shown here. They can harm readability in longer functions.
/* named-results.go */
package main
import "fmt"
func split(sum int) (x, y int) {
x = sum * 4 / 9
y = sum - x
return
}
func main() {
fmt.Println(split(17))
}
The var statement declares a list of variables; as in function argument lists, the type is last.
A var statement can be at package or function level. We see both in this example.
/* variables.go */
package main
import "fmt"
var c, python, java bool
func main() {
var i int
fmt.Println(i, c, python, java)
}
A var declaration can include initializers, one per variable.
If an initializer is present, the type can be omitted; the variable will take the type of the initializer.
/* variables-with-initializers.go */
package main
import "fmt"
var i, j int = 1, 2
func main() {
var c, python, java = true, false, "no!"
fmt.Println(i, j, c, python, java)
}
Inside a function, the := short assignment statement can be used in place of a var declaration with implicit type.
Outside a function, every statement begins with a keyword (var, func, and so on) and so the := construct is not available.
/* short-variable-declarations.go */
package main
import "fmt"
func main() {
var i, j int = 1, 2
k := 3
c, python, java := true, false, "no!"
fmt.Println(i, j, k, c, python, java)
}
Go’s basic types are
bool
string
int int8 int16 int32 int64
uint uint8 uint16 uint32 uint64 uintptr
byte // alias for uint8
rune // alias for int32
// represents a Unicode code point
float32 float64
complex64 complex128
The example shows variables of several types, and also that variable declarations may be “factored” into blocks, as with import statements.
The int, uint, and uintptr types are usually 32 bits wide on 32-bit systems and 64 bits wide on 64-bit systems.
When you need an integer value you should use int unless you have a specific reason to use a sized or unsigned integer type.
/* basic-types.go */
package main
import (
"fmt"
"math/cmplx"
)
var (
ToBe bool = false
MaxInt uint64 = 1<<64 - 1
z complex128 = cmplx.Sqrt(-5 + 12i)
)
func main() {
fmt.Printf("Type: %T Value: %v\n", ToBe, ToBe)
fmt.Printf("Type: %T Value: %v\n", MaxInt, MaxInt)
fmt.Printf("Type: %T Value: %v\n", z, z)
}
Variables declared without an explicit initial value are given their zero_value.
The zero value is:
0 for numeric types,false for the boolean type, and"" (the empty string) for strings./* zero.go */
package main
import "fmt"
func main() {
var i int
var f float64
var b bool
var s string
fmt.Printf("%v %v %v %q\n", i, f, b, s)
}
The expression T(v) converts the value v to the type T.
Some numeric conversions:
var i int = 42
var f float64 = float64(i)
var u uint = uint(f)
Or, put more simply:
i := 42
f := float64(i)
u := uint(f)
Unlike in C, in Go assignment between items of different type requires an
explicit conversion.
Try removing the float64 or uint conversions in the example and see what happens.
/* type-conversions.go */
package main
import (
"fmt"
"math"
)
func main() {
var x, y int = 3, 4
var f float64 = math.Sqrt(float64(x*x + y*y))
var z uint = uint(f)
fmt.Println(x, y, z)
}
When declaring a variable without specifying an explicit type (either by using the := syntax or var = expression syntax), the variable’s type is inferred from the value on the right hand side.
When the right hand side of the declaration is typed, the new variable is of that same type:
var i int
j := i // j is an int
But when the right hand side contains an untyped numeric constant, the new variable may be an int, float64, or complex128 depending on the precision of the constant:
i := 42 // int
f := 3.142 // float64
g := 0.867 + 0.5i // complex128
Try changing the initial value of v in the example code and observe how its type is affected.
/* type-inference.go */
package main
import "fmt"
func main() {
v := 42 // change me!
fmt.Printf("v is of type %T\n", v)
}
Constants are declared like variables, but with the const keyword.
Constants can be character, string, boolean, or numeric values.
Constants cannot be declared using the := syntax.
/* constants.go */
package main
import "fmt"
const Pi = 3.14
func main() {
const World = "世界"
fmt.Println("Hello", World)
fmt.Println("Happy", Pi, "Day")
const Truth = true
fmt.Println("Go rules?", Truth)
}
Numeric constants are high-precision values.
An untyped constant takes the type needed by its context.
Try printing needInt(Big) too.
(An int can store at maximum a 64-bit integer, and sometimes less.)
/* numeric-constants.go */
package main
import "fmt"
const (
// Create a huge number by shifting a 1 bit left 100 places.
// In other words, the binary number that is 1 followed by 100 zeroes.
Big = 1 << 100
// Shift it right again 99 places, so we end up with 1<<1, or 2.
Small = Big >> 99
)
func needInt(x int) int { return x*10 + 1 }
func needFloat(x float64) float64 {
return x * 0.1
}
func main() {
fmt.Println(needInt(Small))
fmt.Println(needFloat(Small))
fmt.Println(needFloat(Big))
}
You finished this lesson!
You can go back to the list of modules to find what to learn next, or continue with the next lesson.
Learn how to control the flow of your code with conditionals, loops, switches and defers.
Go has only one looping construct, the for loop.
The basic for loop has three components separated by semicolons:
The init statement will often be a short variable declaration, and the
variables declared there are visible only in the scope of the for
statement.
The loop will stop iterating once the boolean condition evaluates to false.
Note: Unlike other languages like C, Java, or JavaScript there are no parentheses
surrounding the three components of the for statement and the braces { } are
always required.
/* for.go */
package main
import "fmt"
func main() {
sum := 0
for i := 0; i < 10; i++ {
sum += i
}
fmt.Println(sum)
}
The init and post statements are optional.
/* for-continued.go */
package main
import "fmt"
func main() {
sum := 1
for ; sum < 1000; {
sum += sum
}
fmt.Println(sum)
}
At that point you can drop the semicolons: C’s while is spelled for in Go.
/* for-is-gos-while.go */
package main
import "fmt"
func main() {
sum := 1
for sum < 1000 {
sum += sum
}
fmt.Println(sum)
}
If you omit the loop condition it loops forever, so an infinite loop is compactly expressed.
/* forever.go */
package main
func main() {
for {
}
}
Go’s if statements are like its for loops; the expression need not be
surrounded by parentheses ( ) but the braces { } are required.
/* if.go */
package main
import (
"fmt"
"math"
)
func sqrt(x float64) string {
if x < 0 {
return sqrt(-x) + "i"
}
return fmt.Sprint(math.Sqrt(x))
}
func main() {
fmt.Println(sqrt(2), sqrt(-4))
}
Like for, the if statement can start with a short statement to execute before the condition.
Variables declared by the statement are only in scope until the end of the if.
(Try using v in the last return statement.)
/* if-with-a-short-statement.go */
package main
import (
"fmt"
"math"
)
func pow(x, n, lim float64) float64 {
if v := math.Pow(x, n); v < lim {
return v
}
return lim
}
func main() {
fmt.Println(
pow(3, 2, 10),
pow(3, 3, 20),
)
}
Variables declared inside an if short statement are also available inside any
of the else blocks.
(Both calls to pow return their results before the call to fmt.Println
in main begins.)
/* if-and-else.go */
package main
import (
"fmt"
"math"
)
func pow(x, n, lim float64) float64 {
if v := math.Pow(x, n); v < lim {
return v
} else {
fmt.Printf("%g >= %g\n", v, lim)
}
// can't use v here, though
return lim
}
func main() {
fmt.Println(
pow(3, 2, 10),
pow(3, 3, 20),
)
}
As a way to play with functions and loops, let’s implement a square root function: given a number x, we want to find the number z for which z² is most nearly x.
Computers typically compute the square root of x using a loop. Starting with some guess z, we can adjust z based on how close z² is to x, producing a better guess:
z -= (z*z - x) / (2*z)
Repeating this adjustment makes the guess better and better until we reach an answer that is as close to the actual square root as can be.
Implement this in the func Sqrt provided.
A decent starting guess for z is 1, no matter what the input.
To begin with, repeat the calculation 10 times and print each z along the way.
See how close you get to the answer for various values of x (1, 2, 3, …)
and how quickly the guess improves.
Hint: To declare and initialize a floating point value, give it floating point syntax or use a conversion:
z := 1.0
z := float64(1)
Next, change the loop condition to stop once the value has stopped changing (or only changes by a very small amount). See if that’s more or fewer than 10 iterations. Try other initial guesses for z, like x, or x/2. How close are your function’s results to the math.Sqrt in the standard library?
(Note: If you are interested in the details of the algorithm, the z² − x above is how far away z² is from where it needs to be (x), and the division by 2z is the derivative of z², to scale how much we adjust z by how quickly z² is changing. This general approach is called Newton’s method. It works well for many functions but especially well for square root.)
/* exercise-loops-and-functions.go */
package main
import (
"fmt"
)
func Sqrt(x float64) float64 {
}
func main() {
fmt.Println(Sqrt(2))
}
A switch statement is a shorter way to write a sequence of if - else statements.
It runs the first case whose value is equal to the condition expression.
Go’s switch is like the one in C, C++, Java, JavaScript, and PHP,
except that Go only runs the selected case, not all the cases that follow.
In effect, the break statement that is needed at the end of each case in those
languages is provided automatically in Go.
Another important difference is that Go’s switch cases need not
be constants, and the values involved need not be integers.
/* switch.go */
package main
import (
"fmt"
"runtime"
)
func main() {
fmt.Print("Go runs on ")
switch os := runtime.GOOS; os {
case "darwin":
fmt.Println("macOS.")
case "linux":
fmt.Println("Linux.")
default:
// freebsd, openbsd,
// plan9, windows...
fmt.Printf("%s.\n", os)
}
}
Switch cases evaluate cases from top to bottom, stopping when a case succeeds.
(For example,
switch i {
case 0:
case f():
}
does not call f if i==0.)
Note: Time in the Go playground always appears to start at 2009-11-10 23:00:00 UTC, a value whose significance is left as an exercise for the reader.
/* switch-evaluation-order.go */
package main
import (
"fmt"
"time"
)
func main() {
fmt.Println("When's Saturday?")
today := time.Now().Weekday()
switch time.Saturday {
case today + 0:
fmt.Println("Today.")
case today + 1:
fmt.Println("Tomorrow.")
case today + 2:
fmt.Println("In two days.")
default:
fmt.Println("Too far away.")
}
}
Switch without a condition is the same as switch true.
This construct can be a clean way to write long if-then-else chains.
/* switch-with-no-condition.go */
package main
import (
"fmt"
"time"
)
func main() {
t := time.Now()
switch {
case t.Hour() < 12:
fmt.Println("Good morning!")
case t.Hour() < 17:
fmt.Println("Good afternoon.")
default:
fmt.Println("Good evening.")
}
}
A defer statement defers the execution of a function until the surrounding function returns.
The deferred call’s arguments are evaluated immediately, but the function call is not executed until the surrounding function returns.
/* defer.go */
package main
import "fmt"
func main() {
defer fmt.Println("world")
fmt.Println("hello")
}
Deferred function calls are pushed onto a stack. When a function returns, its deferred calls are executed in last-in-first-out order.
To learn more about defer statements read this blog post.
/* defer-multi.go */
package main
import "fmt"
func main() {
fmt.Println("counting")
for i := 0; i < 10; i++ {
defer fmt.Println(i)
}
fmt.Println("done")
}
You finished this lesson!
You can go back to the list of modules to find what to learn next, or continue with the next lesson.
Learn how to define types based on existing ones: this lesson covers structs, arrays, slices, and maps.
Go has pointers. A pointer holds the memory address of a value.
The type *T is a pointer to a T value. Its zero value is nil.
var p *int
The & operator generates a pointer to its operand.
i := 42
p = &i
The * operator denotes the pointer’s underlying value.
fmt.Println(*p) // read i through the pointer p
*p = 21 // set i through the pointer p
This is known as “dereferencing” or “indirecting”.
Unlike C, Go has no pointer arithmetic.
/* pointers.go */
package main
import "fmt"
func main() {
i, j := 42, 2701
p := &i // point to i
fmt.Println(*p) // read i through the pointer
*p = 21 // set i through the pointer
fmt.Println(i) // see the new value of i
p = &j // point to j
*p = *p / 37 // divide j through the pointer
fmt.Println(j) // see the new value of j
}
A struct is a collection of fields.
/* structs.go */
package main
import "fmt"
type Vertex struct {
X int
Y int
}
func main() {
fmt.Println(Vertex{1, 2})
}
Struct fields are accessed using a dot.
/* struct-fields.go */
package main
import "fmt"
type Vertex struct {
X int
Y int
}
func main() {
v := Vertex{1, 2}
v.X = 4
fmt.Println(v.X)
}
Struct fields can be accessed through a struct pointer.
To access the field X of a struct when we have the struct pointer p we could
write (*p).X.
However, that notation is cumbersome, so the language permits us instead to
write just p.X, without the explicit dereference.
/* struct-pointers.go */
package main
import "fmt"
type Vertex struct {
X int
Y int
}
func main() {
v := Vertex{1, 2}
p := &v
p.X = 1e9
fmt.Println(v)
}
A struct literal denotes a newly allocated struct value by listing the values of its fields.
You can list just a subset of fields by using the Name: syntax. (And the order of named fields is irrelevant.)
The special prefix & returns a pointer to the struct value.
/* struct-literals.go */
package main
import "fmt"
type Vertex struct {
X, Y int
}
var (
v1 = Vertex{1, 2} // has type Vertex
v2 = Vertex{X: 1} // Y:0 is implicit
v3 = Vertex{} // X:0 and Y:0
p = &Vertex{1, 2} // has type *Vertex
)
func main() {
fmt.Println(v1, p, v2, v3)
}
The type [n]T is an array of n values of type T.
The expression
var a [10]int
declares a variable a as an array of ten integers.
An array’s length is part of its type, so arrays cannot be resized. This seems limiting, but don’t worry; Go provides a convenient way of working with arrays.
/* array.go */
package main
import "fmt"
func main() {
var a [2]string
a[0] = "Hello"
a[1] = "World"
fmt.Println(a[0], a[1])
fmt.Println(a)
primes := [6]int{2, 3, 5, 7, 11, 13}
fmt.Println(primes)
}
An array has a fixed size. A slice, on the other hand, is a dynamically-sized, flexible view into the elements of an array. In practice, slices are much more common than arrays.
The type []T is a slice with elements of type T.
A slice is formed by specifying two indices, a low and high bound, separated by a colon:
a[low : high]
This selects a half-open range which includes the first element, but excludes the last one.
The following expression creates a slice which includes
elements 1 through 3 of a:
a[1:4]
/* slices.go */
package main
import "fmt"
func main() {
primes := [6]int{2, 3, 5, 7, 11, 13}
var s []int = primes[1:4]
fmt.Println(s)
}
A slice does not store any data, it just describes a section of an underlying array.
Changing the elements of a slice modifies the corresponding elements of its underlying array.
Other slices that share the same underlying array will see those changes.
/* slices-pointers.go */
package main
import "fmt"
func main() {
names := [4]string{
"John",
"Paul",
"George",
"Ringo",
}
fmt.Println(names)
a := names[0:2]
b := names[1:3]
fmt.Println(a, b)
b[0] = "XXX"
fmt.Println(a, b)
fmt.Println(names)
}
A slice literal is like an array literal without the length.
This is an array literal:
[3]bool{true, true, false}
And this creates the same array as above, then builds a slice that references it:
[]bool{true, true, false}
/* slice-literals.go */
package main
import "fmt"
func main() {
q := []int{2, 3, 5, 7, 11, 13}
fmt.Println(q)
r := []bool{true, false, true, true, false, true}
fmt.Println(r)
s := []struct {
i int
b bool
}{
{2, true},
{3, false},
{5, true},
{7, true},
{11, false},
{13, true},
}
fmt.Println(s)
}
When slicing, you may omit the high or low bounds to use their defaults instead.
The default is zero for the low bound and the length of the slice for the high bound.
For the array
var a [10]int
these slice expressions are equivalent:
a[0:10]
a[:10]
a[0:]
a[:]
/* slice-bounds.go */
package main
import "fmt"
func main() {
s := []int{2, 3, 5, 7, 11, 13}
s = s[1:4]
fmt.Println(s)
s = s[:2]
fmt.Println(s)
s = s[1:]
fmt.Println(s)
}
A slice has both a length and a capacity.
The length of a slice is the number of elements it contains.
The capacity of a slice is the number of elements in the underlying array, counting from the first element in the slice.
The length and capacity of a slice s can be obtained using the expressions
len(s) and cap(s).
You can extend a slice’s length by re-slicing it, provided it has sufficient capacity. Try changing one of the slice operations in the example program to extend it beyond its capacity and see what happens.
/* slice-len-cap.go */
package main
import "fmt"
func main() {
s := []int{2, 3, 5, 7, 11, 13}
printSlice(s)
// Slice the slice to give it zero length.
s = s[:0]
printSlice(s)
// Extend its length.
s = s[:4]
printSlice(s)
// Drop its first two values.
s = s[2:]
printSlice(s)
}
func printSlice(s []int) {
fmt.Printf("len=%d cap=%d %v\n", len(s), cap(s), s)
}
The zero value of a slice is nil.
A nil slice has a length and capacity of 0 and has no underlying array.
/* nil-slices.go */
package main
import "fmt"
func main() {
var s []int
fmt.Println(s, len(s), cap(s))
if s == nil {
fmt.Println("nil!")
}
}
Slices can be created with the built-in make function;
this is how you create dynamically-sized arrays.
The make function allocates a zeroed array
and returns a slice that refers to that array:
a := make([]int, 5) // len(a)=5
To specify a capacity, pass a third argument to make:
b := make([]int, 0, 5) // len(b)=0, cap(b)=5
b = b[:cap(b)] // len(b)=5, cap(b)=5
b = b[1:] // len(b)=4, cap(b)=4
/* making-slices.go */
package main
import "fmt"
func main() {
a := make([]int, 5)
printSlice("a", a)
b := make([]int, 0, 5)
printSlice("b", b)
c := b[:2]
printSlice("c", c)
d := c[2:5]
printSlice("d", d)
}
func printSlice(s string, x []int) {
fmt.Printf("%s len=%d cap=%d %v\n",
s, len(x), cap(x), x)
}
Slices can contain any type, including other slices.
/* slices-of-slice.go */
package main
import (
"fmt"
"strings"
)
func main() {
// Create a tic-tac-toe board.
board := [][]string{
[]string{"_", "_", "_"},
[]string{"_", "_", "_"},
[]string{"_", "_", "_"},
}
// The players take turns.
board[0][0] = "X"
board[2][2] = "O"
board[1][2] = "X"
board[1][0] = "O"
board[0][2] = "X"
for i := 0; i < len(board); i++ {
fmt.Printf("%s\n", strings.Join(board[i], " "))
}
}
It is common to append new elements to a slice, and so Go provides a built-in
append function. The documentation
of the built-in package describes append.
func append(s []T, vs ...T) []T
The first parameter s of append is a slice of type T, and the rest are
T values to append to the slice.
The resulting value of append is a slice containing all the elements of the
original slice plus the provided values.
If the backing array of s is too small to fit all the given values a bigger
array will be allocated. The returned slice will point to the newly allocated
array.
(To learn more about slices, read the Slices: usage and internals article.)
/* append.go */
package main
import "fmt"
func main() {
var s []int
printSlice(s)
// append works on nil slices.
s = append(s, 0)
printSlice(s)
// The slice grows as needed.
s = append(s, 1)
printSlice(s)
// We can add more than one element at a time.
s = append(s, 2, 3, 4)
printSlice(s)
}
func printSlice(s []int) {
fmt.Printf("len=%d cap=%d %v\n", len(s), cap(s), s)
}
The range form of the for loop iterates over a slice or map.
When ranging over a slice, two values are returned for each iteration. The first is the index, and the second is a copy of the element at that index.
/* range.go */
package main
import "fmt"
var pow = []int{1, 2, 4, 8, 16, 32, 64, 128}
func main() {
for i, v := range pow {
fmt.Printf("2**%d = %d\n", i, v)
}
}
You can skip the index or value by assigning to _.
for i, _ := range pow
for _, value := range pow
If you only want the index, you can omit the second variable.
for i := range pow
/* range-continued.go */
package main
import "fmt"
func main() {
pow := make([]int, 10)
for i := range pow {
pow[i] = 1 << uint(i) // == 2**i
}
for _, value := range pow {
fmt.Printf("%d\n", value)
}
}
Implement Pic. It should return a slice of length dy, each element of which is a slice of dx 8-bit unsigned integers. When you run the program, it will display your picture, interpreting the integers as grayscale (well, bluescale) values.
The choice of image is up to you. Interesting functions include (x+y)/2, x*y, and x^y.
(You need to use a loop to allocate each []uint8 inside the [][]uint8.)
(Use uint8(intValue) to convert between types.)
/* exercise-slices.go */
package main
import "golang.org/x/tour/pic"
func Pic(dx, dy int) [][]uint8 {
}
func main() {
pic.Show(Pic)
}
A map maps keys to values.
The zero value of a map is nil.
A nil map has no keys, nor can keys be added.
The make function returns a map of the given type,
initialized and ready for use.
/* maps.go */
package main
import "fmt"
type Vertex struct {
Lat, Long float64
}
var m map[string]Vertex
func main() {
m = make(map[string]Vertex)
m["Bell Labs"] = Vertex{
40.68433, -74.39967,
}
fmt.Println(m["Bell Labs"])
}
Map literals are like struct literals, but the keys are required.
/* map-literals.go */
package main
import "fmt"
type Vertex struct {
Lat, Long float64
}
var m = map[string]Vertex{
"Bell Labs": Vertex{
40.68433, -74.39967,
},
"Google": Vertex{
37.42202, -122.08408,
},
}
func main() {
fmt.Println(m)
}
If the top-level type is just a type name, you can omit it from the elements of the literal.
/* map-literals-continued.go */
package main
import "fmt"
type Vertex struct {
Lat, Long float64
}
var m = map[string]Vertex{
"Bell Labs": {40.68433, -74.39967},
"Google": {37.42202, -122.08408},
}
func main() {
fmt.Println(m)
}
Insert or update an element in map m:
m[key] = elem
Retrieve an element:
elem = m[key]
Delete an element:
delete(m, key)
Test that a key is present with a two-value assignment:
elem, ok = m[key]
If key is in m, ok is true. If not, ok is false.
If key is not in the map, then elem is the zero value for the map’s element type.
Note: If elem or ok have not yet been declared you could use a short declaration form:
elem, ok := m[key]
/* mutating-maps.go */
package main
import "fmt"
func main() {
m := make(map[string]int)
m["Answer"] = 42
fmt.Println("The value:", m["Answer"])
m["Answer"] = 48
fmt.Println("The value:", m["Answer"])
delete(m, "Answer")
fmt.Println("The value:", m["Answer"])
v, ok := m["Answer"]
fmt.Println("The value:", v, "Present?", ok)
}
Implement WordCount. It should return a map of the counts of each “word” in the string s. The wc.Test function runs a test suite against the provided function and prints success or failure.
You might find strings.Fields helpful.
/* exercise-maps.go */
package main
import (
"golang.org/x/tour/wc"
)
func WordCount(s string) map[string]int {
return map[string]int{"x": 1}
}
func main() {
wc.Test(WordCount)
}
Functions are values too. They can be passed around just like other values.
Function values may be used as function arguments and return values.
/* function-values.go */
package main
import (
"fmt"
"math"
)
func compute(fn func(float64, float64) float64) float64 {
return fn(3, 4)
}
func main() {
hypot := func(x, y float64) float64 {
return math.Sqrt(x*x + y*y)
}
fmt.Println(hypot(5, 12))
fmt.Println(compute(hypot))
fmt.Println(compute(math.Pow))
}
Go functions may be closures. A closure is a function value that references variables from outside its body. The function may access and assign to the referenced variables; in this sense the function is “bound” to the variables.
For example, the adder function returns a closure. Each closure is bound to its own sum variable.
/* function-closures.go */
package main
import "fmt"
func adder() func(int) int {
sum := 0
return func(x int) int {
sum += x
return sum
}
}
func main() {
pos, neg := adder(), adder()
for i := 0; i < 10; i++ {
fmt.Println(
pos(i),
neg(-2*i),
)
}
}
Let’s have some fun with functions.
Implement a fibonacci function that returns a function (a closure) that
returns successive fibonacci numbers
(0, 1, 1, 2, 3, 5, …).
/* exercise-fibonacci-closure.go */
package main
import "fmt"
// fibonacci is a function that returns
// a function that returns an int.
func fibonacci() func() int {
}
func main() {
f := fibonacci()
for i := 0; i < 10; i++ {
fmt.Println(f())
}
}
You finished this lesson!
You can go back to the list of modules to find what to learn next, or continue with the next lesson.
This lesson covers methods and interfaces, the constructs that define objects and their behavior.
Go does not have classes. However, you can define methods on types.
A method is a function with a special receiver argument.
The receiver appears in its own argument list between the func keyword and
the method name.
In this example, the Abs method has a receiver of type Vertex named v.
/* methods.go */
package main
import (
"fmt"
"math"
)
type Vertex struct {
X, Y float64
}
func (v Vertex) Abs() float64 {
return math.Sqrt(v.X*v.X + v.Y*v.Y)
}
func main() {
v := Vertex{3, 4}
fmt.Println(v.Abs())
}
Remember: a method is just a function with a receiver argument.
Here’s Abs written as a regular function with no change in functionality.
/* methods-funcs.go */
package main
import (
"fmt"
"math"
)
type Vertex struct {
X, Y float64
}
func Abs(v Vertex) float64 {
return math.Sqrt(v.X*v.X + v.Y*v.Y)
}
func main() {
v := Vertex{3, 4}
fmt.Println(Abs(v))
}
You can declare a method on non-struct types, too.
In this example we see a numeric type MyFloat with an Abs method.
You can only declare a method with a receiver whose type is defined in the same
package as the method.
You cannot declare a method with a receiver whose type is defined in another
package (which includes the built-in types such as int).
/* methods-continued.go */
package main
import (
"fmt"
"math"
)
type MyFloat float64
func (f MyFloat) Abs() float64 {
if f < 0 {
return float64(-f)
}
return float64(f)
}
func main() {
f := MyFloat(-math.Sqrt2)
fmt.Println(f.Abs())
}
You can declare methods with pointer receivers.
This means the receiver type has the literal syntax *T for some type T.
(Also, T cannot itself be a pointer such as *int.)
For example, the Scale method here is defined on *Vertex.
Methods with pointer receivers can modify the value to which the receiver
points (as Scale does here).
Since methods often need to modify their receiver, pointer receivers are more
common than value receivers.
Try removing the * from the declaration of the Scale function on line 16
and observe how the program’s behavior changes.
With a value receiver, the Scale method operates on a copy of the original
Vertex value.
(This is the same behavior as for any other function argument.)
The Scale method must have a pointer receiver to change the Vertex value
declared in the main function.
/* methods-pointers.go */
package main
import (
"fmt"
"math"
)
type Vertex struct {
X, Y float64
}
func (v Vertex) Abs() float64 {
return math.Sqrt(v.X*v.X + v.Y*v.Y)
}
func (v *Vertex) Scale(f float64) {
v.X = v.X * f
v.Y = v.Y * f
}
func main() {
v := Vertex{3, 4}
v.Scale(10)
fmt.Println(v.Abs())
}
Here we see the Abs and Scale methods rewritten as functions.
Again, try removing the * from line 16.
Can you see why the behavior changes?
What else did you need to change for the example to compile?
(If you’re not sure, continue to the next page.)
/* methods-pointers-explained.go */
package main
import (
"fmt"
"math"
)
type Vertex struct {
X, Y float64
}
func Abs(v Vertex) float64 {
return math.Sqrt(v.X*v.X + v.Y*v.Y)
}
func Scale(v *Vertex, f float64) {
v.X = v.X * f
v.Y = v.Y * f
}
func main() {
v := Vertex{3, 4}
Scale(&v, 10)
fmt.Println(Abs(v))
}
Comparing the previous two programs, you might notice that functions with a pointer argument must take a pointer:
var v Vertex
ScaleFunc(v, 5) // Compile error!
ScaleFunc(&v, 5) // OK
while methods with pointer receivers take either a value or a pointer as the receiver when they are called:
var v Vertex
v.Scale(5) // OK
p := &v
p.Scale(10) // OK
For the statement v.Scale(5), even though v is a value and not a pointer,
the method with the pointer receiver is called automatically.
That is, as a convenience, Go interprets the statement v.Scale(5) as
(&v).Scale(5) since the Scale method has a pointer receiver.
/* indirection.go */
package main
import "fmt"
type Vertex struct {
X, Y float64
}
func (v *Vertex) Scale(f float64) {
v.X = v.X * f
v.Y = v.Y * f
}
func ScaleFunc(v *Vertex, f float64) {
v.X = v.X * f
v.Y = v.Y * f
}
func main() {
v := Vertex{3, 4}
v.Scale(2)
ScaleFunc(&v, 10)
p := &Vertex{4, 3}
p.Scale(3)
ScaleFunc(p, 8)
fmt.Println(v, p)
}
The equivalent thing happens in the reverse direction.
Functions that take a value argument must take a value of that specific type:
var v Vertex
fmt.Println(AbsFunc(v)) // OK
fmt.Println(AbsFunc(&v)) // Compile error!
while methods with value receivers take either a value or a pointer as the receiver when they are called:
var v Vertex
fmt.Println(v.Abs()) // OK
p := &v
fmt.Println(p.Abs()) // OK
In this case, the method call p.Abs() is interpreted as (*p).Abs().
/* indirection-values.go */
package main
import (
"fmt"
"math"
)
type Vertex struct {
X, Y float64
}
func (v Vertex) Abs() float64 {
return math.Sqrt(v.X*v.X + v.Y*v.Y)
}
func AbsFunc(v Vertex) float64 {
return math.Sqrt(v.X*v.X + v.Y*v.Y)
}
func main() {
v := Vertex{3, 4}
fmt.Println(v.Abs())
fmt.Println(AbsFunc(v))
p := &Vertex{4, 3}
fmt.Println(p.Abs())
fmt.Println(AbsFunc(*p))
}
There are two reasons to use a pointer receiver.
The first is so that the method can modify the value that its receiver points to.
The second is to avoid copying the value on each method call. This can be more efficient if the receiver is a large struct, for example.
In this example, both Scale and Abs are methods with receiver type *Vertex,
even though the Abs method needn’t modify its receiver.
In general, all methods on a given type should have either value or pointer receivers, but not a mixture of both. (We’ll see why over the next few pages.)
/* methods-with-pointer-receivers.go */
package main
import (
"fmt"
"math"
)
type Vertex struct {
X, Y float64
}
func (v *Vertex) Scale(f float64) {
v.X = v.X * f
v.Y = v.Y * f
}
func (v *Vertex) Abs() float64 {
return math.Sqrt(v.X*v.X + v.Y*v.Y)
}
func main() {
v := &Vertex{3, 4}
fmt.Printf("Before scaling: %+v, Abs: %v\n", v, v.Abs())
v.Scale(5)
fmt.Printf("After scaling: %+v, Abs: %v\n", v, v.Abs())
}
An interface_type is defined as a set of method signatures.
A value of interface type can hold any value that implements those methods.
Note: There is an error in the example code on line 22.
Vertex (the value type) doesn’t implement Abser because
the Abs method is defined only on *Vertex (the pointer type).
/* interfaces.go */
package main
import (
"fmt"
"math"
)
type Abser interface {
Abs() float64
}
func main() {
var a Abser
f := MyFloat(-math.Sqrt2)
v := Vertex{3, 4}
a = f // a MyFloat implements Abser
a = &v // a *Vertex implements Abser
// In the following line, v is a Vertex (not *Vertex)
// and does NOT implement Abser.
a = v
fmt.Println(a.Abs())
}
type MyFloat float64
func (f MyFloat) Abs() float64 {
if f < 0 {
return float64(-f)
}
return float64(f)
}
type Vertex struct {
X, Y float64
}
func (v *Vertex) Abs() float64 {
return math.Sqrt(v.X*v.X + v.Y*v.Y)
}
A type implements an interface by implementing its methods. There is no explicit declaration of intent, no “implements” keyword.
Implicit interfaces decouple the definition of an interface from its implementation, which could then appear in any package without prearrangement.
/* interfaces-are-satisfied-implicitly.go */
package main
import "fmt"
type I interface {
M()
}
type T struct {
S string
}
// This method means type T implements the interface I,
// but we don't need to explicitly declare that it does so.
func (t T) M() {
fmt.Println(t.S)
}
func main() {
var i I = T{"hello"}
i.M()
}
Under the hood, interface values can be thought of as a tuple of a value and a concrete type:
(value, type)
An interface value holds a value of a specific underlying concrete type.
Calling a method on an interface value executes the method of the same name on its underlying type.
/* interface-values.go */
package main
import (
"fmt"
"math"
)
type I interface {
M()
}
type T struct {
S string
}
func (t *T) M() {
fmt.Println(t.S)
}
type F float64
func (f F) M() {
fmt.Println(f)
}
func main() {
var i I
i = &T{"Hello"}
describe(i)
i.M()
i = F(math.Pi)
describe(i)
i.M()
}
func describe(i I) {
fmt.Printf("(%v, %T)\n", i, i)
}
If the concrete value inside the interface itself is nil, the method will be called with a nil receiver.
In some languages this would trigger a null pointer exception,
but in Go it is common to write methods that gracefully handle being called
with a nil receiver (as with the method M in this example.)
Note that an interface value that holds a nil concrete value is itself non-nil.
/* interface-values-with-nil.go */
package main
import "fmt"
type I interface {
M()
}
type T struct {
S string
}
func (t *T) M() {
if t == nil {
fmt.Println("<nil>")
return
}
fmt.Println(t.S)
}
func main() {
var i I
var t *T
i = t
describe(i)
i.M()
i = &T{"hello"}
describe(i)
i.M()
}
func describe(i I) {
fmt.Printf("(%v, %T)\n", i, i)
}
A nil interface value holds neither value nor concrete type.
Calling a method on a nil interface is a run-time error because there is no type inside the interface tuple to indicate which concrete method to call.
/* nil-interface-values.go */
package main
import "fmt"
type I interface {
M()
}
func main() {
var i I
describe(i)
i.M()
}
func describe(i I) {
fmt.Printf("(%v, %T)\n", i, i)
}
The interface type that specifies zero methods is known as the empty_interface:
interface{}
An empty interface may hold values of any type. (Every type implements at least zero methods.)
Empty interfaces are used by code that handles values of unknown type.
For example, fmt.Print takes any number of arguments of type interface{}.
/* empty-interface.go */
package main
import "fmt"
func main() {
var i interface{}
describe(i)
i = 42
describe(i)
i = "hello"
describe(i)
}
func describe(i interface{}) {
fmt.Printf("(%v, %T)\n", i, i)
}
A type_assertion provides access to an interface value’s underlying concrete value.
t := i.(T)
This statement asserts that the interface value i holds the concrete type T
and assigns the underlying T value to the variable t.
If i does not hold a T, the statement will trigger a panic.
To test whether an interface value holds a specific type, a type assertion can return two values: the underlying value and a boolean value that reports whether the assertion succeeded.
t, ok := i.(T)
If i holds a T, then t will be the underlying value and ok will be true.
If not, ok will be false and t will be the zero value of type T,
and no panic occurs.
Note the similarity between this syntax and that of reading from a map.
/* type-assertions.go */
package main
import "fmt"
func main() {
var i interface{} = "hello"
s := i.(string)
fmt.Println(s)
s, ok := i.(string)
fmt.Println(s, ok)
f, ok := i.(float64)
fmt.Println(f, ok)
f = i.(float64) // panic
fmt.Println(f)
}
A type_switch is a construct that permits several type assertions in series.
A type switch is like a regular switch statement, but the cases in a type switch specify types (not values), and those values are compared against the type of the value held by the given interface value.
switch v := i.(type) {
case T:
// here v has type T
case S:
// here v has type S
default:
// no match; here v has the same type as i
}
The declaration in a type switch has the same syntax as a type assertion i.(T),
but the specific type T is replaced with the keyword type.
This switch statement tests whether the interface value i
holds a value of type T or S.
In each of the T and S cases, the variable v will be of type
T or S respectively and hold the value held by i.
In the default case (where there is no match), the variable v is
of the same interface type and value as i.
/* type-switches.go */
package main
import "fmt"
func do(i interface{}) {
switch v := i.(type) {
case int:
fmt.Printf("Twice %v is %v\n", v, v*2)
case string:
fmt.Printf("%q is %v bytes long\n", v, len(v))
default:
fmt.Printf("I don't know about type %T!\n", v)
}
}
func main() {
do(21)
do("hello")
do(true)
}
One of the most ubiquitous interfaces is Stringer defined by the fmt package.
type Stringer interface {
String() string
}
A Stringer is a type that can describe itself as a string. The fmt package
(and many others) look for this interface to print values.
/* stringer.go */
package main
import "fmt"
type Person struct {
Name string
Age int
}
func (p Person) String() string {
return fmt.Sprintf("%v (%v years)", p.Name, p.Age)
}
func main() {
a := Person{"Arthur Dent", 42}
z := Person{"Zaphod Beeblebrox", 9001}
fmt.Println(a, z)
}
Make the IPAddr type implement fmt.Stringer to print the address as
a dotted quad.
For instance, IPAddr{1, 2, 3, 4} should print as "1.2.3.4".
/* exercise-stringer.go */
package main
import "fmt"
type IPAddr [4]byte
// TODO: Add a "String() string" method to IPAddr.
func main() {
hosts := map[string]IPAddr{
"loopback": {127, 0, 0, 1},
"googleDNS": {8, 8, 8, 8},
}
for name, ip := range hosts {
fmt.Printf("%v: %v\n", name, ip)
}
}
Go programs express error state with error values.
The error type is a built-in interface similar to fmt.Stringer:
type error interface {
Error() string
}
(As with fmt.Stringer, the fmt package looks for the error interface when
printing values.)
Functions often return an error value, and calling code should handle errors
by testing whether the error equals nil.
i, err := strconv.Atoi("42")
if err != nil {
fmt.Printf("couldn't convert number: %v\n", err)
return
}
fmt.Println("Converted integer:", i)
A nil error denotes success; a non-nil error denotes failure.
/* errors.go */
package main
import (
"fmt"
"time"
)
type MyError struct {
When time.Time
What string
}
func (e *MyError) Error() string {
return fmt.Sprintf("at %v, %s",
e.When, e.What)
}
func run() error {
return &MyError{
time.Now(),
"it didn't work",
}
}
func main() {
if err := run(); err != nil {
fmt.Println(err)
}
}
Copy your Sqrt function from the earlier exercise and modify it to return an error value.
Sqrt should return a non-nil error value when given a negative number, as it doesn’t support complex numbers.
Create a new type
type ErrNegativeSqrt float64
and make it an error by giving it a
func (e ErrNegativeSqrt) Error() string
method such that ErrNegativeSqrt(-2).Error() returns "cannot Sqrt negative number: -2".
Note: A call to fmt.Sprint(e) inside the Error method will send the program into an infinite loop. You can avoid this by converting e first: fmt.Sprint(float64(e)). Why?
Change your Sqrt function to return an ErrNegativeSqrt value when given a negative number.
/* exercise-errors.go */
package main
import (
"fmt"
)
func Sqrt(x float64) (float64, error) {
return 0, nil
}
func main() {
fmt.Println(Sqrt(2))
fmt.Println(Sqrt(-2))
}
The io package specifies the io.Reader interface,
which represents the read end of a stream of data.
The Go standard library contains many implementations of this interface, including files, network connections, compressors, ciphers, and others.
The io.Reader interface has a Read method:
func (T) Read(b []byte) (n int, err error)
Read populates the given byte slice with data and returns the number of bytes
populated and an error value. It returns an io.EOF error when the stream
ends.
The example code creates a
strings.Reader
and consumes its output 8 bytes at a time.
/* reader.go */
package main
import (
"fmt"
"io"
"strings"
)
func main() {
r := strings.NewReader("Hello, Reader!")
b := make([]byte, 8)
for {
n, err := r.Read(b)
fmt.Printf("n = %v err = %v b = %v\n", n, err, b)
fmt.Printf("b[:n] = %q\n", b[:n])
if err == io.EOF {
break
}
}
}
Implement a Reader type that emits an infinite stream of the ASCII character
'A'.
/* exercise-reader.go */
package main
import "golang.org/x/tour/reader"
type MyReader struct{}
// TODO: Add a Read([]byte) (int, error) method to MyReader.
func main() {
reader.Validate(MyReader{})
}
A common pattern is an io.Reader that wraps another io.Reader, modifying the stream in some way.
For example, the gzip.NewReader function takes an io.Reader (a stream of compressed data) and returns a *gzip.Reader that also implements io.Reader (a stream of the decompressed data).
Implement a rot13Reader that implements io.Reader and reads from an io.Reader, modifying the stream by applying the rot13 substitution cipher to all alphabetical characters.
The rot13Reader type is provided for you.
Make it an io.Reader by implementing its Read method.
/* exercise-rot-reader.go */
package main
import (
"io"
"os"
"strings"
)
type rot13Reader struct {
r io.Reader
}
func main() {
s := strings.NewReader("Lbh penpxrq gur pbqr!")
r := rot13Reader{s}
io.Copy(os.Stdout, &r)
}
Package image defines the Image interface:
package image
type Image interface {
ColorModel() color.Model
Bounds() Rectangle
At(x, y int) color.Color
}
Note: the Rectangle return value of the Bounds method is actually an
image.Rectangle, as the
declaration is inside package image.
(See the documentation for all the details.)
The color.Color and color.Model types are also interfaces, but we’ll ignore that by using the predefined implementations color.RGBA and color.RGBAModel. These interfaces and types are specified by the image/color package.
/* images.go */
package main
import (
"fmt"
"image"
)
func main() {
m := image.NewRGBA(image.Rect(0, 0, 100, 100))
fmt.Println(m.Bounds())
fmt.Println(m.At(0, 0).RGBA())
}
Remember the picture generator you wrote earlier? Let’s write another one, but this time it will return an implementation of image.Image instead of a slice of data.
Define your own Image type, implement the necessary methods, and call pic.ShowImage.
Bounds should return a image.Rectangle, like image.Rect(0, 0, w, h).
ColorModel should return color.RGBAModel.
At should return a color; the value v in the last picture generator corresponds to color.RGBA{v, v, 255, 255} in this one.
/* exercise-images.go */
package main
import "golang.org/x/tour/pic"
type Image struct{}
func main() {
m := Image{}
pic.ShowImage(m)
}
You finished this lesson!
You can go back to the list of modules to find what to learn next, or continue with the next lesson.
Go supports generic programming using type parameters. This lesson shows some examples for employing generics in your code.
Go functions can be written to work on multiple types using type parameters. The type parameters of a function appear between brackets, before the function’s arguments.
func Index[T comparable](s []T, x T) int
This declaration means that s is a slice of any type T that fulfills the
built-in constraint comparable. x is also a value of the same type.
comparable is a useful constraint that makes it possible to use the == and
!= operators on values of the type. In this example, we use it to compare a
value to all slice elements until a match is found. This Index function works
for any type that supports comparison.
/* index.go */
package main
import "fmt"
// Index returns the index of x in s, or -1 if not found.
func Index[T comparable](s []T, x T) int {
for i, v := range s {
// v and x are type T, which has the comparable
// constraint, so we can use == here.
if v == x {
return i
}
}
return -1
}
func main() {
// Index works on a slice of ints
si := []int{10, 20, 15, -10}
fmt.Println(Index(si, 15))
// Index also works on a slice of strings
ss := []string{"foo", "bar", "baz"}
fmt.Println(Index(ss, "hello"))
}
In addition to generic functions, Go also supports generic types. A type can be parameterized with a type parameter, which could be useful for implementing generic data structures.
This example demonstrates a simple type declaration for a singly-linked list holding any type of value.
As an exercise, add some functionality to this list implementation.
/* list.go */
package main
// List represents a singly-linked list that holds
// values of any type.
type List[T any] struct {
next *List[T]
val T
}
func main() {
}
You finished this lesson!
You can go back to the list of modules to find what to learn next, or continue with the next lesson.
Go provides concurrency constructions as part of the core language. This lesson presents them and gives some examples on how they can be used.
A goroutine is a lightweight thread managed by the Go runtime.
go f(x, y, z)
starts a new goroutine running
f(x, y, z)
The evaluation of f, x, y, and z happens in the current goroutine and the execution of f happens in the new goroutine.
Goroutines run in the same address space, so access to shared memory must be synchronized. The sync package provides useful primitives, although you won’t need them much in Go as there are other primitives. (See the next slide.)
/* goroutines.go */
package main
import (
"fmt"
"time"
)
func say(s string) {
for i := 0; i < 5; i++ {
time.Sleep(100 * time.Millisecond)
fmt.Println(s)
}
}
func main() {
go say("world")
say("hello")
}
Channels are a typed conduit through which you can send and receive values with the channel operator, <-.
ch <- v // Send v to channel ch.
v := <-ch // Receive from ch, and
// assign value to v.
(The data flows in the direction of the arrow.)
Like maps and slices, channels must be created before use:
ch := make(chan int)
By default, sends and receives block until the other side is ready. This allows goroutines to synchronize without explicit locks or condition variables.
The example code sums the numbers in a slice, distributing the work between two goroutines. Once both goroutines have completed their computation, it calculates the final result.
/* channels.go */
package main
import "fmt"
func sum(s []int, c chan int) {
sum := 0
for _, v := range s {
sum += v
}
c <- sum // send sum to c
}
func main() {
s := []int{7, 2, 8, -9, 4, 0}
c := make(chan int)
go sum(s[:len(s)/2], c)
go sum(s[len(s)/2:], c)
x, y := <-c, <-c // receive from c
fmt.Println(x, y, x+y)
}
Channels can be buffered. Provide the buffer length as the second argument to make to initialize a buffered channel:
ch := make(chan int, 100)
Sends to a buffered channel block only when the buffer is full. Receives block when the buffer is empty.
Modify the example to overfill the buffer and see what happens.
/* buffered-channels.go */
package main
import "fmt"
func main() {
ch := make(chan int, 2)
ch <- 1
ch <- 2
fmt.Println(<-ch)
fmt.Println(<-ch)
}
A sender can close a channel to indicate that no more values will be sent. Receivers can test whether a channel has been closed by assigning a second parameter to the receive expression: after
v, ok := <-ch
ok is false if there are no more values to receive and the channel is closed.
The loop for i := range c receives values from the channel repeatedly until it is closed.
Note: Only the sender should close a channel, never the receiver. Sending on a closed channel will cause a panic.
Anothernote:* Channels aren’t like files; you don’t usually need to close them. Closing is only necessary when the receiver must be told there are no more values coming, such as to terminate a range loop.
/* range-and-close.go */
package main
import (
"fmt"
)
func fibonacci(n int, c chan int) {
x, y := 0, 1
for i := 0; i < n; i++ {
c <- x
x, y = y, x+y
}
close(c)
}
func main() {
c := make(chan int, 10)
go fibonacci(cap(c), c)
for i := range c {
fmt.Println(i)
}
}
The select statement lets a goroutine wait on multiple communication operations.
A select blocks until one of its cases can run, then it executes that case. It chooses one at random if multiple are ready.
/* select.go */
package main
import "fmt"
func fibonacci(c, quit chan int) {
x, y := 0, 1
for {
select {
case c <- x:
x, y = y, x+y
case <-quit:
fmt.Println("quit")
return
}
}
}
func main() {
c := make(chan int)
quit := make(chan int)
go func() {
for i := 0; i < 10; i++ {
fmt.Println(<-c)
}
quit <- 0
}()
fibonacci(c, quit)
}
The default case in a select is run if no other case is ready.
Use a default case to try a send or receive without blocking:
select {
case i := <-c:
// use i
default:
// receiving from c would block
}
/* default-selection.go */
package main
import (
"fmt"
"time"
)
func main() {
start := time.Now()
tick := time.Tick(100 * time.Millisecond)
boom := time.After(500 * time.Millisecond)
elapsed := func() time.Duration {
return time.Since(start).Round(time.Millisecond)
}
for {
select {
case <-tick:
fmt.Printf("[%6s] tick.\n", elapsed())
case <-boom:
fmt.Printf("[%6s] BOOM!\n", elapsed())
return
default:
fmt.Printf("[%6s] .\n", elapsed())
time.Sleep(50 * time.Millisecond)
}
}
}
There can be many different binary trees with the same sequence of values stored in it. For example, here are two binary trees storing the sequence 1, 1, 2, 3, 5, 8, 13.
A function to check whether two binary trees store the same sequence is quite complex in most languages. We’ll use Go’s concurrency and channels to write a simple solution.
This example uses the tree package, which defines the type:
type Tree struct {
Left *Tree
Value int
Right *Tree
}
Continue description on next page.
1. Implement the Walk function.
2. Test the Walk function.
The function tree.New(k) constructs a randomly-structured (but always sorted) binary tree holding the values k, 2k, 3k, …, 10k.
Create a new channel ch and kick off the walker:
go Walk(tree.New(1), ch)
Then read and print 10 values from the channel. It should be the numbers 1, 2, 3, …, 10.
3. Implement the Same function using Walk to determine whether t1 and t2 store the same values.
4. Test the Same function.
Same(tree.New(1), tree.New(1)) should return true, and Same(tree.New(1), tree.New(2)) should return false.
The documentation for Tree can be found here.
/* exercise-equivalent-binary-trees.go */
package main
import "golang.org/x/tour/tree"
// Walk walks the tree t sending all values
// from the tree to the channel ch.
func Walk(t *tree.Tree, ch chan int)
// Same determines whether the trees
// t1 and t2 contain the same values.
func Same(t1, t2 *tree.Tree) bool
func main() {
}
We’ve seen how channels are great for communication among goroutines.
But what if we don’t need communication? What if we just want to make sure only one goroutine can access a variable at a time to avoid conflicts?
This concept is called mutual_exclusion, and the conventional name for the data structure that provides it is mutex.
Go’s standard library provides mutual exclusion with
sync.Mutex and its two methods:
LockUnlockWe can define a block of code to be executed in mutual exclusion by surrounding it
with a call to Lock and Unlock as shown on the Inc method.
We can also use defer to ensure the mutex will be unlocked as in the Value method.
/* mutex-counter.go */
package main
import (
"fmt"
"sync"
"time"
)
// SafeCounter is safe to use concurrently.
type SafeCounter struct {
mu sync.Mutex
v map[string]int
}
// Inc increments the counter for the given key.
func (c *SafeCounter) Inc(key string) {
c.mu.Lock()
// Lock so only one goroutine at a time can access the map c.v.
c.v[key]++
c.mu.Unlock()
}
// Value returns the current value of the counter for the given key.
func (c *SafeCounter) Value(key string) int {
c.mu.Lock()
// Lock so only one goroutine at a time can access the map c.v.
defer c.mu.Unlock()
return c.v[key]
}
func main() {
c := SafeCounter{v: make(map[string]int)}
for i := 0; i < 1000; i++ {
go c.Inc("somekey")
}
time.Sleep(time.Second)
fmt.Println(c.Value("somekey"))
}
In this exercise you’ll use Go’s concurrency features to parallelize a web crawler.
Modify the Crawl function to fetch URLs in parallel without fetching the same URL twice.
Hint: you can keep a cache of the URLs that have been fetched on a map, but maps alone are not safe for concurrent use!
/* exercise-web-crawler.go */
package main
import (
"fmt"
)
type Fetcher interface {
// Fetch returns the body of URL and
// a slice of URLs found on that page.
Fetch(url string) (body string, urls []string, err error)
}
// Crawl uses fetcher to recursively crawl
// pages starting with url, to a maximum of depth.
func Crawl(url string, depth int, fetcher Fetcher) {
// TODO: Fetch URLs in parallel.
// TODO: Don't fetch the same URL twice.
// This implementation doesn't do either:
if depth <= 0 {
return
}
body, urls, err := fetcher.Fetch(url)
if err != nil {
fmt.Println(err)
return
}
fmt.Printf("found: %s %q\n", url, body)
for _, u := range urls {
Crawl(u, depth-1, fetcher)
}
return
}
func main() {
Crawl("https://golang.org/", 4, fetcher)
}
// fakeFetcher is Fetcher that returns canned results.
type fakeFetcher map[string]*fakeResult
type fakeResult struct {
body string
urls []string
}
func (f fakeFetcher) Fetch(url string) (string, []string, error) {
if res, ok := f[url]; ok {
return res.body, res.urls, nil
}
return "", nil, fmt.Errorf("not found: %s", url)
}
// fetcher is a populated fakeFetcher.
var fetcher = fakeFetcher{
"https://golang.org/": &fakeResult{
"The Go Programming Language",
[]string{
"https://golang.org/pkg/",
"https://golang.org/cmd/",
},
},
"https://golang.org/pkg/": &fakeResult{
"Packages",
[]string{
"https://golang.org/",
"https://golang.org/cmd/",
"https://golang.org/pkg/fmt/",
"https://golang.org/pkg/os/",
},
},
"https://golang.org/pkg/fmt/": &fakeResult{
"Package fmt",
[]string{
"https://golang.org/",
"https://golang.org/pkg/",
},
},
"https://golang.org/pkg/os/": &fakeResult{
"Package os",
[]string{
"https://golang.org/",
"https://golang.org/pkg/",
},
},
}
You can get started by installing Go.
Once you have Go installed, the Go Documentation is a great place to continue. It contains references, tutorials, videos, and more.
To learn how to organize and work with Go code, read How to Write Go Code.
If you need help with the standard library, see the package reference. For help with the language itself, you might be surprised to find the Language Spec is quite readable.
To further explore Go’s concurrency model, watch Go Concurrency Patterns (slides) and Advanced Go Concurrency Patterns (slides) and read the Share Memory by Communicating codewalk.
To get started writing web applications, watch A simple programming environment (slides) and read the Writing Web Applications tutorial.
The First Class Functions in Go codewalk gives an interesting perspective on Go’s function types.
The Go Blog has a large archive of informative Go articles.
Visit the Go home page for more.