This program is using the packages with import paths "fmt" and "math/rand".
Note: The environment in which these programs are executed is deterministic, so each time you run the example program rand.Intn will return the same number.
(To see a different number, seed the number generator; see rand.Seed. Time is constant in the playground, so you will need to use something else as the seed.)
This code groups the imports into a parenthesized, "factored" import statement.
import (
"fmt"
"math"
)In Go, a name is exported if it begins with a capital letter.
A function can take zero or more arguments.
(For more about why types look the way they do, see the article on Go's declaration syntax.)
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 intto
x, y intpackage main
import "fmt"
func add(x, y int) int {
return x + y
}
func main() {
fmt.Println(add(4,9))
}A function can return any number of results.
package main
import "fmt"
func swap(x, y string) (string, string) {
return y, x
}
func main() {
//fmt.Println(swap("hello", "world"))
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.
package main
import "fmt"
func split(sum int) (x, y int) {
x = sum * 5 / 10
y = sum - x
return
}
func main() {
fmt.Println(split(30))
}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.
package main
import (
"fmt"
)
var Google, Apple ,Amazon bool
func main() {
var i int
fmt.Println(i,Google, Apple, Amazon)
}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.
package main
import "fmt"
var i, j int = 3, 4
func main() {
var python, c, java = true, false ,"yes"
fmt.Println(i,j,python,c,java)
}Output:
3 4 true false yesInside 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.
package main
import "fmt"
func main() {
var i, j int = 3, 4
c, python, cplusplus := "c", "python", "cplusplus"
fmt.Println(i, j, c, python, cplusplus)
}Output:
3 4 c python cplusplusGo'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 complex128The 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.
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)
}Output :
Type: bool Value: false
Type: uint64 Value: 18446744073709551615
Type: complex128 Value: (2+3i)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.
package main
import "fmt"
func main() {
var i int
var f float64
var mj bool
var google string
fmt.Printf("%v %v %v %q",i, f, mj, google)
}Output :
0 0 false ""
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.
package main
import (
"fmt"
"math"
)
func main() {
var x, y int = 6, 9
var f float64 = math.Sqrt(float64(x*x + y*y))
var u uint = uint(f)
fmt.Println(x, y, f, u)
}Output :
6 9 10.816653826391969 10
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.
package main
import "fmt"
func main() {
v := 42
fmt.Printf("v is of type %T, value is of %v\n",v,v)
f := 42.00
fmt.Printf("f is of type %T, value is of %f\n",f,f)
}Output :
v is of type int, value is of 42
f is of type float64, value is of 42.000000
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.
package main
import "fmt"
const Pi = 3.14
func main() {
const Word = "Google"
fmt.Println("Hello",Word)
fmt.Println("Happy", Pi, "Day")
}Output :
Hello Google
Happy 3.14 Day
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.)
package main
import "fmt"
const(
// Create a huger number by shifting a 1 bit left 100 places
// In other words, the binary number is 1 followed by 100 zeros.
Big = 1 << 100
// Shift it right again 99 places, so we end with 1 << 1 or 2.
Small = Big >> 99
)
func needInt(x int) int { return 10*x + 1}
func needFloat(x float64) float64 { return x*0.1}
func main() {
fmt.Println(needInt(Small))
fmt.Println(needFloat(Small))
//fmt.Println(needInt(Big)) //overflows int
fmt.Println(needFloat(Big))
}Output :
21
0.2
1.2676506002282295e+29
Go has only one looping construct, the for loop.
The basic for loop has three components separated by semicolons:
the init statement: executed before the first iteration
the condition expression: evaluated before every iteration
the post statement: executed at the end of every iteration
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.
package main
import "fmt"
func main() {
sum := 0
for i := 1; i <= 100; i++ {
sum += i
}
fmt.Printf("Total is %v", sum)
}Output :
Total is 5050
The init and post statements are optional.
package main
import "fmt"
func main() {
sum := 1
for ;sum < 1000; {
sum += sum
//fmt.Println(sum)
}
fmt.Println(sum)
}Output :
1024
At that point you can drop the semicolons: C's while is spelled for in Go.
package main
import "fmt"
func main() {
sum := 1
for sum < 1000 {
sum += sum
}
fmt.Println(sum)
}Output ：
1024
If you omit the loop condition it loops forever, so an infinite loop is compactly expressed.
package main
import "fmt"
func main() {
i := 0
for{
i++
fmt.Println("I love mj",i)
}
}Output :
...
I love mj 616161
I love mj 616162
I love mj 616163
I love mj 61
Go's if statements are like its for loops; the expression need not be surrounded by parentheses ( ) but the braces { } are required.
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(520),sqrt(-520))
}Output :
22.80350850198276 22.80350850198276i
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.)
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(2,8,520),
pow(3,6,520),
)
}Output :
256 520
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.)
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)
}
return lim
}
func main() {
fmt.Println(
pow(2,6,40),
pow(2,5,520),
)
}Output :
64 >= 40
40 32
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.)
package main
import (
"fmt"
"math"
)
func Sqrt(x float64) float64 {
z := float64(1)
for i:= 1; i <= 10; i++{
z -= (z*z - x) / (2*z)
}
return z
}
func main() {
p :=2.0
fmt.Println(Sqrt(p))
fmt.Println(math.Sqrt(p))
}Output :
1.414213562373095
1.4142135623730951
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.
package main
import (
"fmt"
"runtime"
)
func main() {
fmt.Print("Go runs on ")
switch os := runtime.GOOS; os{
case "darwin":
fmt.Println("OS X")
case "linux":
fmt.Println("Linux")
case "windows":
fmt.Println("windows")
default:
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.
package main
import (
"fmt"
"time"
)
func main() {
fmt.Println("When's Saturday")
today := time.Now().Weekday()
fmt.Println(today)
switch time.Saturday{
case today + 0:
fmt.Println("Today.")
case today + 1:
fmt.Println("Tomorrow.")
case today + 3:
fmt.Println("In three days.")
default:
fmt.Println("Too far away.")
}
}Output :
When's Saturday
Wednesday
In three days.
Switch without a condition is the same as switch true.
This construct can be a clean way to write long if-then-else chains.
package main
import (
"fmt"
"time"
)
func main() {
t := time.Now()
fmt.Println(t)
fmt.Println(t.Hour(),t.Minute(),t.Second())
switch {
case t.Hour() < 12:
fmt.Println("Good morning!")
case t.Hour() < 17:
fmt.Println("Good afternoon!")
default:
fmt.Println("Good evening!")
}
}Output :
2020-03-04 21:12:06.503189 +0800 CST m=+0.000079829
21 12 6
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.
package main
import "fmt"
func main() {
defer fmt.Println("world")
fmt.Println("hello")
}Output :
hello
world
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.
package main
import "fmt"
func main() {
fmt.Println("counting")
for i := 1; i <= 5; i++{
defer fmt.Println(i)
}
fmt.Println("done")
}Output :
counting
done
5
4
3
2
1
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 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.
package main
import "fmt"
func main() {
i, j := 4, 5
p := &i // point to i
fmt.Println(*p) // read i through the pointer
*p = 32 // set i through the pointer
fmt.Println("the new value of i ",i) // see the new value of i
p = &j // point to j
*p = *p / 23 // divide j through the pointer
fmt.Println("the new value of j ", j) // see the new value of j
}Output :
4
the new value of i 32
the new value of j 0
A struct is a collection of fields.
package main
import "fmt"
type Vertex struct{
X int
Y int
}
func main() {
fmt.Println(Vertex{1,520,})
}Output :
{1 520}
Struct fields are accessed using a dot.
package main
import "fmt"
type Vertex struct{
X int
Y int
}
func main() {
v := Vertex{1,4}
fmt.Println(v)
v.X = 5
v.Y = 0
fmt.Println(v)
}Output :
{1 4}
{5 0}
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.
package main
import "fmt"
type Vertex struct {
X int
Y int
}
func main() {
v := Vertex{1,10}
fmt.Println(v)
p := &v
(*p).X = 10
p.Y = 100
fmt.Println(v)
}
Output :
{1 10}
{10 100}
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.
package main
import "fmt"
type Vertex struct{
X, Y int
}
var(
v1 = Vertex{X:1} // Y:0 is implicit
v2 = Vertex{} // X:0 and Y:0
p = &Vertex{1,3} // has type *Vertex
)
func main() {
v := Vertex{1,2} // has type Vertex
fmt.Println(v, v1, v2, p, *p)
}Output :
{1 2} {1 0} {0 0} &{1 3} {1 3}
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.
package main
import "fmt"
func main() {
var str [2]string
str[0] = "Hello"
str[1] = "World"
fmt.Println(str[0],str[1])
fmt.Println(str)
primes := [6]int{2,3,5,7,11,13}
fmt.Println(primes)
}Output :
Hello World
[Hello World]
[2 3 5 7 11 13]
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]
package main
import "fmt"
func main() {
primes := [6]int{2, 3, 5, 7, 11, 13}
var s []int = primes[1:4]
fmt.Println(s)
}
Output :
[3 5 7]
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.
package main
import "fmt"
func main(){
names := [4]string {
"mj",
"henry",
"lbq",
"jy",
}
fmt.Println(names)
a := names[0:1]
fmt.Println(a)
b := names[1:4]
fmt.Println(b)
b[0] = "henry20"
fmt.Println(b)
fmt.Println(names)
}Output :
[mj henry lbq jy]
[mj]
[henry lbq jy]
[henry20 lbq jy]
[mj henry20 lbq jy]
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}
package main
import "fmt"
func main() {
q := []int{3, 7, 9, 11, 13}
fmt.Println(q)
r := []bool{false, true, false, true}
fmt.Println(r)
s := []struct{
x int
b bool
}{
{1, false},
{2, false},
{520, true},
}
fmt.Println(s)
}Output :
[3 7 9 11 13]
[false true false true]
[{1 false} {2 false} {520 true}]
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[:]
package main
import "fmt"
func main() {
s := []int{2, 3, 520 ,11, 13}
s = s[1:4]
fmt.Println(s)
s = s[:2]
fmt.Println(s)
s = s[1:]
fmt.Println(s)
}Output :
[3 520 11]
[3 520]
[520]
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.
package main
import "fmt"
func main() {
s := []int{2, 3, 5, 7, 11, 13, 520}
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)
}Output :
len=7 cap=7 [2 3 5 7 11 13 520]
len=0 cap=7 []
len=4 cap=7 [2 3 5 7]
len=2 cap=5 [5 7]
The zero value of a slice is nil.
A nil slice has a length and capacity of 0 and has no underlying array.
package main
import "fmt"
func main() {
var s[] int
if len(s) == 0{
fmt.Println("0")
}
fmt.Println(s,len(s),cap(s))
if s == nil{
fmt.Println("nil!")
}
// panic: runtime error: index out of range [0] with length 0
s[0] = 3
fmt.Println(s)
}Output :
0
[] 0 0
nil!
panic: runtime error: index out of range [0] with length 0
goroutine 1 [running]:
main.main()
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
package main
import "fmt"
func main() {
a := make([]int,5)
println("a",a)
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)
}Output :
a [5/5]0xc000114030
a len=5 cap=5 [0 0 0 0 0]
b len=0 cap=5 []
c len=2 cap=5 [0 0]
d len=3 cap=3 [0 0 0]
Slices can contain any type, including other slices.
package main
import (
"fmt"
"strings"
)
func main() {
// creat a tic-tac-toe board.
board := [][]string{
[]string{"_", "_", "_"},
[]string{"_", "_", "_"},
[]string{"^", "_", "^"},
}
// The player takes turns.
board[0][0] = "x"
board[2][1] = "o"
board[1][2] = "o"
board[1][0] = "o"
board[0][2] = "x"
for i:=0; i < len(board); i++{
fmt.Printf("%s\n", strings.Join(board[i]," "))
}
}Output :
x _ x
o _ o
^ o ^
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.)
package main
import "fmt"
func main() {
var s[]int
printSlices(s)
// append works on nil slices
s = append(s, 0)
printSlices(s)
// the slices grows as need
s = append(s, 1)
printSlices(s)
// we can add more elements at a time
s = append(s, 520,1,1314)
printSlices(s)
}
func printSlices(s []int) {
fmt.Printf("len=%d cap=%d %v\n",len(s), cap(s), s)
}Output :
len=0 cap=0 []
len=1 cap=1 [0]
len=2 cap=2 [0 1]
len=5 cap=6 [0 1 520 1 1314]
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.
package main
import "fmt"
var pow = []int{1, 2, 4, 8, 16, 32, 64}
func main() {
for i , v := range pow{
fmt.Printf("2**%d = %d\n",i ,v)
}
}Output :
2**0 = 1
2**1 = 2
2**2 = 4
2**3 = 8
2**4 = 16
2**5 = 32
2**6 = 64
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
package main
import "fmt"
func main() {
pow := make([]int, 10)
fmt.Println(pow)
// index
for i := range pow{
pow[i] = 1 << uint(i) // == 2**i
}
// value
for _, value := range pow{
fmt.Printf("%d\n", value)
}
}Output :
[0 0 0 0 0 0 0 0 0 0]
1
2
4
8
16
32
64
128
256
512
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.)
package main
import "code.google.com/p/go-tour/pic"
func Pic(dx, dy int) [][]uint8 {
a := make([][]uint8, dy)
for i := range(a){
a[i] = make([]uint8,dx)
}
for i := range(a){
for j:= range(a[i]){
a[i][j] = uint8(i+j) /2
}
}
return a
}
func main() {
pic.Show(Pic)
}Output :
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.
package main
import "fmt"
type Vetex struct{
Lat, Long float64
}
var m map[string]Vetex
func main() {
m = make(map[string]Vetex)
m["mj"] = Vetex{
Lat: 2,
Long: 0,
}
fmt.Println(m)
fmt.Println(m["mj"])
}Output :
map[mj:{2 0}]
{2 0}
Map literals are like struct literals, but the keys are required.
package main
import "fmt"
type Info struct{
Name string
Id int
}
var m = map[string]Info{
"mj":Info{
"mj", 101,
},
"henry":Info{"henry",100,
},
}
func main() {
fmt.Println(m)
}Output :
map[henry:{henry 100} mj:{mj 101}]
If the top-level type is just a type name, you can omit it from the elements of the literal.
package main
import "fmt"
type Info struct{
Id int
Name string
}
var m = map[string]Info{
"henry":{1 ,"henry"},
"mj":{2,"mj"},
}
func main() {
fmt.Println(m)
}Output :
map[henry:{1 henry} mj:{2 mj}]
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]
package main
import "fmt"
func main() {
m := make(map[string]int)
m["mj"] = 520
fmt.Println("The value: ", m["mj"])
m["mj"] = 20
fmt.Println("The value: ",m["mj"])
delete(m,"mj")
fmt.Println("The value: ",m["mj"])
v, ok := m["mj"]
fmt.Println("The value ", v, "Present?", ok)
}Output :
The value: 520
The value: 20
The value: 0
The value 0 Present? false
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.
package main
import (
"golang.org/x/tour/wc"
"strings"
)
func WordCount(s string) map[string]int {
m := make(map[string]int)
a := strings.Fields(s)
for _, v := range(a){
m[v] ++
}
return m
}
func main() {
wc.Test(WordCount)
}Output :
PASS
f("I am learning Go!") =
map[string]int{"Go!":1, "I":1, "am":1, "learning":1}
PASS
f("The quick brown fox jumped over the lazy dog.") =
map[string]int{"The":1, "brown":1, "dog.":1, "fox":1, "jumped":1, "lazy":1, "over":1, "quick":1, "the":1}
PASS
f("I ate a donut. Then I ate another donut.") =
map[string]int{"I":2, "Then":1, "a":1, "another":1, "ate":2, "donut.":2}
PASS
f("A man a plan a canal panama.") =
map[string]int{"A":1, "a":2, "canal":1, "man":1, "panama.":1, "plan":1}
Functions are values too. They can be passed around just like other values.
Function values may be used as function arguments and return values.
package main
import (
"fmt"
"math"
)
func compute(fn func(x, y float64) float64) float64 {
return fn(4, 3)
}
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))
}Output :
13
5
64
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.
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))
}
}Output :
0 0
1 -2
3 -6
6 -12
10 -20
15 -30
21 -42
28 -56
36 -72
45 -90
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, ...).
package main
import "fmt"
// fibonacci is a function that returns
// a function that return an int.
func fibonacci() func() int {
a, b := 0,1
return func() int {
a,b = b, a + b
return a
}
}
func main() {
f := fibonacci()
for i:= 0; i < 10; i++{
fmt.Println(f())
}
}Output :
1
1
2
3
5
8
13
21
34
55
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.