Go learning (II. Built in container)

2.1 Array

go There are also arrays , Next, let's learn about an array .

2.1.1 Define an array

go There are several ways to define an array ：

func main() {
    
    // go Definition of array 
    var arr1 [3]bool    //  There is no initial value   however go By default, the initial value will be assigned 
    arr2 := [3]int{
    1, 2, 3}    //  This is another initial value 
    arr3 := [...]int{
    1,2,3,4,5}    //  This is to determine the number of arrays according to the number defined later 

    var arr4 [4][5]int   //  Two dimensional array ,4 That's ok 5 Column 

    fmt.Println(arr1, arr2, arr3, arr4)

    return
}

go Array of , It is also different from other arrays , Is a number in front , Add type after , It's easy to get confused with other languages .

go It also supports two-dimensional arrays , How many rows are the first in a two-dimensional array , How many columns are following

[false false false] [1 2 3] [1 2 3 4 5] [[0 0 0 0 0] [0 0 0 0 0] [0 0 0 0 0] [0 0 0 0 0]]

2.1.2 Array traversal

Next, let's look at the traversal of arrays ,

• The conventional way

for i:=0; i<len(arr2); i++ {
    
        fmt.Println(arr2[i])
}

This traversal based on length is our normal operation .

• range

for i, v := range arr3 {
    
        fmt.Println(i, v)
}

go A keyword is provided ,range, Is to traverse the values of each container , Not just arrays , And the back slice and map. and range The return value of is still interesting , Returns two values , The first is the subscript , The second is the value of the array .

2.1.3 Arrays as function arguments

Let's take a look at the array as a function parameter , How is it transmitted ：

• Value passed

func funcarr1(arr [3]int) {
        // [3]int  Is a type , No follow c Same first address only 
    arr[0] = 100
}

func main() {
    
    // go Definition of array 
    var arr1 [3]bool    
    arr2 := [3]int{
    1, 2, 3}
    arr3 := [...]int{
    1,2,3,4,5}

    var arr4 [4][5]int   //  Two dimensional array ,4 That's ok 5 Column 

    fmt.Println(arr1, arr2, arr3, arr4)

    for i:=0; i<len(arr2); i++ {
    
        fmt.Println(arr2[i])
    }

    funcarr1(arr2)

    for i, v := range arr2 {
    
        fmt.Println(i, v)
    }

    return
}

According to the printed results , Obviously, this array is just passing values , This is with us c/c++ It's the same , If it is transmitted in this way, it is directly transmitted .

So if this array is large , Is it difficult to copy , So there is an array pointer ：

• Pointer passing

func funcarr2(arr *[3]int) {
    
    arr[0] = 100
}

The result of this print is obviously modified , therefore go It can also be passed to an array .

however go Can you use arrays , Actually in go In language , Generally, arrays are not used , Only slices .

2.2 section

go The famous slice of , Actually, I learned slicing , It feels a bit like ArrayList, But it's different , But with the help of ArrayList It is also meaningful to help us learn slicing .

2.2.1 Introduction of slices

arr5 := [...]int{
    0,1,2,3,4,5,6,7,8,9}

    fmt.Println("s := [2:6]", arr5[2:6])
    fmt.Println("s := [:6]", arr5[:6])
    fmt.Println("s := [2:]", arr5[2:])
    fmt.Println("s := [:]", arr5[:])

Define a arr5 Array of , And then you can use [] Perform various slices , That's an interesting name .

Printed results ：

s := [2:6] [2 3 4 5]
s := [:6] [0 1 2 3 4 5]
s := [2:] [2 3 4 5 6 7 8 9]
s := [:] [0 1 2 3 4 5 6 7 8 9]

Just like what we thought , Is a slice of the sensory subscript .

2.2.2 Slice as function parameter

Above, we wrote an array as a function parameter , Because the type of array needs

func funcarr3(arr []int) {
            //  Do not limit the type 
    arr[0] = 100
}

func main() {
    

    arr5 := [...]int{
    0,1,2,3,4,5,6,7,8,9}

    s := arr5[2:6]
    fmt.Println(s)

    funcarr3(s)

    fmt.Println(s)


    return
}

When the slice is passed, the size is not required , And slice as a function parameter , It's a quote , Therefore, the values in the array can be modified inside the function .

[2 3 4 5]
[100 3 4 5]

I just wanted to use an array as a parameter , Can you pass values to the slice , But it still doesn't work ,go The type of language is very limited .

2.2.3 Extension of slicing

The teacher came up with a difficult problem ：

arr6 := [...]int{
    0,1,2,3,4,5,6,7}

s1 := arr6[2:6]
s2 := s1[3:5]

fmt.Println(s1, s2)

s1 and s2 Value , Printed directly ：

[2 3 4 5] [5 6]

So we see s2 You can get the value , Why is it so , Actually, this slice has a len, And a cap Of , That is, the underlying implementation of the slice has a maximum capacity cap, There is also a valid data len, We can change the code , Print it out ：

arr6 := [...]int{
    0,1,2,3,4,5,6,7}

    s1 := arr6[2:6]
    s2 := s1[3:5]

    fmt.Printf("s1=%v len(s1)=%d cap(s1)=%d\n", s1, len(s1), cap(s1))
    fmt.Printf("s2=%v len(s2)=%d cap(s2)=%d\n", s2, len(s2), cap(s2))

Print the results ：

s1=[2 3 4 5] len(s1)=4 cap(s1)=6
s2=[5 6] len(s2)=2 cap(s2)=3

Does this look like a ArrayList, In fact, it has a capacity , There is also an effective length , So when slicing data , It retains the ability to extend backwards , such s2 To get the value .

If you use append When you add elements , If it exceeds the original array cap, A new and larger underlying array will be allocated , Then copy the value , If no one uses the original array , Just from go Recycle the garbage .

2.2.4 Define slices directly

We usually use it in operation , You should not define an array first , Then change it into sliced , We can define slices directly ：

var s3 []int //  No value , The value is nil
fmt.Println(s3)
s3 = append(s3, 1)

s4 := []int{
    3,4,5}
fmt.Println(s4)

//  Know the length , You need to define a slice 
s5 := make([]int, 10)   
fmt.Println(s5)

//  Need to define cap It's OK, too 
s6 := make([]int, 10, 20)   
fmt.Println(s6)

Directly defined slices have such 4 Ways of planting , However, No. 3 In this way, we will often use , Let's see if it's more like ArrayList 了 .

2.2.5 Slicing operation

Pass the rookie tutorial , We know that in addition to append() There is copy() function .

I don't know any other functions at present , Ha ha ha ha .

2.3 map

go And again map Of , Next, learn to learn map.

2.3.1 Definition

Or just learn one map The definition of ：

m := map[string]string {
    
        "name" : "aaa",
        "course" : "golang",
        "site" : "immoc",
    }
    fmt.Println(m)

    mm := map[string]map[string]string {
    
        "name" : m,
    }
    fmt.Println(mm)


m2 := make(map[string]int)

m3 := make(map[string]int, 10)
fmt.Printf("m2=%v len(m2)=%d\n", m2, len(m2))
fmt.Printf("m3=%v len(m3)=%d\n", m3, len(m3))

It's on it map How to define ,map It can be defined directly , You can also use make, But the use of make You can't set the size , It's not like slicing ,map It has value when inserting , It's worth it .

2.3.2 Traverse

map Traversal should be here , You know. , Yes ,map Traversal uses range.

for k, v := range m {
    
	fmt.Println(k, v)
}

I won't introduce it here .

2.3.3 operation

Let's try the delete operation ：

m["name"]            //  Get value 
delete(m, "name")    //  Delete 

This is not difficult

2.3.4 map Of key

• map Use hash table ,key It has to be comparable

• except slice,map,function All built-in types can be used as key

• Struct Type does not contain the above fields , Have something to do with key

2.3.5 Example

Find the longest string that does not contain duplicate characters ：

（ After that, I wrote , Ha ha ha ）

2.4 Characters and strings

2.4.1 rune

rune Equal to in other languages char, Because it needs to be compatible with Unicode code , So the type is int32, In this way, Chinese can also support .

ss := "hello world  Hello , The world "
for i, c := range []rune(ss) {
    
	fmt.Printf("(i=%d c=%c) ", i, c)
}

Running results ：

(i=0 c=h) (i=1 c=e) (i=2 c=l) (i=3 c=l) (i=4 c=o) (i=5 c= ) (i=6 c=w) (i=7 c=o) (i=8 c=r) (i=9 c=l) (i=10 c=d) (i=11 c= ) (i=12 c= you ) (i=13 c= good ) (i=14 c=,) (i=15 c= the ) (i=16 c= world )

go Language can print out Chinese directly , If like other languages , Strong conversion []byte It's OK, too .

2.4.2 String manipulation

• Use range Traverse pos,rune Yes

• Use utf8.RuneCountInString Get the number of characters （ Support Chinese ）

• Use len Get the byte length （ By byte ）

• Use []byte Get bytes

2.4.3 Other string operations

• Fields,Split,Join

• Contains,Index

• ToLower,ToUpper

• Trim,TrimRight,TrimLeft