Learning go language 0x02: understanding slice

Study Go Language 0x02： To slice Slice The understanding of the

It's just my own understanding , For reference only . If you don't understand it correctly , Please correct . thank you ：）

* example 1

The code comes from Go Language journey , There may be a slight change .

package main

import "fmt"

func main() {
    
	s := []int{
    2, 3, 5, 7, 11, 13}
    fmt.Println(len(s), cap(s),s)

	s = s[1:4]
	fmt.Println(len(s), cap(s),s)

	s = s[:2]
	fmt.Println(len(s), cap(s),s)

	s = s[1:]
	fmt.Println(len(s), cap(s),s)
}

Output ：

6 6 [2 3 5 7 11 13]
3 5 [3 5 7]
2 5 [3 5]
1 4 [5]

Interpretation of output ：
len(s) yes s The actual number of elements in .
cap(s) It's the capacity of the slice , It's from s The first element of begins （ Start with the subscript of the left boundary ）, To Original array The number of the last element in the .

• 6 6 [2 3 5 7 11 13]
  original s by [2 3 5 7 11 13], Capacity is 6, From the subscript 0 To 5.

• 3 5 [3 5 7]
s[1:4] It's from [2 3 5 7 11 13] The value of ,s[1:4] The value of is [3 5 7]. The actual number of elements is 3,s[1:4] The first element in is 3,3 The position in the original array is the subscript 1, From the subscript 1 To the subscript in the original array 5, Is the total 5 A place , So the capacity is 5.
  Or another way to think about it ： The capacity is from the left boundary （ Subscript 1） Start , To the subscript in the original array 5, So the capacity is 5.

• 2 5 [3 5]
s[:2] It's from [3 5 7] The value of , therefore ,s[:2] The value of is [3 5].s[:2] The first element of is 3, In the original array, the subscript is 1, To the original array subscript 5, So the capacity is 5 .
  Or another way to think about it ：s[:2] It doesn't change the left boundary , It's still the left boundary of the last time （ Subscript 1）, So the capacity is still 5.

• 1 4 [5]
s[1:] It's from [3 5] The value of , therefore s[1:] The value of is [5]. Numbers 5 The subscript in the original array is 2, To the subscript in the original array 5, So the capacity is 4 .
  Or another way to think about it ,s[1:] Changed the left boundary again , Cumulative changes 2 Second left boundary , Each time the left boundary is 1, They all remove a number , A total of 2 Number , So the subscript is from 2 Start , Until the subscript of the original array 5, Capacity of 4 .

* example 2

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[:5]
	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=6 cap=6 [2 3 5 7 11 13]
len=0 cap=6 []
len=5 cap=6 [2 3 5 7 11]
len=3 cap=4 [5 7 11]

Understanding of output ：

• len=6 cap=6 [2 3 5 7 11 13]
  s The original value of is {2, 3, 5, 7, 11, 13}, The number of elements is 6, Capacity is 6

• len=0 cap=6 []
s[:0] It's from {2, 3, 5, 7, 11, 13} The value of , But because the right boundary is 0, therefore s[:0] The value of is [], Is an empty array . because s[:0] The left boundary is not changed , That is, from the subscript 0 Start , Until the subscript in the original array 5, So the capacity is 6.

• len=5 cap=6 [2 3 5 7 11]
s[:5] Is from the original array {2, 3, 5, 7, 11, 13} Value , From the subscript 0 To the subscript 4, therefore s[:5] The value of is [2 3 5 7 11]. s[:5] The left boundary is not changed , So the capacity is still 6 .
  s[:5] The reason why it can expand , Because last time s Is the capacity of the 6, Expand to 5 Elements , No excess capacity .
  Capacity can also be understood in this way ：s[:5] The value of is [2 3 5 7 11]. First element 2, The subscript in the original array is 0, Until the subscript of the last element in the original array 5, So the capacity is 6 .

• len=3 cap=4 [5 7 11]
s[2:] Changed the left boundary , From the subscript 2 Start , Removed the subscript 0 and 1 Two numbers of positions , So the capacity is reduced 2, namely cap by 4 . and s[2:] It's from the last time [2 3 5 7 11] Value , therefore s[2:] The value of is [5 7 11].
  Or another way to think about it ：s[2:] The value of is [5 7 11], First element 5 The subscript in the original array is 2, Until the subscript in the original array 5, So the capacity is 4 .

* Reference resources

https://tour.go-zh.org/moretypes/11