Two-dimensional slice

Go supports multiple-dimensional slice, but I only want to introduce two-dimensional slice here. One reason is the two-dimensional slice is usually used in daily life, while multiple-dimensional seems not common. If you often use multiple-dimensional slice, personally I think the code is a little clumsy and not easy to maintain, so maybe you can try to check whether there is a better method; the other reason is the principle behind multiple-dimensional slice is the same with two-dimensional slice, you can also understand it if you know two-dimensional slice well.

Let's the following example:

package main

import "fmt"

func main() {
    s := make([][]int, 2)
    fmt.Println(len(s), cap(s), &s[0])

    s[0] = []int{1, 2, 3}
    fmt.Println(len(s[0]), cap(s[0]), &s[0][0])

    s[1] = make([]int, 3, 5)
    fmt.Println(len(s[1]), cap(s[1]), &s[1][0])
}

I still use gdb to inspect the execution flow:

5       func main() {
(gdb) n
6               s := make([][]int, 2)
(gdb)
7               fmt.Println(len(s), cap(s), &s[0])
(gdb)
2 2 &[]
9               s[0] = []int{1, 2, 3}
(gdb) p &s
$1 = (struct [][]int *) 0xc82003fe70
(gdb) x/24xb 0xc82003fe70
0xc82003fe70:   0x40    0x02    0x01    0x20    0xc8    0x00    0x00    0x00
0xc82003fe78:   0x02    0x00    0x00    0x00    0x00    0x00    0x00    0x00
0xc82003fe80:   0x02    0x00    0x00    0x00    0x00    0x00    0x00    0x00

s is a slice (the start memory address is 0xc82003fe70), but its elements are also slices. Let's check the elements:

(gdb) x/48xb 0xc820010240
0xc820010240:   0x00    0x00    0x00    0x00    0x00    0x00    0x00    0x00
0xc820010248:   0x00    0x00    0x00    0x00    0x00    0x00    0x00    0x00
0xc820010250:   0x00    0x00    0x00    0x00    0x00    0x00    0x00    0x00
0xc820010258:   0x00    0x00    0x00    0x00    0x00    0x00    0x00    0x00
0xc820010260:   0x00    0x00    0x00    0x00    0x00    0x00    0x00    0x00
0xc820010268:   0x00    0x00    0x00    0x00    0x00    0x00    0x00    0x00

All the memory content are 0, nothing exciting! Continue to step by step:

(gdb) n
10              fmt.Println(len(s[0]), cap(s[0]), &s[0][0])
(gdb)
3 3 0xc82000e220
12              s[1] = make([]int, 3, 5)

Now since s contains a valid slice element, check its underlying array:

(gdb) x/48xb 0xc820010240
0xc820010240:   0x20    0xe2    0x00    0x20    0xc8    0x00    0x00    0x00
0xc820010248:   0x03    0x00    0x00    0x00    0x00    0x00    0x00    0x00
0xc820010250:   0x03    0x00    0x00    0x00    0x00    0x00    0x00    0x00
0xc820010258:   0x00    0x00    0x00    0x00    0x00    0x00    0x00    0x00
0xc820010260:   0x00    0x00    0x00    0x00    0x00    0x00    0x00    0x00
0xc820010268:   0x00    0x00    0x00    0x00    0x00    0x00    0x00    0x00

Yeah, the memory has been updated by the pointer, length and capacity of s[0], the same with previous output from fmt.Println. Check the underlying array of s[0]:

(gdb) x/24xb 0xc82000e220
0xc82000e220:   0x01    0x00    0x00    0x00    0x00    0x00    0x00    0x00
0xc82000e228:   0x02    0x00    0x00    0x00    0x00    0x00    0x00    0x00
0xc82000e230:   0x03    0x00    0x00    0x00    0x00    0x00    0x00    0x00

We can see 3 elements: 1, 2, 3.

Following the same method to check the s[1]:

(gdb) n
13              fmt.Println(len(s[1]), cap(s[1]), &s[1][0])
(gdb)
3 5 0xc820010270
14      }
(gdb) x/48xb 0xc820010240
0xc820010240:   0x20    0xe2    0x00    0x20    0xc8    0x00    0x00    0x00
0xc820010248:   0x03    0x00    0x00    0x00    0x00    0x00    0x00    0x00
0xc820010250:   0x03    0x00    0x00    0x00    0x00    0x00    0x00    0x00
0xc820010258:   0x70    0x02    0x01    0x20    0xc8    0x00    0x00    0x00
0xc820010260:   0x03    0x00    0x00    0x00    0x00    0x00    0x00    0x00
0xc820010268:   0x05    0x00    0x00    0x00    0x00    0x00    0x00    0x00
(gdb) x/40xb 0xc820010270
0xc820010270:   0x00    0x00    0x00    0x00    0x00    0x00    0x00    0x00
0xc820010278:   0x00    0x00    0x00    0x00    0x00    0x00    0x00    0x00
0xc820010280:   0x00    0x00    0x00    0x00    0x00    0x00    0x00    0x00
0xc820010288:   0x00    0x00    0x00    0x00    0x00    0x00    0x00    0x00
0xc820010290:   0x00    0x00    0x00    0x00    0x00    0x00    0x00    0x00

Now, we can see s contains all the info of its slice elements, and the elements of s[1] are initialized to 0.