附加到切片中的for循环中具有意外结果

Question

It could be a silly question but when I try to append a []byte slice to a [][]byte slice I get weird results.

Here is my code:

func Normalizer(s string) (ss [][]byte) {

    ss = make([][]byte, 0)
    // norm
    var ia norm.Iter

    ia.InitString(norm.NFC, s)

    for !ia.Done() {

        next := ia.Next()

        fmt.Println(next)
        // [226 128 139]
        // [227 128 129]
        // [39]
        // [226 128 153]
        // [46]
        // [44]
        // [63]
        // [33]
        // [92]
        // [10]
        // [226 128 153]
        // ...
        ss = append(ss, next)

    }
    ia.Done()


    fmt.Println(ss)
    return
}

I'm expecting somethin like this:

// [[226 128 139] [227 128 129] [39] [226 128 153] [46] [44] [63] [33] [92] [10] [226 128 153]...] 

but instead i get this:

// [[226 129 128] [226 129 128] [226] [226 129 128] [226] [226] [226] [226] [226] [226] [226 129 128]...]

and I have no idea why. Help and explanation would be appreciated.

Answer 1

A slice is a struct with a pointer to an underlying array, a length, and a capacity.

type slice struct {
    array unsafe.Pointer
    len   int
    cap   int
}

You are changing the underlying array after you have appended the slice struct. ia.Next() reuses its return buffer.

For example,

package main

import (
    "fmt"

    "golang.org/x/text/unicode/norm"
)

func Normalizer(s string) (ss [][]byte) {
    ss = make([][]byte, 0)
    var ia norm.Iter
    ia.InitString(norm.NFC, s)
    for !ia.Done() {
        next := ia.Next()
        fmt.Println(string(next), &next[0])
        ss = append(ss, next)
    }
    fmt.Println()
    for i := range ss {
        fmt.Println(string(ss[i]), &ss[i][0])
    }
    fmt.Println()
    return
}

func main() {
    ss := Normalizer("abc")
    fmt.Printf("%s
", ss)
}

Output:

a 0xc420092228
b 0xc420092228
c 0xc420092228

c 0xc420092228
c 0xc420092228
c 0xc420092228

[c c c]

Replace a copy of a slice struct

next := ia.Next()

with a new slice struct with a new underlying array

next := append([]byte(nil), ia.Next()...)

For example,

package main

import (
    "fmt"

    "golang.org/x/text/unicode/norm"
)

func Normalizer(s string) (ss [][]byte) {
    ss = make([][]byte, 0)
    var ia norm.Iter
    ia.InitString(norm.NFC, s)
    for !ia.Done() {
        next := append([]byte(nil), ia.Next()...)
        fmt.Println(string(next), &next[0])
        ss = append(ss, next)
    }
    fmt.Println()
    for i := range ss {
        fmt.Println(string(ss[i]), &ss[i][0])
    }
    fmt.Println()
    return
}

func main() {
    ss := Normalizer("abc")
    fmt.Printf("%s
", ss)
}

Output:

a 0xc4200120d0
b 0xc4200120e8
c 0xc420012108

a 0xc4200120d0
b 0xc4200120e8
c 0xc420012108

[a b c]

References:

Slice types

Go Slices: usage and internals

Arrays, slices (and strings): The mechanics of 'append'