remove elements from slice

Sax:

Go doesn't provide any advanced functionality to remove elements from slices. I wrote a function that removes a given value from a slice in the way that is usually suggested here, but it produces very unexpected results.

package main

import "fmt"

type Area struct {
    Cells [2][]uint8
}
func main() {
    var area1 Area
    area1.Cells[1] = []uint8 {5, 6, 7}

    area2 := area1

    area1.Cells[1] = removeValueFromCell(area1.Cells[1], 6)

    fmt.Println(area1.Cells[1])
    fmt.Println(area2.Cells[1])
}


func removeValueFromCell(cell []uint8, value uint8) []uint8{
    var res = cell
    for i := 0; i < len(cell); i++ {
        if cell[i] == value {
            res = append(cell[:i], cell[i+1:]...)
        }
    }
    return res
}

The program outputs:

[5 7] <- as expected

[5 7 7] <- why not [5 6 7] or [5 7] ?
icza ：

The slice value is just the header, pointing to the backing array. Slice headers only contain pointers. So when you copy the slice value, the copy will also point to the same backing array. So if you change the backing array via the original slice header, the copy will also observe the change.

This is your case. you assign area1to area2. A cell is an array of slices. So the array containing the slice headers will be copied, so the slice headers will be copied. The slice header contains a pointer to the backing array, which will not be repeated.

Therefore, there is only one backing array that supports [5, 6, 7]elements . Then call removeValueFromCell(), it will modify this backing array:

Before:
[5, 6, 7]
After:
[5, 7, 7]

Because the element 6has been removed, the rest of the slice (the elements ) will be [7]copied in place of the removed element.

Then assign this new slice header (which correctly contains only 2 elements) to area1.Cells[1].

But the shard value area2.Cells[1]points to the same backing array, and since you didn't touch this shard value, it still has length 3, so it will see all backing arrays changed elements: [5, 7, 7].

Also note that your implementation removeValueFromCell()is wrong because the movable element will behave incorrectly if it will be listed multiple times in the slice. The reason for this is because when you remove an element, the indices of subsequent elements are shifted (decremented by 1), but loop variables don't fix that. The easiest fix is ​​to use a downward loop. See how to remove elements of an array of structs in a loop in golang for details .

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