Decreasing slice capacity

Issue

My question is about slice length and capacity. I’m learning about Go here: https://tour.golang.org/moretypes/11.

(My question was marked as a possible duplicate of this; however, this is not the case. My question is specifically about the cutting off the first few elements of a slice and the implications of that.)

Why does the line s = s[2:] decrease the capacity when s = s[:4] and s = s[:0] do not? The only difference I see is that there is a number before the colon in s = s[2:] while there is a number after the colon in the other two lines.

Is there any way to recover the first two elements that we cut off with s = s[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[:4]
    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)
}

After clicking the Run button, we get the following.

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

Solution

You can read more about slices here. But I think this passage answers your question:

Slicing does not copy the slice’s data. It creates a new slice value that points to the original array. This makes slice operations as efficient as manipulating array indices. Therefore, modifying the elements (not the slice itself) of a re-slice modifies the elements of the original slice.

So you cannot recover the slice data if you are assigning it to the same variable.

The capacity decrease is because by dropping the first 2 elements you are changing the pointer to the new slice (slices are referenced by the pointer to the first element).

How slices are represented in the memory:
def

make([]byte, 5)

memory vis

s = s[2:4]

slice cap decrease

Answered By – Alex Efimov

Answer Checked By – Clifford M. (GoLangFix Volunteer)

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