How Does Slicing a Go Slice with `s = s[2:]` Impact Its Capacity Differently Than `s = s[:0]` or `s = s[:4]`?

Slicing and Capacity Reduction in Go Slices

When you work with slices in Go, it's essential to understand the impact of slicing operations on their capacity. This article addresses a specific question regarding the varying effects of different slicing operations on slice capacity.

Let's consider the following code snippet:

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)
}

In this code, we have a slice of integers s. After printing it initially, we perform various slicing operations and observe their effects on the slice's length and capacity.

Reduced Capacity after 's = s[2:]'

The key question is why the line s = s[2:] reduces the capacity, unlike s = s[:4] and s = s[:0]. The difference lies in the syntax used: [2:] represents slicing starting from index 2 to the end of the slice, effectively removing the first two elements.

Memory and Slicing

Slices reference a portion of an underlying array in memory. When you alter a slice, you modify its pointer to the array.

In the case of s = s[2:], the slice's pointer moves forward, skipping over the first two elements. This effectively reduces the capacity of the slice since it now points to a smaller portion of the underlying array.

Can We Recover Cut-Off Elements?

Unfortunately, you cannot recover the first two elements that were cut off using s = s[2:]. This is because the slice's pointer has moved forward, and the original data is no longer referenced.

Conclusion

Understanding the behavior of slicing operations is crucial when working with Go slices. Different slicing syntax can have varying effects on capacity, and it's essential to be aware of these implications to prevent unexpected results.

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