How Does Dropping Elements from the Beginning of a Go Slice Affect Its Capacity?

Understanding Slice Capacity Changes in Go

When working with Go slices, it's essential to understand their underlying properties, including their capacity. While dropping the last n items from a slice maintains its capacity, dropping the first n items affects the capacity.

The Go Slice Structure

Go slices are implemented as structures consisting of three fields:

<code class="go">type slice struct {
    array unsafe.Pointer
    len   int
    cap   int
}</code>

• array: Pointer to the underlying array backing the slice.
• len: Number of elements in the slice.
• cap: Capacity of the slice, which represents the number of elements it can hold.

Dropping Last n Items

When we drop the last n items from a slice, using the expression s[:len(s)-n], we create a new slice that shares the same underlying array as the original. However, the length of the new slice is reduced, while the capacity remains unchanged. This is because the underlying array has sufficient capacity to accommodate the remaining elements.

Dropping First n Items

On the other hand, when we drop the first n items from a slice, using the expression s[n:], we not only create a new slice with a shorter length but also allocate a new underlying array. The capacity of the new slice is adjusted to accommodate the reduced number of elements.

Example

Consider the following code:

<code class="go">func main() {
    s := []int{2, 3, 5, 7, 11, 13}
    printSlice(s)

    // Drop its last two values
    s = s[:len(s)-2]
    printSlice(s)

    // Drop its first two values
    s = s[2:]
    printSlice(s)
}</code>

The output shows the changes in length, capacity, and the underlying array's pointer as we manipulate the slice:

ptr=0x450000 len=6 cap=6 [2 3 5 7 11 13]
ptr=0x450000 len=4 cap=6 [2 3 5 7]
ptr=0x450008 len=2 cap=4 [5 7]

Conclusion

Understanding the internal structure of Go slices is crucial for effectively manipulating them. By recognizing that dropping the last n items affects the capacity differently than dropping the first n items, developers can optimize their code and avoid potential performance issues related to slice resizing.

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