Why Does Dropping Elements at the Beginning of a Slice Reduce its Capacity?

Go Tour #11: The Mystery of Changing Slice Capacity

In the "Slice the Slice" section of the Go Tour, a seemingly puzzling transformation occurs. After modifying a slice by dropping its first two values, its capacity changes from 6 to 4. This behavior contradicts our initial assumption that capacity should remain constant. Let's delve deeper into the reasons behind this change.

Understanding Slice Capacity

Slices in Go are a data structure that provides an abstraction over arrays. They reference elements from an underlying array but don't own the backing storage. Capacity, on the other hand, represents the size of the underlying array that can hold the slice elements.

The Cause of Capacity Reduction

Dropping elements from the beginning of a slice moves the slice data pointer to the right within the underlying array. As a result, the distance between the slice's current data pointer and the end of the array decreases. This decrease manifests itself as a reduction in capacity.

Why Only the Last Modification Affects Capacity

The other operations, such as slicing to give the slice zero length and extending its length, don't modify the slice data pointer. They only alter the slice's length, which doesn't affect the distance between the data pointer and the end of the array. Therefore, the capacity remains unchanged.

Internal Slice Details

To gain a deeper understanding, we can print the slice header using reflection:

<code class="go">func printSlice(s []int) {
    sh := (*reflect.SliceHeader)(unsafe.Pointer(&s))
    fmt.Printf("header=%+v len=%d cap=%d %v\n", sh, len(s), cap(s), s)
}</code>

The following output demonstrates the changes in the slice header as the modifications are performed:

<code class="text">header=&{Data:272990208 Len:6 Cap:6} len=6 cap=6 [2 3 5 7 11 13]
header=&{Data:272990208 Len:0 Cap:6} len=0 cap=6 []
header=&{Data:272990208 Len:4 Cap:6} len=4 cap=6 [2 3 5 7]
header=&{Data:272990216 Len:2 Cap:4} len=2 cap=4 [5 7]</code>

As we can see, the final modification shifts the data pointer, resulting in a reduced capacity.

Summary

Slice capacity can change when elements are dropped from the beginning of a slice. This is because it reduces the distance between the slice data pointer and the end of the underlying array. Other slice operations like slicing to zero length or extending length don't affect capacity. Understanding these concepts is crucial for working effectively with Go slices.

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