Does Go Slice Expansion Always Double Capacity?

Understanding Slice Expansion in Append

In Go, slices are dynamically sized arrays. When appending an element to a slice, it may need to be enlarged to accommodate the new data. This article explores the algorithm used for slice enlargement and addresses the question of whether its capacity is always doubled.

Enlargement Algorithm

The implementation of the slice enlargement algorithm is available in the Go source code. According to the code committed on Oct 26, 2016, the rules are as follows:

1. If the new length of the slice is more than double its current length, the new capacity is set to match the desired length.
2. Otherwise, if the current length is less than 1024, its capacity is doubled. For lengths greater than or equal to 1024, the capacity is increased by 25%. This step is repeated until the new capacity meets the required length.

Capacity Doubling

Based on the algorithm, the capacity of a slice is not always doubled when enlarged. The capacity is doubled only if the current length is less than 1024 and the new length is not more than double the current length. Otherwise, the capacity is increased proportionally by 25%.

Conclusion

The slice enlargement algorithm in Go follows a specific set of rules. It adjusts the slice's capacity based on the size of the slice and the number of elements to be appended. While the capacity may occasionally be doubled in certain scenarios, it is not always the case.

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