Golang Slices: Is Direct Assignment Always Faster Than Append?

Golang Slice: Append vs. Assignment Performance

In Golang, there are two common ways to append elements to a slice: using the append function or by direct assignment. While both methods achieve the same result, their performance characteristics differ significantly.

Append Operation

The append function appends one or more elements to an existing slice by creating a new slice with sufficient capacity. The new slice is then assigned to the original variable. This involves several steps, including copying the slice header, creating a temporary slice for the variadic parameters, and reslicing the original slice if necessary.

Assignment Operation

Direct assignment, on the other hand, simply assigns a value to an element in the slice. This is a much more straightforward operation that only involves updating the value of a single element.

Performance Comparison

Benchmarking these two operations reveals that direct assignment (a[i] = i) consistently outperforms append (a = append(a, i)). This is primarily because direct assignment involves far fewer steps and does not require the creation of a new slice.

Reason for Performance Difference

The reason for this difference lies in the implementation of the append function. The append function not only adds the new element to the slice but also updates the slice header. This can lead to a significant overhead when performing repeated append operations.

Direct assignment, on the other hand, only modifies the value of a single element in the slice, without affecting the slice header. This makes it a much more efficient operation for single-element appends.

Conclusion

When performance is a concern, direct assignment (a[i] = i) should be favored over the append function for single-element appends. However, if multiple elements need to be appended at once, the append function remains the more convenient and idiomatic approach.

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