What is the Time Complexity of Go's `append` Function and String Concatenation with ` `?

Big O of Append in Go

The built-in append function in Go has a time complexity of O(1) if the destination slice has sufficient capacity, or O(n) if it needs to allocate a new slice with a larger capacity. The function reslices the destination slice, which is a constant time operation, only if it has enough capacity to accommodate the appended elements.

For example, in the following code:

nums := []int{0, 1, 2, 3, 4, 5, 6, 7}
fmt.Println(append(nums[:4], nums[5:]...))

The append function reslices the destination slice nums[:4] because it has sufficient capacity to hold the elements from the second slice nums[5:]. Therefore, the time complexity of this operation is O(1).

String Concatenation Using

Unlike append, string concatenation using the operator always allocates a new string, regardless of whether the original strings have enough capacity. This is because strings in Go are immutable, meaning that they cannot be modified in place.

As a result, string concatenation using has a time complexity of O(n), where n is the total number of characters in the concatenated strings. Each concatenation creates a new string that is a copy of the existing strings, leading to multiple memory allocations and copies during a loop.

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