What is the maximum number of elements you can add using the append method to a given slice without errors or changes to the original data?

In Go, the append method adds elements to a slice, and if the capacity of the original slice allows, the addition occurs without allocating new memory and without changing the original array. The maximum number of elements that can be added without reallocating memory is the difference between the slice's capacity and its current length.

For example, if a slice s has a length of 3 and a capacity of 5, you can add a maximum of 2 elements without allocating new memory.

s := make([]int, 3, 5) // length 3, capacity 5
s = append(s, 1, 2)     // adding 2 elements

If more elements are added than free space in the capacity, Go will create a new array, copy the data, and return a new slice — the original array remains unchanged, but the slice will point to the new array.

Thus, the maximum number of elements that can be added without errors and without changing the original data is cap(s) - len(s).