How do I manually reserve space for an existing slice in Go?

Question

I'm appending the values of a map into an existing slice.

The code's like (the s is a slice that already has some elements in it):

for key, value := range m {
    s = append(s, value)
}

As far as I know, slices in Go double their sizes when needed. I could let it double the capacity of itself, but it'll happen several times a loop, which probably is bad for performance.

In this case, I know the exact space needed, that is, len(m) . How do I “reserve” the space for a slice, like that in C++? I want the reallocation to happen just once.

Answer 1

You allocate memory for an object with make , like ( play ):

s := make([]string, len(m))
for i, v := range m {
    s[i] = v
}

Alternatively, you can create a slice of length zero, but with enough capacity to hold n values. Here, append will never need to be allocate new memory, if the number of items appended does not exceed capacity ( play ).

s := make([]string, 0, len(m))
for _, v := range m {
    s = append(s, v)
}

If you like to dive into slices more visually, this blog post may help:

• https://divan.dev/posts/avoid_gotchas/#arrays-and-slices

If you want to manually enlarge an slices with element you would need to:

• create a new slice of desired capacity (and length)
• use copy to copy elements over

Example on play .