Append two slices in Go

Go ships with a buildin function append that can append elements to the end of a slice and returns the updated slice. Here is an example:

data := []int{ 1,2,3 }
data = append(data, 4)


This will print the following:

[1 2 3 4]

Now I was browsing to some Go code and found something like this:

data := []int{ 1,2,3 }
next := []int{ 4,5,6 }

for _ , v := range next {
    data = append(data, v)

This is not a very good way to append a slice to another slice. Actually it is bad and here is why. The append function appends elements to the end of a slice. If it has sufficient capacity, the destination is resliced to accommodate the new elements. If it does not, a new underlying array will be allocated.

The important part here is that it will allocate a new underlying array everytime there is insufficient capacity. Because the code above only appends a single value at the time, the runtime does not know what to expect, otherwise that it should grow to make room for an single extra value. The whole idea behind slices is to prevent unnecessary allocations.

An easy way to fix this is to pass all the data that needs to be appended at once. This way the runtime has all the information needed to grow only once, if needed.

data := []int{ 1,2,3 }
next := []int{ 4,5,6 }

data = append(data, next...)

Notice the suffix ... added to next. This is Go's way of telling to a variadict that we want to pass multiple arguments instead of a single value. If we remove the ... we will receive an compile error that next an invalid type to append to data. This makes sense because we can't append an []int value to an []int, only int values are allowed.

I guess that the programmer used an for loop to resolve this compile error instead of passing multiple arguments at once.