Appending with append

Learn to add elements to a slice with the append function.

Appending with append

append adds one or more elements to the end of a slice and returns the new slice; the result must always be assigned to a variable:

var sepet []string
sepet = append(sepet, "elma")
sepet = append(sepet, "armut", "kiraz")
diger := []string{"muz", "üzüm"}
sepet = append(sepet, diger...)
fmt.Println(len(sepet))  // 5

If capacity runs out, append allocates a new, larger array and moves the elements there.

Reserving capacity up front

If you know how many elements will be added, reserving capacity with make([]T, 0, n) avoids repeated growth copies:

kareler := make([]int, 0, 5)
for i := 1; i <= 5; i++ {
    kareler = append(kareler, i*i)
}
fmt.Println(kareler, cap(kareler))  // [1 4 9 16 25] 5

make([]int, 5) instead makes a slice of length 5 filled with zeros; appending to it starts at the 6th element.

Exercise Even numbers

Append the even numbers from 1 to 10 with append to a slice started with make([]int, 0, 5). Print the slice and its capacity.

Show example solution

package main

import "fmt"

func main() {
    ciftler := make([]int, 0, 5)
    for i := 1; i <= 10; i++ {
        if i%2 == 0 {
            ciftler = append(ciftler, i)
        }
    }
    fmt.Println(ciftler, cap(ciftler))
}

Test yourself

What happens if you write <code>append(s, 4)</code> without assigning the result?
- A compile error: the result is unused
- 4 is appended to s
- Nothing, it passes silently
- A panic
Show answer

A compile error: the result is unused — append returns a value and Go treats an unused append result as an error; the correct form is s = append(s, 4).
Append a slice
```
a = append(a, b____)
```
Show answer
```
a = append(a, b...)
```
Code and result

append(nil, 1)

[1]
append([]int{1}, 2, 3)

[1 2 3]
make([]int, 0, 5)

len 0, cap 5
append(make([]int, 2), 7)

[0 0 7]

Next lesson: Sub-slice

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