Iterating with range

Learn to loop over slices and maps with range.

Iterating with range

range iterates over slices, arrays, maps, strings and channels. For slices it gives the index and value on each iteration:

meyveler := []string{"elma", "armut", "kiraz"}
for i, m := range meyveler {
    fmt.Println(i, m)
}
for _, m := range meyveler {   // value only
    fmt.Println(m)
}

Since Go 1.22, for i := range 5 also counts from 0 to 4.

range over maps and strings

On a map, range gives the key and value; the order is random and may change on every run. On a string it gives the byte index and the rune:

stok := map[string]int{"kalem": 3, "defter": 5}
for ad, adet := range stok {
    fmt.Println(ad, adet)
}
for i, r := range "çay" {
    fmt.Println(i, string(r))  // 0 ç, 2 a, 3 y
}

If ordered output is needed, collect the map keys into a slice and sort them first.

Exercise Double the slice

Declare s := []int{1, 2, 3}. Double every element in place with a for i := range s loop and print the slice.

Show example solution

package main

import "fmt"

func main() {
    s := []int{1, 2, 3}
    for i := range s {
        s[i] *= 2
    }
    fmt.Println(s)
}

Test yourself

Does <code>for _, v := range s { v *= 2 }</code> change the slice?
- Yes, every element doubles
- Only the first element changes
- It doesn't compile
- No; v is a copy of the element
Show answer

No; v is a copy of the element — To change the slice, write through the index: for i := range s { s[i] *= 2 }.

Skip the index

for ____, m := range meyveler {
    fmt.Println(m)
}

Show answer

for _, m := range meyveler {
    fmt.Println(m)
}

range target

[]string

index, value
map[K]V

key, value
string

byte index, rune
range 5

0..4

Next lesson: switch and fallthrough

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