Range

Consider a Go program that has a slice with indexes 0, 1, 2. With a range expression in a for-loop we iterate over those indexes.

A range can be used on a slice, string or map. The first variable from a range is the index. The second is the element itself (like a slice element or a rune).

Slice example

We have a slice of string elements here—a string slice. This slice has 3 elements. We use range in a for-loop on the slice.

package main

import "fmt"

func main() {
    colors := []string{"blue", "green", "red"}
    // Use slice range with one value.
    // ... This loops over the indexes of the slice.
    for i := range colors {
        fmt.Println(i)
    }
    // With two values, we get the element value at that index.
    for i, element := range colors {
        fmt.Println(i, element)
    }
}
0
1
2
0 blue
1 green
2 red

String

Here is a string range example. We can use 1 or 2 variables in the range clause. The first is the rune index, and the second is the rune value itself.

package main

import "fmt"

func main() {
    // This is a string.
    name := "abcdef"

    // Use range on string.
    // ... This accesses only the indexes of the string.
    for i := range name {
        fmt.Println(i)
    }
    // Use range with two variables on string.
    // ... This is an index and a rune (char of the string).
    for i, element := range name {
        // Convert element to string to display it.
        fmt.Println(i, string(element))
    }
}
0
1
2
3
4
5
0 a
1 b
2 c
3 d
4 e
5 f

Map

Here is a map. We use int keys and string values. We map the English words for some numbers—this could be useful in a real program.

package main

import "fmt"

func main() {
    // An example map.
    words := map[int]string{
        0: "zero",
        1: "one",
        2: "two",
        3: "three",
    }

    // Use range on map to loop over keys.
    for key := range words {
        fmt.Println(key)
    }

    // Range on map can access both keys and values.
    for key, value := range words {
        fmt.Println(key, value)
    }
}
2
3
0
1
3 three
0 zero
1 one
2 two

Range int

In newer versions of Go we can use a range-int loop, which loops from zero to the integer value specified. This repeats a loop body a certain number of times.

package main

import "fmt"

func main() {
    // Repeat a loop a certain number of times with a range int loop.
    for i := range 5 {
        fmt.Println(i)
    }
}
0
1
2
3
4

In Go we use a range expression to enumerate arrays, slices, maps, strings and channels. For most uses, a range can be used with one or two variables.