Range

range iterates over elements in a variety of data structures. Let’s see how to use range with some of the data structures we’ve already learned.

package main

import "fmt"

func main() {

    // Here we use range to sum the numbers in a slice.
    // Arrays work like this too.
    nums := []int{2, 3, 4}
    sum := 0
    for _, num := range nums {
        sum += num
    }
    fmt.Println("sum:", sum)

    // range on arrays and slices provides both
    // the index and value for each entry.
    // Above we didn’t need the index, so we ignored it
    // with the blank identifier _.
    // Sometimes we actually want the indexes though.
    for i, num := range nums {
        if num == 3 {
            fmt.Println("index:", i)
        }
    }

    // range on map iterates over key/value pairs.
    kvs := map[string]string{"a": "apple", "b": "banana"}
    for k, v := range kvs {
        fmt.Printf("%s -> %s\n", k, v)
    }

    // range can also iterate over just the keys of a map.
    for k := range kvs {
        fmt.Println("key:", k)
    }

    // range on strings iterates over Unicode code points.
    // The first value is the starting byte index of the rune
    // and the second the rune itself.
    for i, c := range "go" {
        fmt.Println(i, c)
    }
}

See Strings and Runes for more details.

$ go run range.go
sum: 9
index: 1
a -> apple
b -> banana
key: a
key: b
0 103
1 111