Range over Built-in Types

range iterates over elements in a variety of built-in data structures. Here is range with slices, maps, strings, and integers.

package main

import "fmt"

func main() {

Sum the numbers in a slice. Arrays work the same way.

	nums := []int{2, 3, 4}
	sum := 0
	for _, num := range nums {
		sum += num
	}
	fmt.Println("sum:", sum)

range yields both the index and the value. Above we ignored the index with _.

	for i, num := range nums {
		if num == 3 {
			fmt.Println("index:", i)
		}
	}

The value is a copy; assign through the index to change the slice.

	for i := range nums {
		nums[i] *= 10
	}
	fmt.Println(nums)

range on a map iterates over key/value pairs.

	kvs := map[string]string{"a": "apple"}
	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 a string iterates over Unicode code points. The first value is the byte index of the rune, the second the rune itself.

	for i, c := range "go€" {
		fmt.Println(i, c, string(c))
	}

range over an integer counts from zero.

	for i := range 3 {
		fmt.Print(i, " ")
	}
	fmt.Println()
}

$ go run ./11_range_over_built_in_types
sum: 9
index: 1
[20 30 40]
a -> apple
key: a
0 103 g
1 111 o
2 8364 €
0 1 2