listing20.go

// Code listing 20: https://play.golang.org/p/SkL_AS-1Jd

// _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)
    }
}