Go by Example: Range Iteration

Go 1.23

The `range` keyword provides a clean way to iterate over slices, arrays, maps, strings, and channels. This example shows how to use `range` to access both indexes/keys and values.

Code

package main

import "fmt"

func main() {
    // 1. Iterating over a Slice
    // range returns (index, value)
    nums := []int{2, 4, 6}
    sum := 0
    for _, num := range nums {
        sum += num
    }
    fmt.Println("Sum:", sum)

    // If you need the index:
    for i, num := range nums {
        if num == 4 {
            fmt.Println("Index of 4:", i)
        }
    }

    // 2. Iterating over a Map
    // range returns (key, value)
    kvs := map[string]string{"a": "apple", "b": "banana"}
    for k, v := range kvs {
        fmt.Printf("%s -> %s\n", k, v)
    }

    // 3. Iterating over a String
    // range returns (index, rune)
    // It handles Unicode characters correctly.
    for i, c := range "go" {
        fmt.Printf("idx: %d, char: %c\n", i, c)
    }
    
    // 4. Iterating over a Channel
    // Loops until the channel is closed.
    queue := make(chan string, 2)
    queue <- "one"
    queue <- "two"
    close(queue) // Important: close to terminate the range loop

    for elem := range queue {
        fmt.Println("Channel received:", elem)
    }
}

Explanation

The range keyword is Go's tool for "foreach" style iteration. It works on a variety of data structures: slices, arrays, maps, strings, and channels. The syntax is consistent: for key, value := range collection.

Depending on what you are iterating over, the return values change slightly:

Slices/Arrays: Returns index, value.
Maps: Returns key, value. Note that map iteration order is random.
Strings: Returns index, rune. It decodes UTF-8 characters automatically, so the index might jump by more than 1 for multi-byte characters.
Channels: Returns value only. It iterates until the channel is closed.

A common idiom in Go is using the blank identifier _ (underscore) to ignore values you don't need. For example, if you only want the values of a slice and not the indexes, you write for _, v := range slice.

Code Breakdown

10

Using the blank identifier '_'. Range returns the index first, but we don't need it here. Since Go forbids unused variables, we must assign the index to '_' to discard it.

25

Iterating over a map. This gives you both the key and the value. Remember that the iteration order for maps in Go is randomized to prevent developers from relying on a specific order.

32

Iterating over a string. 'c' is a rune (Unicode code point). If the string contained "Hello, 世界", range would correctly iterate over the Chinese characters, treating them as single units rather than raw bytes.

43

Range over a channel. This loop will block waiting for data and continue running until the 'queue' channel is explicitly closed. If you forget to close the channel, this loop could hang forever (deadlock).

Samplebadu

Go by Example: Range Iteration

Code

Explanation

Code Breakdown