range

The range form of for makes iteration over collections easier. It supplies indexes, keys or values depending on what you iterate.

For a slice, for index, value := range values yields each index and its element value. Use _ when you do not need the index. For a map, range gives keys and values, but map traversal order is not guaranteed.

For strings, range decodes UTF-8 code points; the index is a byte offset, not necessarily a character count. Later string lessons explain that distinction in detail.

Follow this diagram before executing the example. Check where a value or decision changes what runs next.

This example is a complete program. Save it as main.go or paste it into the Go Playground.

Go code

package main

import "fmt"

func main() {
	scores := []int{10, 20, 30}
	sum := 0
	for index, score := range scores {
		fmt.Println("Index", index, "Score", score)
		sum += score
	}
	fmt.Println("Sum:", sum)
}

Output

Index 0 Score 10
Index 1 Score 20
Index 2 Score 30
Sum: 60

The slice has indexes 0, 1 and 2. Range produces each index-value pair in order for the slice, and the loop accumulates 10, 20 and 30 into a final sum of 60.

• Do not rely on a fixed iteration order when ranging over a map.
• The second range value is a copy of the element value in ordinary slice iteration, not a pointer to the original element.

Change the values to 5, 15 and 25. Rewrite the loop as for _, score := range scores when you no longer need the index.

Tip: Edit the example above, predict the output, and run the modified code to test your reasoning.

You have practiced Go range in a small, self-contained program. The important habit is to predict what runs, inspect the actual output and distinguish the language rule from the particular example.

Before continuing, make sure you can explain the program to someone who has not seen it, including the line that changes its result.