While-Style Loops

Go does not have a while keyword. Instead, a for loop with only a condition behaves like a while loop.

Write for count < 3 { ... } when some state changes inside the loop body. The condition is evaluated before every iteration. If it is false from the start, the body never runs.

An infinite loop uses for { ... } and needs a deliberate exit such as break or return when appropriate. Be careful that the loop makes progress toward a stopping condition.

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() {
	fuel := 3
	for fuel > 0 {
		fmt.Println("Fuel units:", fuel)
		fuel--
	}
	fmt.Println("Tank empty")
}

Output

Fuel units: 3
Fuel units: 2
Fuel units: 1
Tank empty

The condition is initially true at 3, then the loop prints the current fuel and decreases it. The values printed are 3, 2 and 1. When fuel reaches 0, the condition becomes false and the program exits the loop.

• If the body never changes a relevant value, a condition-only loop can run forever.
• while (fuel > 0) is not Go syntax; use for fuel > 0.

Start fuel at 5 and decrease it by 2 on each iteration. Predict which fuel values appear and why the loop ends.

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

You have practiced Go While-Style Loops 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.