Loops: for is the only loop

Why one loop is enough

Other languages have for, while and do-while. Go keeps only for, but it can be written in a few shapes that cover every need. Fewer keywords means less to learn and less to argue about.

Shape 1: the counter loop

for i := 1; i <= 5; i++ {
	fmt.Println("Roll number", i)
}

Shape 2: condition only (a while loop)

balance := 1000
months := 0
for balance > 0 {
	balance -= 300
	months++
}
fmt.Println(months, balance) // 4 -200

Shape 3: forever, with break

attempt := 0
for {
	attempt++
	if attempt == 3 {
		fmt.Println("connected on attempt", attempt)
		break
	}
}

for range: walking over things

range gives you each item of a collection. For a slice it gives the index and a copy of the value. Use the blank identifier _ to ignore one of them.

fruits := []string{"mango", "banana", "guava"}
for i, f := range fruits {
	fmt.Println(i, f)
}
for _, f := range fruits {
	fmt.Println(f)
}

for i := range 3 {
	fmt.Println("try", i) // try 0, try 1, try 2
}

break, continue and labels

for i := 1; i <= 10; i++ {
	if i%2 == 0 {
		continue // skip even numbers
	}
	if i > 7 {
		break    // stop the loop completely
	}
	fmt.Print(i, " ")
}
// prints: 1 3 5 7

In nested loops, break only leaves the innermost loop. A label (a name followed by a colon) on the outer loop lets you break out of both.

outer:
for _, row := range [][]int{{1, 2}, {3, 4}} {
	for _, v := range row {
		if v == 3 {
			break outer
		}
		fmt.Println(v)
	}
}

Go 1.22 change: a new loop variable each round

Before Go 1.22, a for loop had ONE variable i reused every round. Closures or goroutines that captured i all saw its final value, a famous bug. Since Go 1.22 each round gets its own copy, so the bug is gone for modules whose go.mod says go 1.22 or newer. You will still see old code with i := i inside loops; that line was the fix for the old behaviour.