Repeating Things

Loops let you run the same code multiple times. Go has one loop keyword: for. It handles everything.

The classic for loop

The most common form has three parts separated by semicolons:

for i := 0; i < 5; i++ {
    fmt.Println(i)
}
// Output: 0, 1, 2, 3, 4

Here's what each part does:

• i := 0 — init: runs once before the loop starts
• i < 5 — condition: checked before each iteration. If false, the loop stops
• i++ — post: runs after each iteration (adds 1 to i)

So this loop starts i at 0, prints it, adds 1, checks if it's still less than 5, and repeats.

Return early, fall back after the loop

Loops get useful when you check something on each iteration. A loop that's searching for a value often wants two exits: return as soon as you find a match, and return a fallback value after the loop if you never did.

func firstMultiple(target, limit int) int {
    for i := 1; i < limit; i++ {
        if i%target == 0 {
            return i
        }
    }
    return -1 // nothing matched
}

The return -1 only runs when the loop finishes without finding a match. That's the fallback.

Watch out for this bug: putting the fallback inside an else.

// BUG: exits on the first iteration every time
for i := 1; i < limit; i++ {
    if i%target == 0 {
        return i
    } else {
        return -1
    }
}

The else branch runs on iteration 1 and returns before the loop can check any other value. The failure case belongs after the loop, not paired with the success case.