for: the only loop

Go has a single loop construct: for. No while, no do-while, no foreach (that arrives in the next lesson with range). A single keyword covers all cases thanks to three forms.

The three forms of for

Classic three-clause form

Identical to C/Java:

for i := 0; i < 5; i++ {
    fmt.Println(i)
}

i has scope limited to the for. The three clauses are optional (you can omit any of them) but the ; remain if you omit an intermediate one.

"while" form: condition only

In Go, while is simply a for with a single bool expression:

i := 0
for i < 5 {
    fmt.Println(i)
    i++
}

Infinite form

Without clauses, the loop never terminates (unless break/return):

for {
    if exitCondition() {
        break
    }
    doWork()
}

It is the pattern for event handling loops, servers, retry policies.

break, continue and labels

break exits the innermost loop, continue skips to the next iteration:

for i := 0; i < 10; i++ {
    if i%2 == 0 {
        continue       // salta i pari
    }
    if i > 7 {
        break          // ferma il ciclo
    }
    fmt.Println(i)     // 1 3 5 7
}

To exit nested loops there are labels (rare but useful):

Esterno:
for i := 0; i < 3; i++ {
    for j := 0; j < 3; j++ {
        if i*j == 4 {
            break Esterno   // esce da entrambi
        }
    }
}

No , operator like in C

In Go you cannot write for i, j := 0, 10; i < j; i, j = i+1, j-1: the post expression does not accept multiple assignments. Use Go's multiple assignment syntax:

for i, j := 0, 10; i < j; i, j = i+1, j-1 {
    fmt.Println(i, j)
}

This instead works because i, j = i+1, j-1 is a single multiple assignment.

Try it

// (a) for i := 0; i < 10; i++ { ... }
// (b) for cond { ... }
// (c) for { ... }

Recap

• A single keyword for with three forms: classic, while, infinite.
• break / continue optionally with a label for nested loops.
• Label Name: before the for, used sparingly.
• No , as an operator: use the multiple assignment i, j = ..., ....