For loops

Zap has two for forms: a C-style loop and a for ... in loop. Both use a block and support break and continue.

C-style for

A C-style loop contains an initialization, a Bool condition, and an update:

for var index: Int = 0; index < 5; index = index + 1 {
    println(toString(index));
}

The order is:

1. initialize index;
2. check the condition;
3. run the body;
4. run the update;
5. check the condition again.

The loop variable is scoped to the loop:

for var index: Int = 0; index < 3; index = index + 1 {
    println(toString(index));
}

// `index` is not available here.

Choose the update explicitly

The update can move by more than one or count backwards:

for var even: Int = 0; even <= 10; even = even + 2 {
    println(toString(even));
}

for var countdown: Int = 3; countdown > 0; countdown = countdown - 1 {
    println(toString(countdown));
}

The compiler requires the update target to be the same variable introduced by the loop initializer.

Iterate over an array

Use for value in values when the index is not needed:

var ports: [3]Int = {80, 443, 8080};
for port in ports {
    println(toString(port));
}

The loop visits elements in index order and binds one value for each iteration.

Iterate over a list

The same form works with List<T>:

var jobs = new List<String>();
jobs.push("compile");
jobs.push("test");

for job in jobs {
    println(job);
}

Use an indexed loop when you need to replace an element or compare its position. See List for collection operations.

break and continue

for value in ports {
    if value == 443 {
        continue;
    }
    if value > 500 {
        break;
    }
    println(toString(value));
}

Both statements affect the nearest loop. continue proceeds to the next element or update step; break leaves the loop.

Choose between while and for

Use for when the counter or collection describes the iteration. Use while when the next iteration depends on state produced by the body or by an external operation.