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Foreach

D features a foreach loop which allows for less error-prone and more readable iteration.

Element iteration

Given an array arr of type int[] it is possible to iterate over the elements using a foreach loop: 

foreach (int e; arr) {
    writeln(e);
}

The first parameter in a foreach loop is the variable name used for each iteration. Its type can be inferred automatically like so: 

foreach (e; arr) {
    // typeof(e) is int
    writeln(e);
}

The second field must be an array - or a special iterable object called a range which will be introduced in the next section.

Access by reference

Elements will be copied from the array or range during iteration. This is acceptable for basic types, but might be a problem for large types. To prevent copying or to enable in-place mutation, ref can be used: 

foreach (ref e; arr) {
    e = 10; // overwrite value
}

Iterate n times

D allows us to write iterations which should be executed n times, more concisely with the .. syntax: 

foreach (i; 0 .. 3) {
    writeln(i);
}
// 0 1 2

The last number in a .. b is excluded from the range, thus the loop body is executed 3 times.

Iteration with index counter

For arrays, it's also possible to access a separate index variable. 

foreach (i, e; [4, 5, 6]) {
    writeln(i, ":", e);
}
// 0:4 1:5 2:6

Reverse iteration with foreach_reverse

A collection can be iterated in reverse order with foreach_reverse: 

foreach_reverse (e; [1, 2, 3]) {
    writeln(e);
}
// 3 2 1

In-depth

rdmd playground.d