void main()
{
    import std.stdio : writeln;
    int[] arr = [1, 3, 4];
    for (int i = 0; i < arr.length; i++)
    {
        arr[i] *= 2;
    }
    writeln(arr); // [2, 6, 8]
}

void main()
{
    import std.stdio : writeln;
    int[] arr = [1, 3, 4];
    int i = 0;
    while (i < arr.length)
    {
        arr[i++] *= 2;
    }
    writeln(arr); // [2, 6, 8]
}

void main()
{
    import std.stdio : writeln;
    int[] arr = [1, 3, 4];
    int i = 0;
    assert(arr.length > 0, "Array must contain at least one element");
    do
    {
        arr[i++] *= 2;
    } while (i < arr.length);
    writeln(arr); // [2, 6, 8]
}

Foreach allows a less error-prone and better readable way to iterate collections. The attribute ref can be used if we want to directly modify the iterated element.

void main()
{
    import std.stdio : writeln;
    int[] arr = [1, 3, 4];
    foreach (ref el; arr)
    {
        el *= 2;
    }
    writeln(arr); // [2, 6, 8]
}

The index of the iteration can be accessed too:

void main()
{
    import std.stdio : writeln;
    int[] arr = [1, 3, 4];
    foreach (i, el; arr)
    {
        arr[i] = el * 2;
    }
    writeln(arr); // [2, 6, 8]
}

Iteration in reverse order is possible too:

void main()
{
    import std.stdio : writeln;
    int[] arr = [1, 3, 4];
    int i = 0;
    foreach_reverse (ref el; arr)
    {
        el += i++; // 4 is incremented by 0, 3 by 1, and 1 by 2
    }
    writeln(arr); // [3, 4, 4]
}

void main()
{
    import std.stdio : writeln;
    int[] arr = [1, 3, 4, 5];
    foreach (i, el; arr)
    {
        if (i == 0)
            continue; // continue with the next iteration
        arr[i] *= 2;
        if (i == 2)
            break; // stop the loop iteration
    }
    writeln(arr); // [1, 6, 8, 5]
}

Labels can also be used to break or continue within nested loops.

void main()
{
    import std.stdio : writeln;
    int[] arr = [1, 3, 4];
    outer: foreach (j; 0..10) // iterates with j=0 and j=1
        foreach (i, el; arr)
        {
            arr[i] *= 2;
            if (j == 1)
                break outer; // stop the loop iteration
    }
    writeln(arr); // [4, 6, 8] (only 1 reaches the second iteration)
}