For

The for construct it's usually used for iteration over collections and for counting.

The following loop counts from 0 to 9:

for (var i = 0; i < 10; i++) {
    say i
}

Alternatively, using a simpler form of the for-loop:

for (0 ..^ 10) { |i|
    say i
}

To iterate over an array, Sidef supports yet another form of the for loop:

for i in [1,2,3,4] {
    say i
}

The for-in loop is a generalization of the for loop, supporting iteration over any object that respond to the iter method, which must return a block as the iterator, and the block should return one value at a time when it's called, or nil when there are no other values to return, which will break the loop.

Alternatively, the object may also respond to the to_a method, which must return an object of type Array. Using this knowledge, we can iterate over Hash objects because the Hash class implements the to_a method:

for i,k in Hash(a => 1, b => 2) {
    say "<#{i}> <#{k}>"
}

The for-in loop can also be used in iterating over matrices:

for i,j,k in [
    %w(a b c),
    %w(d e f),
] {
    say "<#{i}> <#{j}> <#{k}>"
}