In the GLaDOS language, you can express a loop in the following way:

<while> ::= 'while' (<expression>) { <statement> }

The while keyword is followed by an expression, which is then followed by a block of statements. The block of statements is executed while the expression evaluates to True. If the expression evaluates to False, the block of statements is skipped.

You can also express a loop with the for keyword:

<for> ::= 'for' (<range>) { <statement> }

The for keyword is followed by a range, which is then followed by a block of statements. The block of statements is executed for each value in the range.