0.22 REPEAT-UNTIL constructs

The counterpart of WHILE-DO constructs is the REPEAT-UNTIL construct. This executes the body, then checks a condition at 'UNTIL'. If the expression evaluates to FALSE, it branches back to the top of the body (marked by 'BEGIN') again. It executes at least once. This program calculates the largest common divisor.

: lcd
           swap over mod                  \ body
           dup 0=                         \ condition
     until drop . ;

If you enter "27 21 lcd" the programs will answer "3".