| \ BEGIN ... WHILE ... REPEAT. Halts with 21 5 111 0 on the stack (bottom to top). | |
| \ Euclid's algorithm: loop while the second number is nonzero. | |
| : GCD ( a b -- gcd ) BEGIN DUP WHILE SWAP OVER MOD REPEAT DROP ; | |
| \ Number of decimal digits of a positive number. | |
| : DIGITS ( n -- count ) 0 SWAP BEGIN DUP WHILE 10 / SWAP 1 + SWAP REPEAT DROP ; | |
| \ Collatz: steps from n down to 1. | |
| : STEPS ( n -- steps ) | |
| 0 SWAP BEGIN DUP 1 <> WHILE | |
| DUP 2 MOD IF 3 * 1 + ELSE 2 / THEN SWAP 1 + SWAP | |
| REPEAT DROP ; | |
| \ LEAVE inside a WHILE body leaves the enclosing DO loop at once. | |
| : FIRST ( -- 0 ) 0 10 0 DO BEGIN -1 WHILE LEAVE REPEAT 1 + LOOP ; | |
| 1071 462 GCD 12345 DIGITS 27 STEPS FIRST | |