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General Loops

The endless loop is the most simple one:

: endless ( -- )
  0 begin
    dup . 1+
  again ;
endless

Terminate this loop by pressing Ctrl-C (in Gforth). begin does nothing at run-time, again jumps back to begin.

A loop with one exit at any place looks like this:

: log2 ( +n1 -- n2 )
\ logarithmus dualis of n1>0, rounded down to the next integer
  assert( dup 0> )
  2/ 0 begin
    over 0> while
      1+ swap 2/ swap
  repeat
  nip ;
7 log2 .
8 log2 .

At run-time while consumes a flag; if it is 0, execution continues behind the repeat; if the flag is non-zero, execution continues behind the while. Repeat jumps back to begin, just like again.

In Forth there are many combinations/abbreviations, like 1+. However, 2/ is not one of them; it shifts its argument right by one bit (arithmetic shift right):

-5 2 / .
-5 2/ .

assert( is no standard word, but you can get it on systems other then Gforth by including compat/assert.fs. You can see what it does by trying

0 log2 .

Here's a loop with an exit at the end:

: log2 ( +n1 -- n2 )
\ logarithmus dualis of n1>0, rounded down to the next integer
  assert( dup 0 > )
  -1 begin
    1+ swap 2/ swap
    over 0 <=
  until
  nip ;

Until consumes a flag; if it is non-zero, execution continues at the begin, otherwise after the until.

Assignment:

Write a definition for computing the greatest common divisor.

Reference: Simple Loops.