3.18 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 than 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.