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 a number of combinations/abbreviations, like
`1+`

. However, `2/`

is not one of them; it shifts its
argument right by one bit (arithmetic shift right), and viewed as
division that always rounds towards negative infinity (floored
division), like Gforth’s `/`

(since Gforth 0.7), but unlike
`/`

in many other Forth systems.

-5 2 / . \ -2 or -3 -5 2/ . \ -3

`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 zero, execution continues at
the `begin`

, otherwise after the `until`

.

Assignment:Write a definition for computing the greatest common divisor.

Reference: Simple Loops.