# A simple GCD implementation in gforth

by Timmy Jose

I have been working through gforth’s excellent Forth tutorials, and while it can be terse (almost cryptic) in places, I have found it rather smooth sailing so far. Given that Starting Forth is the usual book recommended to beginners, this may be a surprising choice. My reasoning is quite clear - I plan to work through both “Starting Forth” as well as “Thinking Forth”. In fact, I had actually begun with the former, but quickly realised that the book’s age did not lend itself too well to a modern Forth interpreter. Even so, the gforth manuals have been excellent so far.

In any case, one of the exercises in this tutorial series was to write a simple GCD calculator. So here is my solution, and I am rather pleased to be writing actual Forth code at last! Possibly Forth suits some quirk in my brain. Heh.

```
: gcd ( u1 u2 -- gcd )
begin
dup 0<> while
2dup < if swap then
2dup mod rot drop
repeat
drop ;
```

Sample run:

```
18 24 gcd . 6 ok
24 18 gcd . 6 ok
24 17 gcd . 1 ok
12 36 gcd . 12 ok
10 10 gcd . 10 ok
```

Interestingly enough, the Forth compiler seems to convert the inner `if`

to a `while`

:

```
see gcd
: gcd
BEGIN dup 0<>
WHILE 2dup <
WHILE swap
THEN
2dup mod rot drop
REPEAT
drop ; ok
```