Technical Musings

Thoughts, Ideas, and Experimentation

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23 February 2020

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 )  
      dup 0<> while
        2dup < if swap then
        2dup mod rot drop
    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
         2dup mod rot drop
  drop ; ok

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