GCD

The Factor IRC channel (#concatenative) can be a helpful resource when trying to learn how to code in Factor. Today, someone asked for help implementing the Greatest Common Denominator algorithm in Factor. There are a variety of algorithms for solving for the GCD of two numbers (and many are documented on the Wikipedia entry).

One possible solution uses the Euclidean Algorithm, which is implemented like so:

def gcd(a, b):
    if b = 0:
        return a
    else:
        return gcd(b, a % b)

We can translate that fairly directly to a recursive Factor word:

: gcd ( a b -- c )
    dup zero?
    [ drop ]
    [ [ mod ] keep swap gcd ]
    if ;

It's worth noting that Factor has a builtin word gcd in the math vocabulary calculating this problem.

To learn how it works:

( scratchpad ) USE: math

( scratchpad ) \ gcd help

To see how it is implemented:

( scratchpad ) USE: math

( scratchpad ) \ gcd see

Beginning Factor

One of the Factor bloggers posted a good introduction to stack mechanics, shuffling, and combinators.

• Beginning Factor: Introduction
• Beginning Factor: Shufflers & Combinators

wp.factor

I came across a benchmark for comparing languages today. It did not contain a version for Factor, so I thought I would contribute one.

The idea is fairly straightforward, and in the words of the author:

read stdin, tokenize into words
for each word count how often it occurs
output words and counts, sorted in descending order by count

My attempt is below:

USING: arrays assocs kernel io math math.parser 
prettyprint sequences splitting sorting ;

IN: wp

: count-words ( assoc string -- assoc' )
    " " split harvest [ over inc-at ] each ;

: sort-assoc ( assoc -- seq )
    >alist sort-values reverse ;

: print-results ( seq -- )
    [ number>string "    " glue print ] assoc-each ;

: wp ( -- )
    H{ } clone
    [ [ count-words ] each-line ]
    [ sort-assoc print-results ]
    bi drop ;

MAIN: wp

You can run this from factor by putting it in a file called wp.factor and running from the shell:

cat file.txt | ./factor -run=wp

Clamp

One occasionally useful snippet of code when working with numbers is the clamp function for restricting a number to a range of values (either the minimum, maximum, or somewhere in-between).

In a language like Python or C or Java, one might write this function:

def clamp(a, value, b):
    return max(a, min(b, value))

In Factor, this is written in a similar, but more terse, manner:

: clamp ( a value b -- x )
    min max ;

Since the arguments are located on the stack, the "min" word operates on "value" and "b", placing the "result" on the stack, then the "max" word operates on "a" and "result", placing the final result on the stack.

Simple. Elegant.

Factor

Factor is a high-level language written by Slava Pestov with many interesting characteristics.  

It is stack-based with high-level features including dynamic types, extensible syntax, macros, and garbage collection.  On a practical side, Factor has a mature library, supports many different platforms, and has been extensively documented.  Similar in many ways to other concatenative programming languages, the syntax could be considered terse, but powerful.

It is a very different language than most with a lot of potential for its future.  

This blog will contain articles and code related to Factor, and should serve as both a learning guide and exploration of this language.