FACTORIALRATIO

Ratio of factorials, as in (a!b!c!...) / (d!e!f!g!...)

Description

FACTORIALRATIO - ratio of factorial numbers
 
    FR = FACTORIALRATIO (A,B) calculates the ratio between products of
    factorials specified by A and B. A and B are lists of positive
    integers, with N and M elements, respectively. The number of values in
    each list may differ. The factorial ratio is based on the following formula:
 
               (A1! * A2! * ... * AN!)
       FR = -------------------------
               (B1! * B2! * ... * BM!)
 
     However, an approach is taken that increases the accuracy when A or B
     contain larger values, as mentioned in the help of FACTORIAL.
 
     Examples:
 
       f1 = factorialratio([10 8 6],[9 6 5]) ;
       f2 = 10 * 8 * 7 * 6 ; % by hand
       f3 = (factorial(10) * factorial(8) * factorial(6)) ./ ...
             (factorial(9) * factorial(6) * factorial(5)) ;
       disp('f1, f2, f3 = ') ; disp([f1 f2 f3])
 
 
       % Also for larger values FACTORIALRATIO works correctly
       f1 = factorialratio(150,143) ;
       f2 = prod(144:150) ; % by hand
       % where FACTORIAL lacks precision
       f3 = factorial(150) ./ factorial(143) ;
       disp('f1, f2, f3 = ') ; disp([f1 f2 f3])
       disp(' Differences with f2 = ') ; disp([f1-f2 f2-f2 f3-f2])
 
       % and even for extreme numbers things turn out well
       f1 = factorialratio(30000,29998) ;
       f2 = (30000 * 29999) ; % by hand
       % whereas calls to FACTORIAL fail miserably ...
       f3 = factorial(30000) ./ factorial(29998) ;
       disp('f1, f2, f3 = ') ; disp([f1 f2 f3])
 
     Note that, for instance, factorialratio(200,1) also fails, of course.
     For such occasions other engines have to be used.
      
     See also prod, factorial, gammaln

• MATLAB release: MATLAB 6.5 (R13)

[John D'Errico]

Good help as you would expect from Jos. Well written. An interesting solution for the computation itself.

[John D'Errico]

Sorry, I forgot to point out what I feel to be one of the best features of this code - the copious internal comments that explain how it works.

I've often argued that the best MATLAB code should aim for a 1-1 ratio of comments to lines of code. That is, each line of code should have an associated comment line, at least in those places where the code is doing something tricky. This code goes beyond my aim. People interested in how to write good comments should look at what the author did here and learn from it.