myfactor

This function finds the factors of very large numbers (up to 10^14)

Description

To find the prime factors of any number up to 2^32 (about 4.3*10^9), you can use MATLAB's built-in function factor.m

For integers greater than 2^32, try this program, which guarantees a result up to 10^14, thus extending the domain of inputs 23,000-fold.

(This file is an adaptation of MATLAB's function primes.m)

Example:

myfactor(2^32+1)
ans = [1 641 6700417]

• MATLAB release: MATLAB 7.2 (R2006a)

[John D'Errico]

Yes, this does extend the factor code to a larger range.

There are a few problems with the code. It is a little sloppy, generating a few variables for no purpose at all. For example, quotient and result are two variables that are set in the code, but then never used again. Look at the results of mlint on your code. It will streamline your code, but also maybe find some bugs too.

Myfactor also lacks an H1 line. This is nice when you need to use lookfor. Perhaps 6 months from now, when you cannot remember the name of this function, nor why somebody called it myfactor. lookfor will help you, but ONLY if the author bothers to write a descriptive first comment line with good keywords in it.

More irritating, myfactor ONLY runs for certain inputs. If you call it with a small number, it returns only the irritating message

myfactor(123456)
 
For numbers up to 2^32, use MATLAB's built-in function factor.m

Yes, you must first test your number to see if it is smaller than 2^32-1, otherwise myfactor issues this snotty remark. Would it really have been that difficult to just call the Matlab factor instead? Or simpler, just remove this silly test completely? If you do so, the code actually works. So why be so stubborn and obnoxious?

Next, I did remove the offending, irritating test, so that I could compare this code to factor. Factor was generally faster, by about 10-20% for most random inputs that I tried, but there were some cases where myfactor was the faster of the two.

My irritation almost drove me to rate this a 2. Perhaps this deserves a 2.5 rating. If you modify the code yourself to make it useable, I suppose that brings it up to 3.