Question

M-File

fprintf('\n\nn!\n')
for i=[1:length(timearray)]
    fprintf('%s \t %d \n', stringarray{i},factorial(timearray(i)))
end

Output

n!
microsecond      1 
minute       Inf 
hour         Inf 
day          Inf 
month        Inf 
year         Inf 
century      Inf 

I’m trying to calculate the number of steps an algorithm that runs at n! takes at the above intervals (assuming 1 step = 1 microsecond). However, I am unable to get fprintf to display meaningful results.

Switching to fprintf('%s \t %bu \n', stringarray{i},factorial(timearray(i))) gave me some numbers, but I suspect they are wrong.

%bu Output

n!
microsecond      04607182418800017408 
minute       09218868437227405312 
hour         09218868437227405312 
day          09218868437227405312 
month        09218868437227405312 
year         09218868437227405312 
century      09218868437227405312 

Disclaimer: I wrote this program to solve a homework problem, however the homework never specified to write a program.

Thanks for any help!

Mike

Answer 1

The problem is likely not with printing per se, but that the factorial computation itself is overflowing. Try just evaluating the value instead of printing it in the loop; you’ll find that the factorial function overflows after 170!.

One simple way to circumvent this is by returning the log-factorial instead:

log_factorial = @(n) sum(log(1:n));

Then you can compare values on vastly different scales without worrying about overflow. Note: the above function is not vectorized so it will only work with a single value input; if you need to work with array values then there appear to be other solutions already available.