CODEWITHSUNDEEP
pythonc
Koldito
Koldito

Reputation: 169

Same function in Python and C, different results

I had the following function written in Python as part of a larger simulation:

#!/usr/bin/python
counter = 1
while (counter < 10000):
    oldpa = .5
    t = 1
    while (t < counter):
        newpa = ((oldpa * t) + 1) / (t + 1)
        t = t + 1
        oldpa = newpa
    counter = counter + 1
    print str(counter) + "\t" + str(oldpa)

Then, I started rewriting the simulation in C so that it would run faster (and also to give myself an excuse to spend time learning C). Here's my C version of the above function.

#include <stdio.h>
main()
{
  int counter, t;
  float oldpa, newpa;
  counter = 1;
  while ( counter < 10000 )
    {
      oldpa = .5;
      t = 1;
      while ( t < counter )
        {
          newpa = ((oldpa * t) + 1) / (t + 1);
          t = t + 1;
          oldpa = newpa;
        }
      counter = counter + 1;
      printf("%d\t%f\n", counter, oldpa);
   }
}

Now, here is the funny thing. When I run the Python function, the result converges to 0.999950, but when I run the C function, it converges to 0.999883. This difference is actually negligible for the purposes of my simulation, but I still want to know why I get different results

Upvotes: 2

Views: 189

Answers (1)

Mike DeSimone
Mike DeSimone

Reputation: 42805

Floating-point values in Python are almost always IEEE-754 double precision, corresponding to a C or C++ double. If you want a lot more precision, check out the decimal module.

Upvotes: 3

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