Permutations and Combinations - Runtime Failure

Question

This program takes 2 numbers from user input, asks them whether they'd like to find out the permutations or combinations, and then outputs the result. Here's the code.

#include "std_lib_facilities.h"

int permutation(int first, int second)
{
int top_fac;
int bottom_fac;
for (int i = first-1; i >= 1; --i)
    top_fac *=i;
for (int i2 = (first-second)-1; i2>=1; --i2)
    bottom_fac *= i2;
return (top_fac/bottom_fac);
}

int combination(int first, int second)
{
int bottom_fac;
for (int i = second-1; i>=1; --i)
    bottom_fac *= i;
return permutation(first, second)/(bottom_fac);
}

int main()
{
cout << "Enter two numbers.\n";
int first = 0;
int second = 0;
cin >> first >> second;
cout << "Now choose permutation(p) or combination(c).\n";
string choice;
cin >> choice;
if (choice == "p") 
   cout << "Number of permutations: " << permutation(first,second) << endl;
else if (choice == "c")
   cout << "Number of combinations: " << combination(first,second) << endl;
else
   cout << "p or c stupid.\n";
keep_window_open("q");
}

When I try to run the program, and I choose p or c, I get a "permutations_combinations.exe has stopped working" message. I tried to catch an error, but nothing is coming up. Any ideas?

Thanks in advance.

Answer 1

Make sure you initialize top_fac and bottom_fac.

Answer 2

You're not initializing the local variables top_fac and bottom_fac inside of your functions. Unlike other languages, local variables are NOT initialized to anything in particular in C or C++. The values that they receive are whatever garbage happens to be on the stack when you call the function. You should explicitly initialize top_fac and bottom_fac to 1 at the beginning of the permutation() and combination() functions.

I'm guessing that bottom_fac is accidentally getting initialized to 0, and then you're dividing by 0 at the end of the function, which is causing the runtime failure you're seeing.

Answer 3

Uninitialized variables.

Answer 4

I'm guessing your inputs are causing one of your for-loops to infinitely loop. Check those out carefully.

Answer 5

A suggestion - learn how to use your debugger. Even if you're developing on Linux you've got an IDE that will help you step through your code - http://monodevelop.com/ (among others). Setting a break point at the top of your functions would have shown you your variables are not being initialized.