[jmoore_20200]

Ok, I'm having some issues with miscalculation here and i cannot figure it out?

example:

when i do

long numberTest = 14*13*12*11*10*9*8*7*6*5*4*3*2*1;
cout << numberTest;
cin.get();

it outputs a calculation completely different from my calculator and PC calculator.

when i do anything less the 12 or so it computes and outputs fine

long numberTest = 10*9*8*7*6*5*4*3*2*1;
cout << numberTest;
cin.get();

what its going on here?
some sort of coercion converting it to the incorrect number?
wrong data type? I'm a bit lost.

Any help would be greatly appreciated

[chinchang]

The result of 14! is greater than the range of long data type.

Data type ranges

Use long long instead.

[JackOfAllTrades]

The value of LONG_MAX is 2147483647
The value of the the factorial is 87178291200

Can you see the problem?

[CTphpnwb]

Try this:

	long numberTest = (long)14*13*12*11*10*9*8*7*6*5*4*3*2*1;

These are my INT_MAX and LONG_MAX values:
2147483647
9223372036854775807

[jmoore_20200]

Ok, here's what i'm trying to do...

Compare the value of 14! to the value produced by solving Stirlings Approximation formula:

n approximately equal to e^-14 * n^n * sqrt(2(PI)n)

I understand that the value of 14! exceeds the data type I used. But it seems no matter which data type I use i cannot generate the proper result
using this formula:

14*13*12*11*10*9*8*7*6*5*4*3*2*1

I can however generate the correct answer usings stirlings formula...

Heres my code so far:

#include <iostream> 
#include <string> 
#include <iomanip> 
#include <cmath> 

using namespace std;

int main() 
{
const double PI = 3.14159265;
double stirlingsFormula =  (1 / (pow(2.71828, 14.00))) * (pow(14.00, 14.00)) * (sqrt(2*PI*14.00));
double stirlingsFormulaExp = (exp(-14.00)) * (pow(14.00, 14.00)) * (sqrt(2*PI*14.00)); // just using exponential function here to simplify code later on
long exactFact = 14*13*12*11*10*9*8*7*6*5*4*3*2*1;
cout << stirlingsFormula << endl;
cout << stirlingsFormulaExp << endl;
cout << float(exactFact) << endl;
cout << exactFact;
cin.get();
}

as you see the two are very different. I can work out the correct answer with paper and pencil and on a calculator.. but something i just cannot seem to figure out how to store 14! properly, which data type to use to display it either as a decimal value, integer or in scientific notation! 

Thanks for the help so far!!!!!!!!!!!!!!!!!

Using a long long or (long) doesn't change the result. it still is incorrect.

I've tried float, double, long double, long, and long long.

This post has been edited by jmoore_20200: 06 February 2011 - 02:50 PM

[CTphpnwb]

Here's what I'm talking about:

#include <iostream>
using namespace std;
long factorial(long);
long recurse_fact(long);

int main (int argc, char * const argv[]) {
	long N = 15;
	cout << N<<"! = " << factorial(N) << endl;
	cout << N<<"! = " << recurse_fact(N) << endl;
	long numberTest = (long)14*13*12*11*10*9*8*7*6*5*4*3*2*1;
	cout << numberTest << endl;
	cout << INT_MAX << "     " << LONG_MAX << endl;
    return 0;
}

long factorial(long x)
{
	long fact = x;
	for (long i = x-1; i > 1; i--) {
		fact *= i;
	}
	return fact;
}

long recurse_fact(long x)
{
	if (x > 2) {
		return x * recurse_fact(x-1);
	} else {
		return 2;
	}
}

and my output:

15! = 1307674368000
15! = 1307674368000
87178291200
2147483647     9223372036854775807