Thread: How to fix a simple problem with division of numbers

Dear all,

I am writing a simple program that performs some statistical calculations. The code looks like:

Code:

#include <stdio.h>
#include <math.h>

int x, n;
float f_A;
float chi2 = 0;
int genotype[4];
float expected[4];


int main(void)
{

// code here

n = genotype[1]+genotype[2]+genotype[3];
f_A = ((2*genotype[1])+(genotype[2]))/(n*2);

// code here

return 0;
}

This code doesn't work well, because the variable f_A is not computed. In other words, the code fails to calculate correctly f_A.

If I, nevertheless, replace the formula

f_A = ((2*genotype[1])+(genotype[2]))/(n*2);


to

f_A = ((2*genotype[1])+(genotype[2]));
f_A = f_A/(n*2);

the code gives correct results.

Am I missing some very basic information about C programming?

Thank you for any tips.

cheers!

Tiago

You are performing integer division here:

f_A = ((2*genotype[1])+(genotype[2]))/(n*2);

You have to instruct the compiler that it's floating point division, by putting a cast operator
to one of the operands :

f_A = (float) ((2*genotype[1])+(genotype[2])) / (n*2);

Thank you very much! worked like a charm!

also make sure if n should = 0,to handle it appropriately.

I've crashed many of my programs with unforseen division by zero.

Should use double.

Code:

double x, n;
double f_A;
float chi2 = 0;
double genotype[4];
double expected[4];


int main(void)
{

// code here

n = genotype[1]+genotype[2]+genotype[3];
f_A = ((2*genotype[1])+(genotype[2]))/(n*2);

Otherwise with all this integer division you are very dependent on the order of multiplication, division.

The most common mistake is A*B/C . It is assumed that A*B comes before division by C - not guaranteed with Microsoft!!!

Sorry - forgot that n*2 should be n* (double)2!!!!

Originally Posted by johnggold

Sorry - forgot that n*2 should be n* (double)2!!!!

Wouldn't n*2.0 be easier? (Which is part of iMalc's point.)

The most common mistake is A*B/C . It is assumed that A*B comes before division by C - not guaranteed with Microsoft!!!

Why should it matter in this case?

Code:

A*(B/C)==(A*B)/C