Two different answers for same expression in C-CodePudding

I have an expression which does the same calculation. When I try to do the whole calculation in a single expression and store it in variable "a", the expression calculates the answer as 0. When I divide the equations in two different parts and then calculate it, it gives the answer -0.332087. Obviously, -0.332087 is the correct answer. Can anybody explain why is this program misbehaving like this?

#include<stdio.h>

void main(){
    double a, b, c;
    int n=0, sumY=0, sumX=0, sumX2=0, sumY2=0, sumXY=0;
    n = 85;
    sumX = 4276;
    sumY = 15907;
    sumX2 = 288130;
    sumY2 = 3379721;
    sumXY = 775966;
    a = ((n*sumXY) - (sumX*sumY)) / ((n*sumX2)-(sumX*sumX));
    b = ((n*sumXY) - (sumX*sumY));
    c = ((n*sumX2) - (sumX*sumX));
    printf("%lf\n", a);
    printf("%lf\n", b/c);
}

Output:
0.000000
-0.332097

CodePudding user response：

In first equation i.e. a = ((nsumXY) - (sumXsumY)) / ((nsumX2)-(sumXsumX)) both numerator and denominator will give integer results and the value stored in a will also be integer as integer/integer is integer. In second and third expression as you are solving them individually both b and c will be stored in double and double/double will result in a double i.e. a decimal value. This problem can be solved by using type casting.

CodePudding user response：

In your program

a = ((n*sumXY) - (sumX*sumY)) / ((n*sumX2)-(sumX*sumX));

all the variables in the right hand side are of type int, so it will produce a result of type int. The true answer -0.332097 is not a int value, so it will be converted to a valid int value, namely 0. And this 0 is assigned to variable a.

But when you do

b = ((n*sumXY) - (sumX*sumY));
c = ((n*sumX2) - (sumX*sumX));
printf("%lf\n", b/c);

The variable b and c are of type double, so the expression b/c produce a double typed value and the true answer -0.332097 is a valid double value. Thus this part of your code give a right result.

CodePudding user response：

Change all your INTEGERS to DOUBLES. That should solve the problem

CodePudding user response：

The type of 1st expression is int whereas in 2nd expression it is double.