Thread: Calculating an integral

i have to write a program to calculate the area under cos^2(x) but no matter what i do i keep getting 0

I've tried simplifying it down to this

Code:

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


int main(int argc, char* argv[])
 { 
  FILE         *input;
  double 	   b;
  const char    inp_fn[]="px270prog3a.dat";
  int            i;
  double       x[10000], f[10000], integral;
  
  input = fopen(inp_fn, "r");

  b=1;

	for(i=0; i <10000; i++)
		{
			fscanf(input, "%f %f", &x[i], &f[i]); 
				
		}

   integral=0;

	for(i=0; x[i]<=b; i++)
		{
		integral += f[i-1]*(x[i]-x[i-1]);
			
		}
		
		printf("Area = %.4lf \n", integral);
			
return(0);
}

but it's still always 0.

Any help?

Take a look at your last for() loop... Is x[i] always going to be less than b, which is defined as 1 earlier on?

If x[i] is a positive number, the loop will exit leaving you with 0 in your integral variable.

Code:

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


int main(int argc, char* argv[])
 { 
  FILE         *input;
  const char    inp_fn[]="cos.dat";
  int            i;
  double       x[10], f[10], integral;
  
  input = fopen(inp_fn, "r");


	for(i=0; i <10; i++)
		{
			fscanf(input, "%f" "%f", &x[i], &f[i]); 
			printf(" x= %lf", x);
			printf(" f= %lf", f);
		}
		
printf("Here in loop, counter=%d\n", i ); 

   integral=0;

	for(i=1; i<10; i++)
		{
		integral += f[i-1]*(x[i]-x[i-1]);
			printf("Here in loop, counter=%d\n", i ); 
			printf("Area = %.4lf \n", integral);
		}
				
printf("Here in loop, counter=%d\n", i ); 
		printf("Area = %.4lf \n", integral);
			
return(0);
}

i've changed it to this. and it's printing out x and f as huge numbers which are different in the file but im not sure what's wrong with the fscanf stuff?

ok, i changed x,f to x[i] and f[i] and it's giving me smaller but still not correct numbers