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.

> for(i=0; x[i]<=b; i++)
b is 1, how many times around this loop does it go?

Use a debugger to find out where the code goes. How many times does it execute each loop?

Practice on a file with only 10 lines, not 10000 lines.

Add some printf statements if you can't manage a debugger.
Eg.
printf("Here in loop, counter=%d\n", i );

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

All your %f's are screwed up in your printf/scanf calls (scanf in particular)

Code:

---

$ gcc -W -Wall -ansi -pedantic -O2 foo.c
foo.c: In function ‘main’:
foo.c:18: warning: format ‘%f’ expects type ‘float *’, but argument 3 has type ‘double *’
foo.c:18: warning: format ‘%f’ expects type ‘float *’, but argument 4 has type ‘double *’
foo.c:19: warning: ISO C90 does not support the ‘%lf’ gnu_printf format
foo.c:19: warning: format ‘%lf’ expects type ‘double’, but argument 2 has type ‘double *’
foo.c:20: warning: ISO C90 does not support the ‘%lf’ gnu_printf format
foo.c:20: warning: format ‘%lf’ expects type ‘double’, but argument 2 has type ‘double *’
foo.c:31: warning: ISO C90 does not support the ‘%lf’ gnu_printf format
foo.c:35: warning: ISO C90 does not support the ‘%lf’ gnu_printf format
foo.c:18: warning: ignoring return value of ‘fscanf’, declared with attribute warn_unused_result
foo.c:6: warning: unused parameter ‘argc’
foo.c:6: warning: unused parameter ‘argv’

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