Thread: Quadratic equation with input from another file - help please

Hey I am new to the whole C programming and I am trying to write simple code that computes the roots of the quadratic equation and prints the coefficients and the roots into a seperate text file, roots.txt. The coefficients are found in a text file input_equations.txt. Here is my text file

1 -3 2
1 0 -4.84
1 0 1

And here is my program, I am getting wacky numbers in the roots.txt file and I cant seem to work it out.

Code:

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

int compute_roots(double a, double b, double c, double *root1, double *root2)
{
     double disc;
     disc = pow(b,2) - (4.0*a*c);
	
     if (disc < 0)
     {
	return (0);
     }
     
     if (disc >= 0)
     {
	*root1 = (-1.0*b+sqrt(disc))/(2.0*a);
	*root2 = (-1.0*b-sqrt(disc))/(2.0*a);
	return (1);
      }
}

int main()
{
     double a,b,c,root1,root2;
	
     FILE *inp, outp;
     inp = fopen("input_equations.txt", "r");
     outp = fopen("roots.txt", "w");
	
	
	
     while (fscanf(inp, "%f %f %f", &a, &b, &c) != EOF)	
     {
	fprintf(outp, "%f %f %f ", a, b, c);
	if(compute_roots(a, b, c, &root1, &root2) == 1)
	                fprintf(outp, "root1 = %g root2 = %g\n", root1, root2);
	else
		fprintf(outp, "The roots are not real numbers.\n");
     }
	
     fclose(inp);
     fclose(outp);
	
     return(0);
}

It compiles fine, but the output into the roots.txt file is not giving the right coefficients. Any help would be great, I just can't seem to find the bug! Thanks alot!

mike_g is on the right track, but he took the left turn at Albuquerque. The problem is not with printf, but with scanf: in order to read in values to a double variable, you need to use the %lf specifier.

well i changed the %f to %lf (i cant believe i did that by the way) but the numbers are still coming out weird. I think maybe because it is scanning for doubles, but some of the numbers are ints. Could that be a problem? And how would I fix it so that it wouldnt matter what kind of number I am scanning for. And now when the roots are printed out into the output file, numbers are no longer printed but rather NaN or Inf is printed. Im pretty confused about it. I appreciate any help.

Hey i figured it out tabstop and mike_g. Thanks for all the help!

Originally Posted by mrusnak

well i changed the %f to %lf (i cant believe i did that by the way) but the numbers are still coming out weird. I think maybe because it is scanning for doubles, but some of the numbers are ints. Could that be a problem? And how would I fix it so that it wouldnt matter what kind of number I am scanning for. And now when the roots are printed out into the output file, numbers are no longer printed but rather NaN or Inf is printed. Im pretty confused about it. I appreciate any help.

Lack of decimal points doesn't matter. You have to scan for whatever number your variable is supposed to hold. My output (after changing outp from a FILE to a FILE *):

1.000000 -3.000000 2.000000 root1 = 2 root2 = 1
1.000000 0.000000 -4.840000 root1 = 2.2 root2 = -2.2
1.000000 0.000000 1.000000 The roots are not real numbers.

So what was the final working code? I fail to see the errors.

mrusnak you still around?