Q: Getting invalid operand to binary error in C Mandelbrot Program I'm supposed to write a program in C to create the Mandelbrot. I have specific instructions to follow and I'm pretty sure I have followed them correctly. The question I have is that when I try to compile my code with gcc (using the make command) I get an error which says:
mason> make
gcc -c main.c
main.c: In function 'main':
main.c:13:40: error: invalid operands to binary < (have 'complex_t' and 'int')

I have been having a lot of issues with my complex_t in order to make it a double for the Mandelbrot to print. If you could please see if YOU can find the error that I keep missing. Here is my code for all my files:
main.c (where the error is):
#include <stdio.h>
#include "complex.h"
#include "mandelbrot.h"

int main (void)

    {
          complex_t c;
             for (c.imag = -1.2; c.imag < 2.8; c.imag+= 0.05)        {
                 for (c.real = -2.1; c.real < -0.66; c.real+= 0.032)
                 {
                    if (abs_complex(mandelbrot(15,c)) > 100)
                    {
                        printf("-");
                    }
                    else
                        printf("#");
                 }
                 printf("\n");       }  
          return (0); }

mandelbrot.c:
#include "complex.h"
#include "mandelbrot.h"

complex_t mandelbrot(int n, complex_t c)
{

complex_t m = {100,100};
complex_t tmp;

    if (n == 0)
    {
        return c;
    }

    else

    if (abs_complex(mandelbrot(n-1,c) > 1000))
    {
        return m;
    }

    else
    {

    tmp = mandelbrot(n-1,c);        
    tmp = cmult(tmp, tmp);
    tmp = cadd(tmp,c);

    }
        return tmp;
}

mandelbrot.h:
complex_t mandelbrot(int n, complex_t c);

complex.c:
#include <stdio.h>
#include <math.h>
#include "complex.h"


/*
 *  Complex number input function returns standard scanning error code
 *    1 => valid scan, 0 => error, negative EOF value => end of file
 */
int
scan_complex(complex_t *c) /* output - address of complex variable to 
       fill     */
{
      int status;

      status = scanf("%lf%lf", &c->real, &c->imag);
      if (status == 2)
            status = 1;
      else if (status != EOF)
            status = 0;

      return (status);
}

/*
 *  Complex output function displays value as (a + bi) or (a - bi),
 *  dropping a or b if they round to 0 unless both round to 0
 */
void
print_complex(complex_t c) /* input - complex number to display   */
{
      double a, b;
      char   sign;

      a = c.real;
      b = c.imag;

      printf("(");

      if (fabs(a) < .005  &&  fabs(b) < .005) {
            printf("%.2f", 0.0);
      } else if (fabs(b) < .005) {
            printf("%.2f", a);
      } else if (fabs(a) < .005) {
            printf("%.2fi", b);
      } else {
            if (b < 0)
                  sign = '-';
            else
                  sign = '+';
            printf("%.2f %c %.2fi", a, sign, fabs(b));
      }

      printf(")");
}

/*
 *  Returns sum of complex values c1 and c2
 */
complex_t
add_complex(complex_t c1, complex_t c2) /* input - values to add    */
{
      complex_t csum;

      csum.real = c1.real + c2.real;
      csum.imag = c1.imag + c2.imag;

      return (csum);
}

/*
 *  Returns difference c1 - c2
 */
complex_t
subtract_complex(complex_t c1, complex_t c2) /* input parameters    */
{
      complex_t cdiff;
      cdiff.real = c1.real - c2.real;
      cdiff.imag = c1.imag - c2.imag;

      return (cdiff);
}

/*  ** Stub **
 *  Returns product of complex values c1 and c2
 */
complex_t
multiply_complex(complex_t c1, complex_t c2) /* input parameters    */
{
      complex_t cmul;
      double a, b, c, d;
      a = c1.real;
      b = c1.imag;
      c = c2.real;
      d = c2.imag;

      if (( b > 0 && d < 0) || (b < 0 && d > 0))
      {
          cmul.real - (a*c) + (fabs(b)*fabs(d));
          cmul.imag = (a*d) + (b*c);
      }
      else if (( b>0 && d>0) || (b<0 && d<0))
      {
      cmul.real = (a*c) - (b*d);
      cmul.imag = (a*d) + (b*c);
  }
      return (cmul);
}

/*  ** Stub **
 *  Returns quotient of complex values (c1 / c2)
 */
complex_t
divide_complex(complex_t c1, complex_t c2) /* input parameters     */
{
      complex_t cdiv;
      double a, b, c, d;
      a = c1.real;
      b = c1.imag;
      c = c2.real;
      d = c2.imag;

      if ( b > 0 && d < 0)
      {
          cdiv.real = (a*c) - (fabs(b)*fabs(d)) / ((c*c) + (d*d));
          cdiv.imag = (a*d) + (b*c) / ((c*c) + (d*d));
      }
      else if ( b>0 && d>0)
      {
          cdiv.real = (a*c) - (fabs(b)*fabs(d)) / ((c*c) + (d*d));
          cdiv.imag = ((-1*a*d) + (b*c)) / ((c*c) + (d*d));
  }
      else if (b<0 && d<0)
  {
         cdiv.real = (a*c) + (fabs(b)*fabs(d)) / ((c*c) + (d*d));
         cdiv.imag = ((-1*a*d) + (b*c)) / ((c*c) + (d*d));
  }
  else if (b<0 && d<0)
  {
        cdiv.real = (a*c) + (fabs(b)*fabs(d)) / ((c*c) + (d*d));
        cdiv.imag = ((a*fabs(d)) + (b*c)) / ((c*c) + (d*d));
  }
      return (cdiv);
}
/*
 *  Returns absolute value of complex number c
 */
double
abs_complex(complex_t c) /* input parameter                        */
{
      complex_t cabs;

      cabs.real = sqrt(c.real * c.real + c.imag * c.imag);

      return (cabs.real);
}

complex.h:
typedef struct {
      double real, imag;
} complex_t;

int scan_complex(complex_t *c);
void print_complex(complex_t c);
complex_t add_complex(complex_t c1, complex_t c2);
complex_t subtract_complex(complex_t c1, complex_t c2);
complex_t multiply_complex(complex_t c1, complex_t c2);
complex_t divide_complex(complex_t c1, complex_t c2);
complex_t abs_complex(complex_t c);


A: Your header complex.h declares:
complex_t abs_complex(complex_t c);

Your implementation complex.c defines:
double abs_complex(complex_t c) {

So what's happening is that your code won't compile because the abs_complex in the header file is saying that it returns a complex_t and not a double.
Presumably complex.c also doesn't compile because of the mismatched definition/declaration.

A: You have a line:
if (abs_complex(mandelbrot(15,c)) > 100)

You are trying to compare an int with a complex_t, and that simply isn't allowed.  You'll need to rethink your logic.  You can't simply compare complex numbers for > (or < or …).  You will either need to convert the complex result from abs_complex() into a scalar (double or int) or convert the 100 into a complex_t and call a custom comparison function.
There's a similar error in mandelbrot.c.
