Printing an entire character array after looping through every point in a rectangular area

Question

I'm using a double for-loop in order to check every point (coordinate pair) in a rectangular area from (-2.0, -1.12) to (0.47, 1.12) to see whether it belongs to the Mandelbrot set. If it does, I want to print a 1. Likewise, if it does not, I want to print a 0. The basic idea is to print, line by line, an array of characters that displays a simplified Mandelbrot set.

This is my main function:

#include <stdio.h>
#include "complex.h"
#include "mandelbrot.h"

#define STEP_X   0.06175
#define STEP_Y   0.07466                

int main(void){

    int i = 0;
    char arr[50];
    complex_t c, abs, max;
    max.real = 10000;
    max.imag = 0;

    for (c.imag = -1.12; c.imag <= 1.12; c.imag += STEP_Y){
            for (c.real = -2.0; c.real <= 0.47; c.real += STEP_X){
                    abs = abs_complex(mandelbrot(c,15));
                    if (abs.real < max.real){ 
                            arr[i] = 1;
                            i++;
                    } 
                    else{
                            arr[i] = 0;
                            i++;
                    }
            }
            printf("%s", arr);
            i = 0;  
    }
}                                         

The program compiles just fine, but does not produce an output. I know I must not be printing the array the right way, but for the life of me I can not figure out how to do it.

Any feedback, hints, or tips would be greatly appreciated.

Thanks in advance!

Answer 1

The problems you are having are two-fold. (1) you are copying decimal values to arr (e.g. 0 and 1) instead of ASCII characters ('0' and '1'). Decimal 0 and 1 are non-printable. Ironically decimal 0 is the nul-terminating character, so if if (abs.real >= max.real) for i == 0 arr holds the empty-string.

Second you call printf without having insured the final character is the nul-terminating character. (you can do this by default by initializing char arr[MAXC] = ""; and insuring your loop is limited to i + 1 < 50 && c.real <= 0.47 or you can simply affirmatively terminate arr with arr[i] = 0; before calling i = 0; (or move your declaration of i inside the first for loop and initialize).

This is untested (I don't have your local headers), but it looks like you intended:

#include <stdio.h>
#include "complex.h"
#include "mandelbrot.h"

#define MAXC     50
#define STEP_X   0.06175
#define STEP_Y   0.07466                

int main(void){

    complex_t c, abs, max;
    max.real = 10000;
    max.imag = 0;

    for (c.imag = -1.12; c.imag <= 1.12; c.imag += STEP_Y) {

        int i = 0;             /* declare/initialize i & arr here */
        char arr[MAXC] = "";   /* set to all zero */

        for (c.real = -2.0; 
            i + 1 < MAXC && c.real <= 0.47;  /* limit to 49 chars max */
            c.real += STEP_X) {

            abs = abs_complex (mandelbrot (c,15));

            if (abs.real < max.real) 
                arr[i++] = '1';   /* assign character '1' */
            else
                arr[i++] = '0';   /* assign character '0' */
        }
        arr[i] = 0;               /* nul-terminate line */
        printf ("%s\n", arr);     /* output line */
    }
    return 0;
}

Give it a try and let me know if you have further questions.