debug output

(gdb) display zigzag_array 
1: zigzag_array = {"F\000A\000N\000\000\000\000\000", "OBROE\000\000\000\000\000", "O\000J\002S\000@\003\000\000@"}

zigzag.c

#include <stdio.h>
#include <stdbool.h>
#include <string.h>

char * convert(char * input_str, int num_rows) {
    int str_i = 0;
    int row_i = 0;
    int col_i = 0;
    int str_len = strlen(input_str);

    char zigzag_array[num_rows][str_len];

    bool down = true;
    
    // form the zigzag 
    while (str_i < strlen(input_str)) {
        char current_char = input_str[str_i];
        zigzag_array[row_i][col_i] = current_char;

        // change directions if we hit the bottom row
        if (row_i == num_rows-1 && down)
            down = false;
        // change directions if we hit the top row
        if (row_i == 0 && !down)
            down = true;
        
        if (down)
            row_i++;
        else {
            col_i++;
            row_i--;
        }
        
        str_i++;
    }

    char result_str[str_len];
    int result_i = 0;

    for (int i = 0; i < num_rows; i++) {
        for (int j = 0; j < str_len; j++) {
            char zig_char = zigzag_array[i][j];
            if (zig_char) {
                result_str[result_i] = zig_char;
                result_i++;
            }
        }
    }

}

int main() {
    char input_str[] = "FOOBARJONES";
    int num_rows = 3;

    convert(input_str, num_rows);

    return 0;
}