Project Euler Palindrome #4 with C

Question

I found few posts regarding this problem using C. Most of the elements in my code work on their own but the iteration at the beginning is causing problems for some reason. First, I'm getting an "exited with non-zero status" error message. When I run the program with a smaller range for a and b, I don't get that message. I'm guessing there's a problem with the rev_array and for_array variables I created. I'm sure I'm doing something really dumb right here so I apologize in advance for that.

But when I use a smaller range for a and b (like 10 to 25), the program is still showing that all two-digit numbers (even 11, 22, 33, 44) are not the same forward and backward. I used printf to check for this.

I made a similar program that used fixed values for a and b instead of iterating over a range of values and it worked fine. But I couldn't figure out why this one isn't working.

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

int max;
int a;
int b;
int prod;
int m = 0;
int rev_array[10000];
int for_array[10000];
int c;
int d;
int same = 0;

int main(void)
{
  // iterate over all 3 digit numbers in lines 19-21
  for(a = 10; a <= 25; a++)
  {
    for(b = 10; b <= 25; b++)
    {
      max = 0;
      prod = a * b;
      /* thanks to Zach Scrivena for the following formula converting an integer to an array of integers posted on stackoverflow on February 5, 2009 under the subject "convert an integer number into an array"
      */
      int n = prod;
      while(n != 0)
      {
        rev_array[m] = n % 10;
        n /= 10;
        m++;
      }
      /* thanks to Jordan Lewis for the following int length formula posted to stackoverflow on June 18, 2010 in response to "Finding the length of an integer in C"
      */
      int length = floor(log10(abs(prod))) + 1;
      // create the forward array of the ints in prod 
      for(c = length - 1, d = 0; c >= 0; c--, d++)
      {
        for_array[d] = rev_array[c];
      }
      // compare the forward and reverse arrays to see if they match exactly
      for(int e = 0; e < length; e++)
      {
        if(for_array[e] != rev_array[e])
        {
          // if they don't match then set same equal to 1 for following step
          same = 1;
        }
      }
      /* if prod is greater than max and the forward and reverse arrays are identical, then replace max with prod
      */
      if(prod > max && same == 0)
      {
        max = prod;
      }
      // reset same and repeat the process
      same = 0;
    }
  }
  // print the final, greatest number that fits the preceding criteria
  printf("new max: %i \n", max);
  return 0;
}

Project Euler problem #4 is an entry level question after the first three "baby steps". The idea of these sites is to solve it yourself. That is why they are called "challenge sites". They are personal challenges. — Weather Vane, Aug 22 '17 at 21:57
You also don't need to calculate `length`, since `m` should contain the length after the `while` loop (but you need to reset it to `0` at the top). And all these variables should be local, and most of them (like `m`, `n`, `prod`) should be defined inside the inner loop, with a limited scope. `max` is the only one which needs to be preserved between iterations. — vgru, Aug 22 '17 at 22:43

score 0 · Accepted Answer · answered Aug 23 '17 at 15:30

Answers provided in comments:

You need to reset m to zero each time. – Johnny Mopp

You also don't need to calculate length, since m should contain the length after the while loop (but you need to reset it to 0 at the top). And all these variables should be local, and most of them (like m, n, prod) should be defined inside the inner loop, with a limited scope. max is the only one which needs to be preserved between iterations. – Groo

Project Euler Palindrome #4 with C

1 Answers1