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C Exercises: Find the largest palindrome made from the product of two 3-digit numbers

C Programming Practice: Exercise-20 with Solution

A palindrome is a word, number, phrase, or other sequence of characters which reads the same backward as forward, such as taco cat or madam or racecar or the number 10801. The largest palindrome made from the product of two 2-digit numbers is 9009 = 91 × 99.
Write a C programming to find the largest palindrome made from the product of two 3-digit numbers.

C Code:

#include <stdio.h>
static int test_palindromic(unsigned int n);
int main(void)
{
  unsigned int i, j, max = 0;
  for (i = 100; i <= 999; i++) {
    for (j = 100; j <= 999; j++) {
      unsigned int p = i*j;
      if (test_palindromic(p) && p > max) {
        max = p;
      }
    }
  }
  printf("%u\n", max);
  return 0;
}
int test_palindromic(unsigned int n)
{
  unsigned int reversed_num = 0, t = n;
  while (t) {
    reversed_num = 10*reversed_num + (t % 10);
    t /= 10;
  }
  return reversed_num == n;
}

Sample Output:

906609

Flowchart:

C Programming Flowchart: Find the largest palindrome made from the product of two 3-digit numbers.

C Programming Code Editor:

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What is the difficulty level of this exercise?

C Programming: Tips of the Day

C Programming - Why do all the C files written by my lecturer start with a single # on the first line?

In the very early days of pre-standardised C, if you wanted to invoke the preprocessor, then you had to write a # as the first thing in the first line of a source file. Writing only a # at the top of the file affords flexibility in the placement of the other preprocessor directives.

From an original C draft by the great Dennis Ritchie himself:

12. Compiler control lines

[...] In order to cause [the] preprocessor to be invoked, it is necessary that the very first line of the program begin with #. Since null lines are ignored by the preprocessor, this line need contain no other information.

That document makes for great reading (and allowed me to jump on this question like a mad cat).

I suspect it's the lecturer simply being sentimental - it hasn't been required certainly since ANSI C.

Ref : https://bit.ly/2Mb8OVZ