Problems with Palindromes in C

Question

I've written some code in C to try adn find whether or not a number is a Palindrome. The rule is that two 3 digit numbers have to be multiplied together and you have to find the highest palindrome. the answer should be 906609 but my code only gets to 580085.

the code:

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

/* Intialise */
void CalcPalin();
int CheckPalin(int number);

/* Functions */
void CalcPalin()
{
    int result = 0;
    int palin = 0;
    int FNumber = 0;
    int FNumber2 = 0;

    int number = 99;
    int number2 = 100;

    while(number2 < 1000)
    {
        number += 1;

        /*times together - calc result*/
        result = number * number2;

        if(CheckPalin(result) == 1)
        {
            palin = result;
            FNumber = number;
            FNumber2 = number2;
        }

        if(number == 999)
        {
            number = 99;
            number2 += 1;
        }
    }
    printf(" Result = %d, by Multiplying [%d] and [%d]", palin, FNumber, FNumber2 );
}

int CheckPalin(int number)
{
    int checknum, checknum2 = 0;

    checknum = number;
    while(checknum)
    {
        checknum2 = checknum2 * 10 + checknum % 10;
        checknum /= 10;
    }

    if( number == checknum2)
        return 1;
    else
        return 0;
}

int main( void)
{
    CalcPalin();
    return EXIT_SUCCESS;
}

Im pretty sure its a stupid answer and im over looking something simple but i cant seem to find it. Any help would be great

its not homework, i thought it would be interesting to get used to C with these types of questions. — SD1990, Mar 05 '12 at 10:37
For readability, consider rewriting the main loop with for loops instead: `for(number2=100; number2 < 1000; number2++) for(number1=99; number1 < 1000; number1++)`. — Lundin, Mar 05 '12 at 10:51
i initially thought of that, but then my brain took me another way so i followed that, Im going to rework my code tht way thought thanks — SD1990, Mar 05 '12 at 12:33

score 4 · Accepted Answer · answered Mar 05 '12 at 10:40

4

You have not tested whether the current result is higher than one old result. Add this check.

// test new result is higher than old palin before setting this as palin
if(CheckPalin(result) == 1 && palin < result)

answered Mar 05 '12 at 10:40

taskinoor

45,586
12
116
142

thanks that worked prefectly, ill give you teh answer in 4 minutes when it allows me too... – SD1990 Mar 05 '12 at 10:43

score 0 · Answer 2 · answered Mar 05 '12 at 10:40

Your algorithm print :

 Result = 580085, by Multiplying [583] and [995]

It seems that you should find a way to increment more the 1st number. There is many possibility between 583 and 999 in order to get to 906609.

EDIT : In fact, you are looking for 993 * 913 = 906609.

Problems with Palindromes in C

2 Answers2