Running program until values reach ULLONG_MAX

This is a discussion on Running program until values reach ULLONG_MAX within the C Programming forums, part of the General Programming Boards category; Greetings. I am writing a program that is supposed to keep trying find a palindrome by summing up number and ...

  1. #1
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    Running program until values reach ULLONG_MAX

    Greetings. I am writing a program that is supposed to keep trying find a palindrome by summing up number and it's reversal (123 + 321) until it either finds one, or reaches limit of unsigned long long int.
    For small numbers, it seems to be working.
    However I have been told it is not proven possible that one can find palindrome with this algorithm for number 196 yet my program finds one, witch implies an error.

    Code:
    #include <limits.h>
    const unsigned long long int a = ULLONG_MAX;
    I tried this and then checking if the numbers I am working with are still less than a, to no avail. Could you please give me a hint? Thank you!

  2. #2
    and the hat of wrongness Salem's Avatar
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    You want us to give you a hint, based on one line of code?

    Code:
    $ cat foo.c
    #include<stdio.h>
    #include<string.h>
    #include<limits.h>
    void foo ( ) {
        unsigned char i = 0;
        while ( i <= UCHAR_MAX ) i++;    
    }
    
    int main()
    {
        return 0;
    }
    $ gcc -Wall -Wextra foo.c
    foo.c: In function ‘foo’:
    foo.c:6:5: warning: comparison is always true due to limited range of data type [-Wtype-limits]
    > However I have been told it is not proven possible that one can find palindrome with this algorithm for number 196 yet my program finds one, witch implies an error.
    Are you using the right printf format (for example)?
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
    I support http://www.ukip.org/ as the first necessary step to a free Europe.

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    Sorry about that, but I am trying to avoid someone saying I let other people do my homework so I hoped this will be enough

    Input number:
    196
    Computed palindrome: 16623445888854432661 (iterations: 226)

    wich is a contradiction to Jason Doucette - World Records - 196 Palindrome Quest, Most Delayed Palindromic Number

  4. #4
    and the hat of wrongness Salem's Avatar
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    Well the generated numbers here -> Jason Doucette - 196 Palindrome Quest
    FAR FAR exceed what can be stored in an unsigned long long (on my machine, it's only just 20 decimal digits)

    You just stumbled into an answer through ignoring numeric overflow.

    This program has continued the above sequence to 32,000,000 iterations. This resulted in a number over 13,000,000 digits long, which has yet to produce a palindrome. It took just over 283 days of calculations on a 266 MHz and a 400 MHz desktop machine, running at separate times.
    You need to implement some kind of bignum library (like GMP).

    Not to mention a hell of a lot of CPU power, and a lot of patience.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
    I support http://www.ukip.org/ as the first necessary step to a free Europe.

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    Run this program (or add the according printf()-call inside your loop) and you will notice that after the 40th iteration you have an overflow and all following calculations are wrong:
    Code:
    #include <stdio.h>
    
    unsigned long long palindrome(unsigned long long n);
    
    int main(void)
    {
        int i;
        unsigned long long number = 196;
    
        for (i = 0; i < 50; i++)
        {
            printf("%3d %22llu\n", i, number);
            number += palindrome(number);
        }
        return 0;
    }
    
    unsigned long long palindrome(unsigned long long n)
    {
        unsigned long long p = 0;
        while (n > 0)
        {
            p *= 10;
            p += n % 10;
            n /= 10;
        }
        return p;
    }
    Bye, Andreas

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    Well thank you for trying to help but I want the program to stop the moment it reaches limit of unsigned long long int and say 'Couldn't find the palindrome.'; my
    Code:
    if ((number >= ULLONG_MAX)||(reversed >= ULLONG_MAX)) {printf("Couldn't find the palindrome..\n"); return(1);}
    does not work.

  7. #7
    and the hat of wrongness Salem's Avatar
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    > if ((number >= ULLONG_MAX)||(reversed >= ULLONG_MAX))
    Think about this.

    Say MAX is 10.
    and you want to detect whether 6+5 overflows.
    Doing 6+5 does no good, since it wraps around and you end up comparing 1 with 10.

    So instead of
    if ( 6 + 5 >= 10 )
    you need to write
    if ( 6 >= 10 - 5 )
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
    I support http://www.ukip.org/ as the first necessary step to a free Europe.

  8. #8
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    If the sum of two unsigned integers is less than one or both of the original numbers, the result overflowed.

    It other words: c = a + b overflows using unsigned integers of the same type, if and only if (c < a) || (c < b).

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