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Program produces different results on different computers

This is a discussion on Program produces different results on different computers within the C Programming forums, part of the General Programming Boards category; I was doing problem 10 on project euler Project Euler The following code produces two different answers on different systems: ...

  1. #1
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    Program produces different results on different computers

    I was doing problem 10 on project euler Project Euler

    The following code produces two different answers on different systems:
    Code:
    #define INPUT 2000000
    Bool isPrime(const int number)
    {
      int i, sqr = sqrt(n);  
      if(n%2==0) return 0;
      for(i=3;i<=sqr; i+=2) 
      {
           if (n%i==0) return 0;
       }
           return 1;
    }
    
    
    int main()
    {
       int i;
       int sum = 0;
       for( i=0; i <= INPUT; ++i )
       {
          if( isPrime( i ) )
          {
             sum = sum + i;
          }
    
    
       }
       printf( "The sum of prime number less than %d is: %d.\n", INPUT, sum );
    }
    Computer A:
    Linux 3.0.0-30-generic #47-Ubuntu SMP Wed Jan 2 22:39:01 UTC 2013 i686 i686 i386 GNU/Linux
    gcc (Ubuntu/Linaro 4.6.1-9ubuntu3) 4.6.1
    Ubuntu 11.10


    Computer B:
    Linux 3.2.0-31-virtual #50-Ubuntu SMP Fri Sep 7 16:36:36 UTC 2012 x86_64 x86_64 x86_64 GNU/Linux
    gcc (Ubuntu/Linaro 4.6.3-1ubuntu5) 4.6.3
    Ubuntu 12.04

    Compiled by: gcc program.c -o run -lm

    Computer A generates this answer: 1179908154 (wrong)
    Computer B generates this answer: 142913828922 (right)

    I was hoping someone could explain why this is. My guess might be because they are running on different architectures. But I really don't know.

    Thank you

  2. #2
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    Your guess of different architectures is more or less correct. It looks like computer A is a 32-bit system (or at least it's running a 32-bit OS), so the maximum value that an int can store is around ~2,000,000,000 (~2 billion). Note, the correct answer is 142,913,828,922 (~142 billion), which wont fit in a regular int. You are seeing the effects of the summation "wrapping back around to 0". Computer B seems to be a 64-bit system, so a regular int can hold up to 9,223,372,036,854,775,807 (9 quadrillion), which is enough for the correct answer.

    You could do it on computer A if you use C's fixed-width integer type int64_t (make sure you #include <stdint.h>).

    If you ever want to make this more extensible, to sum primes beyond 2 million, you will need a big num library, like The GNU MP Bignum Library.
    Salem and AndiPersti like this.

  3. #3
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    Computer A:
    Linux 3.0.0-30-generic #47-Ubuntu SMP Wed Jan 2 22:39:01 UTC 2013 i686 i686 i386 GNU/Linux
    gcc (Ubuntu/Linaro 4.6.1-9ubuntu3) 4.6.1
    Ubuntu 11.10


    Computer B:
    Linux 3.2.0-31-virtual #50-Ubuntu SMP Fri Sep 7 16:36:36 UTC 2012 x86_64 x86_64 x86_64 GNU/Linux
    gcc (Ubuntu/Linaro 4.6.3-1ubuntu5) 4.6.3
    Ubuntu 12.04

    As anduril462 says, watch your machine architecture.
    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.
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    Thanks for the replies!

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