Thread: How To Fix This Issue In A Program For Finding Prime Numbers

  1. #1
    Registered User
    Join Date
    Jan 2012

    How To Fix This Issue In A Program For Finding Prime Numbers

    This is a program testing for prime numbers in a specific range,using an .in file and writing to an .out one.
    My question is why after a certain range of number e.g.when i set 234 to 234353 it outputs giberish?Also any ideas to NOT have a space character after the last number would be apreciated because my method obviously is wrong for some reason..
    #include <iostream>
    #include <stdio.h>
    #include <math.h>
    using namespace std;
    bool ISPRIME(int K)
          if (K == 2)
                return false;
                 if (K % 2 == 0)
                return false;
                if (K%3==0&&K!=3)
                return false;
                if (K%5==0&&K!=5)
                return false;
                if (K%7==0&&K!=7)
                return false;
          for (int d = 11; d <= (int)sqrt((double)K); d=d+2)
                if (K % d == 0)
                      return false;
          return true;
    int main(){
    int N;
    int M;
    FILE *read;
    read = fopen ( "" , "r" ) ;
    fscanf ( read , "%d" , &N ) ;
    fscanf ( read , "%d" , &M ) ;
    int K = ( N < M ) ? (N+1) : (M+1) ;
    int d = ( N < M ) ? (M-1) : (N-1) ;
    FILE *write;
    while (K<=d)
                 if (ISPRIME(K)==true&&K==d) 
                  else if(ISPRIME(K))
    fprintf(write," ");}
    return 0;

  2. #2
    Registered User
    Join Date
    Jun 2005
    Your compiler probably supports 16-bit integers. The maximum value of a signed 16-bit integer is about 32767. If you attempt to work with larger values than an int can hold, the result is undefined behaviour.

    I'll leave the exercise of avoiding a space after the last number as an exercise. You will learn more by puzzling out a solution yourself, than you would by being given a solution.

    Unrelated to your questions: 2 is prime, but your function does not detect that. There are better ways of computing the square root of an integer than sqrt(). If you do a search for similar topics, you will find better approaches.
    Right 98% of the time, and don't care about the other 3%.

    If I seem grumpy or unhelpful in reply to you, or tell you you need to demonstrate more effort before you can expect help, it is likely you deserve it. Suck it up, Buttercup, and read this, this, and this before posting again.

  3. #3
    Join Date
    Oct 2007
    Inside my computer
    Unrelated to your problem, but important. Your indentation is horrible. You need to fix that. Also don't use fopen, fscanf, fclose, fgets, etc in C++. Use streams (see tutorial on site if you aren't familiar with them).
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

  4. #4
    Registered User kaspari22's Avatar
    Join Date
    Jul 2008
    Czech Republic, Doubravice
    Once I solve it like this:

    #include <cstdlib>
    #include <iostream>
    using namespace std;
    int main()
        for(int i = 2; i <= 100 ; i++)
        int b = 1;
        for(int j = 2;j<=i;j++)
                else if(i % j == 0)
                b =0;
                cout<< i << " ";
                cout << endl;
        return 0;
    So why not only this:

     for(int i = 253; i <= 234353 ; i++)

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