Prime detector

This is a discussion on Prime detector within the C++ Programming forums, part of the General Programming Boards category; Hello everyone, i'm new to c++. I'm currently trying to make a program that finds the first prime above 1 ...

  1. #1
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    Prime detector

    Hello everyone, i'm new to c++. I'm currently trying to make a program that finds the first prime above 1 billion, but i can't make it work. it just prints all numbers from 1 billon to infinity. This is my code:

    Code:
    #include <iostream>
    #include <math.h>
    using namespace std;
    
    int prime(int n);
    
    int main () {
        int i;
        
        for (i = 100000000;; i++) {
            if (prime(i))
               cout << "Dette er er det første primtal over 1 milliard: " << i << endl;   
               break;
        }
        cin.get();
        return 0;               
    
    }
    
    int prime(int n) {
        int i;
        double sqrt_of_n = sqrt(n);
        
        for (i = 2; i <= sqrt_of_n; i++) {
            if (n % i == 0)
            
               return false;
        }
        return true;
    }
    I hope you can help me!

  2. #2
    Cat without Hat CornedBee's Avatar
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    Hmm ... I'd expect this code to print nothing at all, actually. You're missing braces for the if, and the break is therefore always executed.
    All the buzzt!
    CornedBee

    "There is not now, nor has there ever been, nor will there ever be, any programming language in which it is the least bit difficult to write bad code."
    - Flon's Law

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    Quote Originally Posted by CornedBee View Post
    Hmm ... I'd expect this code to print nothing at all, actually. You're missing braces for the if, and the break is therefore always executed.
    What should i change to make it work? I started learning the language a couple of days ago, and i'm a bit unsure on the syntax still.

  4. #4
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    Code:
    #include <iostream>
    #include <math.h>
    using namespace std;
    
    int prime(int n);
    
    int main () {
        int i;
        
        for (i = 100000000;; i++) {
            if (prime(i))
            {
               cout << "Dette er er det f&#248;rste primtal over 1 milliard: " << i << endl;   
               break;
            }
        }
        cin.get();
        return 0;               
    
    }
    
    int prime(int n) {
        int i;
        double sqrt_of_n = sqrt(n);
        
        for (i = 2; i <= sqrt_of_n; i++) {
            if (n &#37; i == 0)
            
               return false;
        }
        return true;
    }

  5. #5
    Cat without Hat CornedBee's Avatar
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    Code:
            if (prime(i)) {
               cout << "Dette er er det f&#248;rste primtal over 1 milliard: " << i << endl;   
               break;
    }
    This is the minimum. Though I still don't know why it would print all numbers.
    All the buzzt!
    CornedBee

    "There is not now, nor has there ever been, nor will there ever be, any programming language in which it is the least bit difficult to write bad code."
    - Flon's Law

  6. #6
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    Quote Originally Posted by CornedBee View Post
    Code:
            if (prime(i)) {
               cout << "Dette er er det første primtal over 1 milliard: " << i << endl;   
               break;
    }
    This is the minimum. Though I still don't know why it would print all numbers.
    I'm sorry the code i posted before wasn't the one that printed all numbers, that was an earlier version of the program. As you said, this program did absolutely nothing. Thanks for the help both of you, i'll see if it works now.

  7. #7
    Banned master5001's Avatar
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    It will work.... but like CornedBee said, you may not end up with exactly what you want. That is not how the algorithm you are trying to encorporate even works.... You have merged two ways of determining primality into one half-assed one.

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    >>You have merged two ways of determining primality into one half-assed one.
    Slightly harsh wording don't you think lol?

  9. #9
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    Quote Originally Posted by master5001 View Post
    It will work.... but like CornedBee said, you may not end up with exactly what you want. That is not how the algorithm you are trying to encorporate even works.... You have merged two ways of determining primality into one half-assed one.
    If it won't do what i want it to, why? And what is wrong in how i find a prime? I'd like criticism, if my mistakes aren't corrected i won't get better at c++. Would you please continue to help me by explaining?

  10. #10
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    double sqrt_of_n = sqrt(n);
    I recognize this line of code anywhere. C++ Without Fear!!!

  11. #11
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    Quote Originally Posted by lruc View Post
    double sqrt_of_n = sqrt(n);
    I recognize this line of code anywhere. C++ Without Fear!!!
    You sound like that's the spawn of the devil

    And yes, that's the book i'm learning from.

  12. #12
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    Quote Originally Posted by Elswyyr View Post
    You sound like that's the spawn of the devil

    And yes, that's the book i'm learning from.
    No way. I loved that book! Good choice. Just realize the fact that the book doesn't cover everything so to get information on things like templates and stl, you will need a more advanced book.

  13. #13
    Malum in se abachler's Avatar
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    Quote Originally Posted by Raigne View Post
    >>You have merged two ways of determining primality into one half-assed one.
    Slightly harsh wording don't you think lol?
    Harsh, and wrong. That method will work. He is just checking all the numbers below the square root and if there is no remainder after devision the number is obviously composite. If it makes it through every number then it is obviously prime.

    There are far mroe highly optimized methods in use, but for only finding one prime number that one probably executes ina few ms so its fine.
    Until you can build a working general purpose reprogrammable computer out of basic components from radio shack, you are not fit to call yourself a programmer in my presence. This is cwhizard, signing off.

  14. #14
    Banned master5001's Avatar
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    If I am not mistaken does this algorithm not also produce pseudo primes as well? *shrugs* I don't care one way or another.

  15. #15
    Cat without Hat CornedBee's Avatar
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    Nope, no pseudo-primes. Only true primes.
    All the buzzt!
    CornedBee

    "There is not now, nor has there ever been, nor will there ever be, any programming language in which it is the least bit difficult to write bad code."
    - Flon's Law

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