Is it prime? Logic error

This is a discussion on Is it prime? Logic error within the C++ Programming forums, part of the General Programming Boards category; I've got a logic error in here and I am having trouble figuring it out. Can someone please point out ...

  1. #1
    Registered User Kudose's Avatar
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    Is it prime? Logic error

    I've got a logic error in here and I am having trouble figuring it out.

    Can someone please point out the obvious as I cannot see it.

    Code:
    #include <iostream>
    #include <math.h>
    using namespace std;
    
    int main(){
    
        int i, n, squirt;
        bool is_prime = true;
    
        cout << "Enter a number: ";
        cin >> n;
    
        squirt = sqrt(static_cast<double>(n));
    
        for(i = 2; i <= squirt; i++){
            if(i % squirt == 0){
                is_prime = false;
                break;
            }
        }
    
        cout << n << " is ";
        if(!is_prime)
            cout << "not ";
        cout << "a prime number.";
    
        cout << endl;
    
        return 0;
    }
    TIA!
    IDE + Complier: Code::Blocks w/ GCC
    PC: AMD Phenom x4 2.2Ghz w/ 8G RAM
    OSes: CentOS 6.2 x64 & Win 7 Ultimate x64

  2. #2
    - - - - - - - - oogabooga's Avatar
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    Look carefully at line 16. It's completely out-to-lunch.

  3. #3
    Registered User Kudose's Avatar
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    Quote Originally Posted by oogabooga View Post
    Look carefully at line 16.
    Thanks!

    Quote Originally Posted by oogabooga View Post
    It's completely out-to-lunch.
    Not sure what this means.

    Referring to previously working code, I fixed it with this:

    Code:
    #include <iostream>
    #include <math.h>
    using namespace std;
    
    int main(){
    
        int i, n, squirt;
        bool is_prime = true;
    
        cout << "Enter a number: ";
        cin >> n;
    
        squirt = sqrt(static_cast<double>(n));
    
        for(i = 2; i <= squirt; i++){
            if(n % squirt == 0){
                is_prime = false;
                break;
            }
        }
    
        cout << n << " is ";
        if(!is_prime)
            cout << "not ";
        cout << "a prime number.";
    
        cout << endl;
    
        return 0;
    }
    IDE + Complier: Code::Blocks w/ GCC
    PC: AMD Phenom x4 2.2Ghz w/ 8G RAM
    OSes: CentOS 6.2 x64 & Win 7 Ultimate x64

  4. #4
    - - - - - - - - oogabooga's Avatar
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    Line 16 is still wrong.

    out to lunch, Slang . not paying attention or tending to business; negligent: You must have been out to lunch when you wrote that weird report.

  5. #5
    Registered User Kudose's Avatar
    Join Date
    Jun 2006
    Posts
    92
    Quote Originally Posted by oogabooga View Post
    Line 16 is still wrong.
    Hard to break it, but I did.

    This doesn't seem to break:

    Code:
    #include <iostream>
    #include <math.h>
    using namespace std;
    
    int main(){
    
        int i, n, squirt;
        bool is_prime = true;
    
        cout << "Enter a number: ";
        cin >> n;
    
        squirt = sqrt(static_cast<double>(n));
    
        for(i = 2; i <= squirt; i++){
            if(n % i == 0){
                is_prime = false;
                break;
            }
        }
    
        cout << n << " is ";
        if(!is_prime)
            cout << "not ";
        cout << "a prime number.";
    
        cout << endl;
    
        return 0;
    }
    Thanks for your assistance.
    IDE + Complier: Code::Blocks w/ GCC
    PC: AMD Phenom x4 2.2Ghz w/ 8G RAM
    OSes: CentOS 6.2 x64 & Win 7 Ultimate x64

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