Programming Forums - Prime Numbers

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This exercise took me donkey's years to do (I am quite new to this). I have a solution, but for some reason, '1' isn't picked up as a prime number.

The exercise is this (taken from Bruce Eckel's guide, chapter 3)

2. Write a program that uses two nested for loops and the modulus operator (%) to detect and print prime numbers.

This is my solution, with the numbers 1 - 100

int main(int nNumberofArgs, char* pszArgs[])

{

    int number = 0;

    

    for (int i = 0; i <= 100; i++)

    {

        int modulus = 1;

        int primecount = 0;

        

        for (int i = 0; i <= number; i++)

        {

                double answer = number % modulus;

                        if (answer == 0)

                        {

                          primecount++;

                        }  

                modulus++;

        }        

    

        if (primecount == 2)

        {

            cout << number << endl;

        } 

        number++;

    }

    

    system("PAUSE");

    return 0; 

}

Any tips on how to do this better would be much appreciated, as my code doesn't 'feel' too clean (the variable's and there positions in the program seem wrong). Can it be done with just for loops?

Also, is it normal to struggle with these exercises so early on? I am trying to read through the chapters and then attempt the tasks, but occasionally find certain tasks very difficult and time consuming, thus making me frustrated and slowing down the learning process. Is the best way to tackle this guide to work through all the exercises in each chapter?

Cheers,

'One' isn't normally included as a prime. There's another really recent thread regarding primes on the forum; last day or two. You might want to do a search ("prime" should make a nice keyword). My example there uses bitfields for nefarious reasons, but the principles are the same.

I dunno why you have so much code there..

int isprime(int num) {

    for(int a=2;a<=num;a++) { if(!(num%a)) { return false; } a++; }

    return true;

}

Anyway, 1 isn't counted as a prime number because it's not one. Prime numbers are numbers divisible only by 1 and themselves- think logically for a moment and think about that, and why 1 wouldn't be a prime. :P

Quote:

I dunno why you have so much code there..

because I'm still learning.

Thanks for that slightly more efficient version riddle. Much Obliged.

Ask Riddle to go back five years and pluck one of his programs as an example :D .

Quote:

Originally Posted by DaWei

Ask Riddle to go back five years and pluck one of his programs as an example :D .

psha! Five years ago, I couldn't do much in C++ (I don't think my skills went beyond making a calculator and temperature scale converter). But hey, I had mad skills in QBasic! Bleh.. now, I'm almost ashamed to say I knew QBasic.

On numerous occassions I've had to to back and maintain someone's old code. Every time I asked who the stupid SOB was it turned out to be me!