Prime numbers program

Question

I\'m currently trying out some questions just to practice my programming skills. ( Not taking it in school or anything yet, self taught ) I came across this problem which re

Answer 1

Since you will need larger prime number values for later questions, I suggest you follow dreeves advice, and do a sieve. It is a very useful arrow to have in your quiver.

Answer 2

#include <cstdio>
#include <iostream>
#include <cstdlib>
#include <fstream>

using namespace std;

int main()
{
ifstream trial;
trial.open("C:\\Users\\User\\Documents\\trial.txt");
int prime, e;
trial>>prime;
ofstream write; 
write.open("C:\\Users\\User\\Documents\\answer.txt");
int num[10000], currentPrime, c, primePrint;
bool check;
currentPrime=0;
num[currentPrime] = 2;
currentPrime=1;

for(int currentInt=2; currentInt<=1000000; currentInt++) 
{check = false; 
for( int arrayPrime=0; arrayPrime<currentPrime; arrayPrime++) 
        { c=num[arrayPrime];
            if ((currentInt%c)==0) { check = true;// second filter based on previous recorded primes in array
                    break;}
               }


 if (!check)
    { e=currentInt;  
    if( currentInt!= 2 ) { 
    num[currentPrime]= currentInt;}
    currentPrime = currentPrime+1;}
    if(currentPrime==prime)
    {
    write<<e<<endl;
    break;}
    }
trial.close();
write.close();
return 0;
}

This is the finalized version base on my original code. It works perfectly and if you want to increase the range of prime numbers simply increase the array number. Thanks for the help =)

Answer 3

For one, you'd have less code (which is always a good thing!) if you didn't have special cases for 3, 5 and 7.

Also, you can avoid the special case for 2 if you just set num[b] = 2 and only test for divisibility by things in your array.

Answer 4

It looks like as you go around the main for() loop, the value of b increases.

Then, this results in a crash because you access memory off the end of your array:

                for (int d = 0; d <= b; d++) {
                    c = num[d];

I think you need to get the algorithm clearer in your head and then approach the code again.

Answer 5

This should also be of interest to you: http://en.wikipedia.org/wiki/Primality_test

Answer 6

Here is my code. When working on a big number, it's very slow! It can calculate all prime numbers with in the number you input!

#include <iostream>
#include <fstream>
#include <cmath>
using namespace std;

int main()
{
    int m;
    int n=0;
    char ch;
    fstream fp;
    cout<<"What prime numbers do you want get within?  ";
    if((cin>>m)==0)
    {
                 cout<<"Bad input! Please try again!\n";
                 return 1;
    }
    if(m<2)
    {
        cout<<"There are no prime numbers within "<<m<<endl;
        return 0;
    }
    else if(m==2)
    {
        fp.open("prime.txt",ios::in|ios::out|ios::trunc);//create a file can be writen and read. If the file exist, it will be overwriten.
        fp<<"There are only 1 prime number within 2.\n";
        fp<<"2\n";
        fp.close();
        cout<<"Congratulations! It has worked out!\n";
        return 0;
    }
    else
    {
        int j;
        int sq;
        fp.open("prime.txt",ios::in|ios::out|ios::trunc);
        fp<<"2\t\t";
        n++;
        for(int i=3;i<=m;i+=2)
        {
            sq=static_cast<int>(sqrt(i))+1;
            fp.seekg(0,ios::beg);
            fp>>j;
            for(;j<sq;)
            {
                if(i%j==0)
                {
                    break;
                }

                else
                {
                    if((fp>>j)==NULL)
                    {
                        j=3;
                    }
                }
            }
            if(j>=sq)
            {
                fp.seekg(0,ios::end);
                fp<<i<<"\t\t";
                n++;
                if(n%4==0)
                    fp<<'\n';
            }
        }
        fp.seekg(0,ios::end);
        fp<<"\nThere are "<<n<<" prime number within "<<m<<".\n";
        fp.close();
        cout<<"Congratulations! It has worked out!\n";
        return 0;
    }
}