Highly Efficient Prime Generator

//
//  main.cpp
//  Pi Generator
//
//  Created by Nikolaj Ignatieff Schwartzbach on 2/6/13.
//  Copyright (c) 2013 Nikolaj Ignatieff Schwartzbach. All rights reserved.
//

#include <iostream>
#include <math.h>
#include <time.h>

using namespace std;

int limit=0;
int p=2;
int nextPrime=5;
void writeNumber(int ix){
    int j , i = 0, k = 0;
    if( ix < 0){cout << '-';ix *= -1;}
    int temp = ix;
    int p = 1;
    while( temp > 0){temp /= 10;p *= 10;i++;}
    j = i % 3;
    p /= 10;
    while( i > 0 ){
        cout << char ((ix/p) +'0');
        ix %= p;p /= 10;i--;k++;j--;
        if ((k % 3 == 0 && i > 0)||(j == 0 && i > 2)){cout <<".";k = 0;}}
}
int main(int argc, const char * argv[])
{
    clock_t t1, t2;
    int isPrime, i, nxtSQR, limit;
    cout << "Amount of primes to find: ";
    cin >> limit;
    if(cin.fail()) cout << "Non-numeric input detected. Action aborted.";
    cout << "Finding ";
    writeNumber(limit);
    cout << " primes..." << endl << endl;
    int* primes = new int[limit];
    primes[0]=2;
    primes[1]=3;
    t1 = clock();
    while(p<limit){
        nxtSQR = sqrt(nextPrime);
        isPrime=true;
        for(i=0;i<limit;i++){
            if(primes[i]==0 || primes[i]>nxtSQR) break;
            if(nextPrime % primes[i] == 0){isPrime=false;break;}}
        if(isPrime){primes[p]=nextPrime;p++;}        nextPrime+=2;
        if(nextPrime % 5 == 0)nextPrime+=2;
    }
    t2 = clock();
    float diff = ( (float)t2 - (float)t1) / 1000000 ;
    cout << "Execution Time: " << diff << " seconds.\n";
    cout << "Highest prime:  ";
    writeNumber(primes[limit-1]);
    cout << endl;
    system( "read -n 1 -s -p \"Press any key to continue...\"" );
    return 0;
}