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Prime Number Generator

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Submitted on: 4/1/2016 10:51:50 PM
By: Jake R. Pomperada  
Level: Beginner
User Rating: By 1 Users
Compatibility: C++ (general)
Views: 1918
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     A simple program that I wrote using C++ as my programming language that will ask the user to give a number and then the program will generate the corresponding prime numbers based from the number give by our user. I wrote a function for prime number generation in this sample program of ours. Add me at Facebook my address is jakerpomperada@gmail.com and jakerpomperada@yahoo.com My mobile number here in the Philippines is 09173084360.
 
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//**************************************
// Name: Prime Number Generator
// Description:A simple program that I wrote using C++ as my programming language that will ask the user to give a number and then the program will generate the corresponding prime numbers based from the number give by our user. I wrote a function for prime number generation in this sample program of ours.
Add me at Facebook my address is jakerpomperada@gmail.com and jakerpomperada@yahoo.com
My mobile number here in the Philippines is 09173084360.
// By: Jake R. Pomperada
//**************************************

#include <iostream.h>
 int check_prime(int x)
 {
 int var1=0,var2=0,flag=0;
 cout << "\n\n";
 cout << "\t List of Prime Numbers";
 cout << "\n\n";
 for(var1 = 2; var1 <= x; var1++){
		flag = 1;
		for(var2 = 2; var2 < var1; var2++){
			if((int)var1 % (int)var2 == 0){
				flag = 0;
			}
		}
	
		if(flag == 1){
			cout << var2;
			cout << " ";
		}
	}
 }
 main(){
	int values=0;
	cout << "\t\tPrime Number Generator Version 1.0";
	cout << "\n\n";
	cout << "Enter a Number : ";
	cin >> values;
	check_prime(values);
	 cout << "\n\n";
	 system("pause");
	}


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4/23/2016 2:37:09 PMWayne

This is mechanically correct but really poorly implemented. For instance, the for loop walks var2 all the way up to var1 looking for numbers that will go into it evenly. This works but there will never be a number greater than (var1 / 2) that will go in evenly, making all the checks greater than or equal to (var1 / 2) wasteful. If a very large '"values" is passed in, this becomes even more obvious. also, not every number from 2 up has to be checked, only the primes. Also, once a number has been found to be NOT a prime, quit looking. This makes "flag" obsolete.
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