black hole sun
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Hi guys, I am writing a simple program to print out 100 prime numbers in C++ (and no, I'm not doing 100 cout's 
). This is what I have so far, but for some unfathomable reason, it isn't working right. It compiles, but try to run it and you'll see what I mean.
Please, if you want to help, don't rewrite my program. just tell me what is wrong with my implementation, it is probably very simple but I just dont see the problem, and my code is well commented and indented. Thanks for any help.
Just for reference, this below code is something I worked on with a fellow classmate of mine that determined whether or not an inputted number was prime. The above code is derived from this code, which works, and because it is a derived work I dont want to deviate too far from the original.
I am a C++ newbie as you can tell, so go easy on me and don't do something wacky like put a pointer or array in there thanks
). This is what I have so far, but for some unfathomable reason, it isn't working right. It compiles, but try to run it and you'll see what I mean.Please, if you want to help, don't rewrite my program. just tell me what is wrong with my implementation, it is probably very simple but I just dont see the problem, and my code is well commented and indented. Thanks for any help.
Code:
//prints the first 100 prime numbers
// FIXME: for some reason this doesn't work. And I cant see why not...
#include <iostream>
using namespace std;
bool isPrime(const unsigned int);
int main(void)
{
// start printing prime numbers from three
int testNum = 3;
cout << "I will now output, for your viewing pleasure, 100 prime numbers!" << endl;
cout << "2, "; // manually print 2, the first prime number
// i is only incremented if testNum really is prime
for(int i = 1; i <= 100; )
{
bool j = isPrime(testNum);
// if isPrime returns true...
if(j == true)
{
cout << testNum << ", ";
testNum++;
i++;
}
}
return 0;
}
bool isPrime(const unsigned int num)
{
int dividedBy = num;
// Assume the number is prime; therefore all I need to do
// is test for non-prime characteristics, like if the number is even
// or divisble by something.
while(dividedBy > 2)
{
// first, check if the number is even
if(num % 2 == 0)
return false;
// now, check if the number can be divided by anything,
// because if the % returns 0,
// then the number is NOT prime
dividedBy--;
int test = num % dividedBy;
if(test == 0)
return false;
}
// if we get this far without returning false, then
// the number must be prime
return true;
}Just for reference, this below code is something I worked on with a fellow classmate of mine that determined whether or not an inputted number was prime. The above code is derived from this code, which works, and because it is a derived work I dont want to deviate too far from the original.
Code:
// asks for inputted number, then determines if the number is
// prime or or not prime.
#include <iostream>
using namespace std;
void getPrime(const unsigned int);
bool PRIME = true; //assume true
int main(void)
{
start: unsigned int number;
cout << "Please input a number and I'll tell you if it's prime." << endl;
cin >> number;
getPrime(number);
if (PRIME == true)
cout << "Prime!" << endl;
cout << "Look at another one?" << endl;
char goAgain;
cin >> goAgain;
if(goAgain == 'y' || goAgain == 'Y')
{
// reset PRIME before we go again
PRIME = true;
goto start;
}
return 0;
}
void getPrime(const unsigned int num)
{
// first, check if 0, 1, or 2 was entered
if(num == 0 || num == 1)
{
cout << "Not prime.";
return;
}
else if (num == 2)
{
cout << "Prime!" << endl;
return;
}
int div = num;
// Since PRIME is assumed true, all we do in this
// while() is test and make sure the number is indeed prime
while(div > 2)
{
if(num % 2 == 0)
{
cout << "Not prime." << endl;
PRIME = false;
return;
}
div--;
int test = num % div;
if(test == 0)
{
PRIME = false;
cout << "Not prime." << endl;
return;
}
}
}I am a C++ newbie as you can tell, so go easy on me and don't do something wacky like put a pointer or array in there thanks
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