Prime Number Program

sterlingsilver10289 · May 7th, 2006, 11:15 PM

While reading this book, I came upon an idea to create a simple C++ program that would list all the prime numbers between two inputted numbers, and I figured this is possible with the codes I learned in this book only. I'm having problems though. Here's the chunk of my code that checks for prime numbers. numStart and numEnd are obviously the imputted numbers. My logic is that I start a loop of all the numbers between the start and end, and inside that loop I take every number below my number from the first loop, and divide them. I then check if that answer is an integer, and if it is, then the program does nothing. I'm unsure what to do to tell the program to display counter if check is never equal to 1. Can you help? This is what I have so far:

Code:

int counter;
int counter2;
float ansFloat;
int ansInt;
float check;
check = 1;
for (counter = numStart; counter <= numEnd; counter++)
{
for (counter2 = 2; counter2 < counter; counter2++)
{
ansFloat = counter / counter2;
ansInt = counter / counter2;
check = ansInt / ansFloat;
while (check == 1)
{
// Don't do anything.
}
}
}

ciderpunx · June 30th, 2006, 09:53 AM

If I've understood your question correctly, and assuming you're using java

if (check != 1) {
  System.out.println(counter);
}

If it's c or c++ you'd import STDIO at the tope of your file and do:

if ($check != 1) {
  printf("%d",$counter);
}

Incidentally you may want to check out the modulo operator (%) which gives the remainder of a division so:

3%2; // would be 1
4%2; // would be 0

This would save you creating all those temporary variables and running three instructions per number you print, which could slow things down on a large set of numbers ;)

You may also want to look at a mathematical method for finding the prime numbers in a set, called the "Net of Erostothenes"

Cheers,
Charlie

--
Don't Stand on your head - you'll get footprints in your hair
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