Need Some Coaching, Display 100 Prime Numbers

This is a discussion on Need Some Coaching, Display 100 Prime Numbers within the C Programming forums, part of the General Programming Boards category; I've been working at this for awile and I need some help. I can print the first 100 non-prime numbers ...

  1. #1
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    Need Some Coaching, Display 100 Prime Numbers

    I've been working at this for awile and I need some help. I can print the first 100 non-prime numbers but not the first 100. What am I missing? This nested loop business is killing me. Here is what I have:

    Code:
    #include<stdio.h>
    
    int main (void)
    {
         int num, test;
    	 int prime = 0;
    	 int not_prime = 0;
    
    	 for (num = 2; num < 100; num++)
    	 {
    		
    		 for (test = 2; test < (num - 1); test++)
    			
    		{
    					if (num % test == 0)
    				{
    					printf ("%d\n", num);
    					break;
    				}
    		 }
    		//printf ("%d\n", num);	
    			
    		}
    	 }

  2. #2
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    I can print the first 100 non-prime numbers but not the first 100
    Huh?

    Also, clean up your whitespace. Makes it easier to read...for everyone!

  3. #3
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    Not the first 100 prime.

  4. #4
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    Do you want to print out non-prime numbers? I don't know what your requirements are. You never stated them clearly.

    Regardless, you need to work out on paper how you'd do this as if you were a computer. Flowchart it , then write the code.

    Throwing loops together haphazardly is a recipe for disaster.

  5. #5
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    I want to print the first 100 prime numbers. I am able to do the all of the non primes from 0 to 100 but cannot figure out how to get the primes. This is what I think I need to do.

    Make a loop that goes from 2 to 100.
    Make another loop that will test each value by using modulus 2 to the number -1
    If any 0's are found, the number is not prime
    If no zeroes are found the number is prime

    I can't figure out how to catch the primes. Actually I have caught them a couple times but they print for every trip through the loop. I can;t figure out how to just catch them one at a time.

  6. #6
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    See bold
    Quote Originally Posted by dolfaniss View Post

    Make a loop that goes from 2 to 100.
    Make another loop that will test each value by using modulus 2 to the number -1
    If any 0's are found, the number is not prime <---set a flag variable (say, prime_found ) and break from the loop either directly, or by the test at the top of the loop
    If no zeroes are found the number is prime

    I can't figure out how to catch the primes. Actually I have caught them a couple times but they print for every trip through the loop. I can;t figure out how to just catch them one at a time.

  7. #7
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    Something like this (not tested)
    Code:
        prime_found = FALSE;
        for (test = 2; test < (num - 1); test++) {
            if (num % test == 0) {
                prime_found = TRUE;
                break;
            }
        }
        if (prime_found) {
            printf ("%d\n", num);
        }

  8. #8
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    Thanks Mike, I will play around and see what I can come up with.

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