• C

Prime Number Conundrum...

I am trying to write a program that will allow input of two numbers (min and max) and then determine all of the prime numbers between the two numbers.

I can not figure out how to do this...

I know what prime numbers are but I can not figure out a quick way to implement this with code... I was thinking about using the % to see if there was a remainder but I could not figure out how to go about it...

Any help?

Thanks,

Mapper
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Author Commented:
Edited text of question.
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Commented:
There is no "shortcut" algorithm for determining whether a number is prime. That is one of reasons for the continuing interest in prime numbers. Straightforward algorithms all involve dividing each candidate number (say n) by numbers from 2 to n/2. This can be approached a number of ways. There may be shortcuts, but only in cases that fulfill specific conditions.

The general case is not hard to program. It could take a lot of computing time, but that depends on the size of numbers being considered.

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Author Commented:

I know I only need to be concerned with numbers up to the sqrt of the max value, but the logic to implementing it is what causes me confusion - I am new to C and I get confused easily when confronted with figuring out and then implementing the logic behind the code...

I will test to make sure that the min is > 1 and the max is > min - this will let me know that I have a valid set of numbers (2 to 20 ) - then I know that basically it's all odds except those divisible by 5 -- So, I will need a while to make sure scanf is returning only 2 numbers - then determine the min and max values are valid - then do I start dividing by the min into the min using the modulus?  This is where I am getting foggy...

Thanks,

Mapper
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Commented:
Sure. A basic test would look like:

testing numbers m through n:

int prime;

for(i=m; i<=n; ++i)
{   prime=1;   /* assert i is prime */
for(j=2; j<=i/2; ++j)
{   if(i%j==0)prime=0;
}  /* if i is divisible by j it is */
/* evidently not prime           */

if(prime)printf("%d is prime\n",i);
}

You can, of course, as you have pointed out, optimize this by various means: check j only up to the square root of i. Only check the odd numbers between m and n (be careful not to exclude 2 though!). Only check odd numbers that don't end in 5. etc. etc.
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Author Commented:

Thanks! Once again the anxiety brought about by learning something new has blinded me to the obvious!  I wish I could learn to have more faith in my abilities...

Thanks again,  I appreciate the nudge...

Mapper
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