[C] Testing whether integer is 'Prime' or 'Not Prime'

[evo0o]

Basically I've this homework assignment for my beginner C class, I managed to compile the program but am unable to clear the auto-marker (coursemarker)

Any one got some clues as to why?

The program is required to read a user input integer and calculate whether the integer is prime or not prime and output the respective lines as in the code below. An entry of 0 will result in the program exiting.

Pardon the noob // comments :p

[boogerjones]

Can you explain the problem better? Are you just talking about the gray box at the end of the code? And when you post code, post the text instead of an image. Or upload the main.c file and I'll remove it with a text or hex-editor.

And a couple comments on the code. You calculate the square-root of "num" with each iteration of the loop; just do it once and store it in a variable. I'm not sure if the compiler optimizes that out or not. Second, all even numbers greater than 2 are not prime; that could help.

[evo0o]

Btw is there any better way to solve whether it's a prime or not?

[Andre S. Veteran]

The algorithm is fine, but this will get you fired:

while (start<(sqrt(num))

sqrt is an expensive calculation, so you don't want to re-compute it each time through the loop. Compute it once, store it in a local variable and use that for the loop condition.

There are plenty of ways to check for primality. For small primes, the fastest way would be to store all known primes in a sorted array and search (using a binary search) for the input number in that array. You could improve your existing algorithm by testing only for odd numbers, since no even number greater than 2 is prime. More advanced methods (efficient for large primes) are based on probabilities. Check out http://en.wikipedia.org/wiki/Primality_test

[boogerjones]

The best way depends on the greatest number that might be tested in a program; it also depends on your tolerance for a false-positive. The Miller-Rabin test is popular; make sure to read the section about making it deterministic. Since the maximum value that may be tested by your program is INT_MAX, you could easily optimize it using the parameters in the Wikipedia article.

[Xilo]

Apart from the sqrt problem and checking for evens, your program is fine. I would use a for loop instead of a while loop though.

Does the check program pass based on your actual source code or output?

[evo0o]

Oops, I figured out what my error was, the program was looking for continuous user entry till '0' is keyed in. I put a while loop in the main function and ta-da!

Thanks for the help guys, I'm still in the process of refining my code to check for primes.

[boogerjones]

  On 08/03/2010 at 07:25, evo0o said:

I'm still in the process of refining my code to check for primes.

Other than taking the sqrt() function out of the while loop, you really don't need to change anything. Since you're limiting yourself to 32-bit signed integers, the slowest-case scenario for your test is running the while() loop about 46000 times. That will happen quickly on any CPU.

[BGM]

  On 08/03/2010 at 04:30, Dr_Asik said:

The algorithm is fine, but this will get you fired:

wtf... wouldn't wanna work where you work! :O

[Jebadiah]

Most people don't realize that: (Prime number > 3) % 6 = 1 or 5 always. (% is the operator which gives you the remainder of the division.)

5 % 6 = 5

7 % 6 = 1

11 % 6 = 5

13 % 6 = 1

17 % 6 = 5

... and so on.

The above calculation can be used as a precondition for testing whether a number is prime, because it helps avoid the use of expensive calculations for testing each and every number for primality with the expensive while loop.

You should also consider hard-coding your program for numbers<=3 because 2 and 3 are prime and 0 and 1 are non-prime.

[Argi]

  On 09/03/2010 at 01:18, BGM said:

wtf... wouldn't wanna work where you work! :o

It's a minor difference in code but as mentioned about having it that would means that sqrt() is evaluated each time the condition is check when it only needs to be done once. If we're talking about large prime numbers that would mean a lot of extra computation time wasted.

[Rohdekill]

I would have just asked the instructor to name one real-world example where I would need to calculate if a number is prime.

[tiagosilva29]

  On 20/03/2010 at 12:38, Rohdekill said:

I would have just asked the instructor to name one real-world example where I would need to calculate if a number is prime.

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