Efficient way to calculate prime numbers

In several puzzles/coding competitions we may need to check for prime numbers and this is required to be done in considerable time to save precious run time. The traditional approach is,



This will however will loop through all elements below x to see if a factor exists. Example, for finding if 13 is prime, we check if its divisible by 2,3,4,....12. This can be optimized to loop through only half the elements since the highest factor is going to be num/2 as,


But by analysis you can see that for a number say 100, the factors are (1*100),(2*50),(4*25),(5*20),(10*10),(20*5),(25*4),(50*2),(100*1). For finding out if 100 is prime or not we just need to check if 100 is divisible by any number not greater than 10 i.e., it is sufficient to check from 2 to 10. The reason being if its divisible by 4, its also divisible by 25.

Hence the best approach to check if a number is prime or not will be



Final prime number program in Ruby (bonus)


This approach will check for factors in very minimized number of loops.

Cheers!!
Bragaadeesh