Quick C# Query

ronkmonster

What does this line of code do?

if ((testForPrime & 1) == 0)

It's part of prime number checker but I'm not sure what this part checks
testForPrime is a long or int

ob

Bitwise And operator, some good explanations here:

http://weblogs.asp.net/alessandro/archive/2007/10/02/bitwise-operators-in-c-or-xor-and-amp-amp-not.aspx

http://dotnetperls.com/prime-number

ronkmonster

The second link is where I got the algorithm from.

Is that code just checking if the number to test is not an even number by checking bits?

if number is odd and could be a prime, then continue with rest of checks.

If so, is this the fastest way
or would something like number%2!=0 be just as fast

padraig_f

if ((testForPrime & 1) == 0)

Yeah he's testing if it's an even number.

It's an optimisation, it allows him to skip even numbers in his for-loop later on:

for (int i = 3; i <= num; [b]i += 2[/b])

If so, is this the fastest way or would something like number%2!=0 be just as fast

The answer to this is, "it's C#, who cares what's faster". You should go for the one that's more readable, so yeah number % 2 == 0 is better. If he wants to use number & 1 == 0 he should've really added a comment to say it's an even-number check. You might argue that an experienced coder should know this, but they'd usually still have to stop and ask themselves if an even-number check was the intention. Code readability is more important than minor optimisations, as poor readability makes bugs hard to find.

Webmonkey

C# will probably optimize it anyways so yeah readability is better.