
syntax thingy x % 2 != 0 amd x % 3 != 0
Code:
def prms(x):
return x % 2 != 0 and x % 3 != 0
r = filter(prms,range(2,25))
print r
that'll return prime numbers from 2 to 25.
but i dont understand this line
return x % 2 != 0 and x % 3 != 0
is 0 flase in this case? what does the % sign mean.
please explain.
thank you

Someone else may give you a more complete answer, but I believe the line is testing for remainders in the division of x by 2 and 3. If the remainder for either division is 0 then the number cannot be prime.

remainder is 0? i dont get that.

It's so simple, you will be wacking yourself on the head.
x % 2 #returns remainder of x / 2.
Prime numbers, by definition, are only divisible by 1 and themselves (duh, right?), so if "x" is evenly divisible by 2 and/or 3 then it cannot be prime and is not passed to "r"

=) i figured that part out. i just dont get the syntax.
i dont get the % and != 0 parts

I've never seen anything like this before, it looks like an ifreturn combination. where did you see this?
% is a mathimatical operator, i dont really know how to describe it but i've used it in PHP, Perl and Python when working on the web to create alternate row colors. you'd do a loop and check the iterator number to see if it % 2.
!= 0 means if whatever is not equal to 0 (also faulse).
Hope this helps, it really is a strance bit of code i hae to say. Wasn't awar this was even possible but
Mark.

% is the modulus operator. It returns the remainder of two numbers when one is divided by the other. For Example. 7/3 is 2 with a remainder of 1. So if you said x = (7/3) then x would be equal to 2, it drops the remainder. To find the remainder you would say 7%3 which is 1 because the remainder of 7/3 is 1. So, y = 7%3 would set y equal to 1.

Thanks a lot damon, good to know .
Mark.


interesting, thanks for the info.
July 10th, 2003, 06:24 PM

this line
Code:
return x % 2 != 0 and x % 3 != 0
will return 1 if both expressions evaluate to true, or 0 otherwise (python 2.2 uses 1 and 0 for booleans, 2.3 uses the words True or False)
July 22nd, 2003, 03:53 AM

Just remember, using
Code:
x%2 != 0 and x %3 !=0
to see if x is a prime number will only work up to, and not including, 25.
Last edited by percivall; July 22nd, 2003 at 04:00 AM.
July 22nd, 2003, 12:29 PM

July 22nd, 2003, 12:57 PM

Once you reach 25, there are more numbers (factors) to consider in determining if a number is prime.
ie 25/2 != 0 and 25/3 != 0, but 25/5 == 0, so 25 is not prime, even though it fits the criteria of that line of code. Below 25, all the nonprime numbers are either even (divisible by 2) or divisible by 3, so that code works.
July 22nd, 2003, 01:22 PM

oh. they never explained that
thanks for pointing that out.