On Jul 15, 12:28 pm, "Andreas Tawn" <[EMAIL PROTECTED]> wrote: > >defn noob wrote: > >> isPrime works when just calling a nbr but not when iterating on a > >> list, why? adding x=1 makes it work though but why do I have to add > >> it? > >> Is there a cleaner way to do it? > > >> def isPrime(nbr): > >> for x in range(2, nbr + 1): > >> if nbr % x == 0: > >> break > >> if x == nbr: > >> return True > >> else: > >> return False > > >>>>> [isPrime(y) for y in range(11)] > > >> Traceback (most recent call last): > >> File "<pyshell#45>", line 1, in <module> > >> [isPrime(y) for y in range(11)] > >> File "C:\Python25\Progs\blandat\myMath.py", line 9, in isPrime > >> if x == nbr: > >> UnboundLocalError: local variable 'x' referenced before assignment > > >>>>> map(isPrime, range(100)) > > >> Traceback (most recent call last): > >> File "<pyshell#38>", line 1, in <module> > >> map(isPrime, range(100)) > >> File "C:\Python25\Progs\blandat\myMath.py", line 9, in isPrime > >> if x == nbr: > >> UnboundLocalError: local variable 'x' referenced before assignment > >>>>> isPrime(10) > >> False > >>>>> isPrime(11) > >> True > > >> adding x=1 makes it work though: > > >> def isPrime(nbr): > >> x=1 > >> for x in range(2, nbr + 1): > >> if nbr % x == 0: > >> break > >> if x == nbr: > >> return True > >> else: > >> return False > > >>>>> [isPrime(y) for y in range(11)] > >> [False, True, True, True, False, True, False, True, False, False, > >> False] > >> -- > >>http://mail.python.org/mailman/listinfo/python-list > > >======================================== > >Yep - "local variable 'x' referenced before assignment" is correct. > >You state: for x in range... but x doesn't exist until initialized. > > To save a loop, initialize x=2 (the minimum value) and loop executes > > on pass one. > >In a straight 'C' program > > ( for (x=1, x=(nbr+1), x++) etc... ) > > the x is initialized and forceably incremented. > > seems Python does not auto initialize but does auto increment. > > I think a better explanation is that in your original function, x only > existed while the for loop was running.
The for loop never ran. > As soon as execution hit the break statement, It never hit the break statement, the first call from [isPrime(y) for y in range(11)] attempted to do for x in range(2,1). > x ceased to exist. Something has to exist before it can cease to exist. > When you attempted to reference it > in the next line, Python has no variable called x so it complains that x > hasn't been initialised. Right conlusion but false premise. > > A more idiomatic way to write it... > > def isPrime(nbr): > if nbr <= 1: > return False > for x in xrange(2, nbr+1): > if not nbr % x: > return x == nbr > > Cheers, > > Drea -- http://mail.python.org/mailman/listinfo/python-list
