>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. As soon as execution hit the
break statement, x ceased 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.
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
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