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#1
February 11th, 2013, 06:13 AM
 Jack J
Registered User

Join Date: Feb 2013
Posts: 1
Time spent in forums: 1 h 24 m 16 sec
Reputation Power: 0
I fixed my code, but why does this old version do an infinite loop?

I finally figured out how to fix this code and why my fixed version works, but I still do not really know why the broken version went into an infinite loop with an unchanging value of both n and checkNum. I would have expected it, if it didn't work, to increment too quickly since I had a next(n) in it twice. I guess my question is then, why does this code not work? The first function generates the next prime. The second function stores that prime into a list.
Code:
```def nextPrime(primeList):
checkNum = 3
while True:
for i in primeList:
if checkNum % i == 0:
break
if i > math.sqrt(checkNum):
yield checkNum
break
checkNum += 2

primeList = [2]
n = next(nextPrime(primeList))
while n <= limit:
primeList.append(n)
return primeList```

This following code is the fixed version which works fine. I hope I wrote it nicely enough, though I could have put in a few comments.
Code:
```def nextPrime(primeList):
checkNum = 3
while True:
for i in primeList:
if checkNum % i == 0:
break
if i > math.sqrt(checkNum):
yield checkNum
break
checkNum += 2

primeList = [2]
i = 0
n = nextPrime(primeList)
while i <= limit:
i = next(n)
primeList.append(i)
return primeList

I guess I do not understand why you need 1) the function itself, 2) a variable to refer to the function - in this case n, and 3) a variable to store the current state of n - in this case i. It makes more sense to me to just be able to use the function and a variable to store the current state.

#2
February 11th, 2013, 10:15 AM
 b49P23TIvg
Contributing User

Join Date: Aug 2011
Posts: 3,364
Time spent in forums: 1 Month 2 Weeks 3 Days 10 h 28 m 48 sec
Reputation Power: 383
n didn't change.
Insert a print statement (and be prepared to interrupt):
Code:
```>>> def nextPrime(primeList):
...     checkNum = 3
...     while True:
...         for i in primeList:
...             if checkNum % i == 0:
...                 break
...             if i > math.sqrt(checkNum):
...                 yield checkNum
...                 break
...         checkNum += 2
...
>>>
...     primeList = [2]
...     n = next(nextPrime(primeList))
...     while n <= limit:
...         primeList.append(n)
...         print(primeList)  ###################### here
...     return primeList
...

>>> import math
[2, 3]
[2, 3, 3]
[2, 3, 3, 3]
[2, 3, 3, 3, 3]
[2, 3, 3, 3, 3, 3]
^C```
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