I fixed my code, but why does this old version do an infinite loop?

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February 11th, 2013, 06:13 AM

Jack J

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Join Date: Feb 2013

Posts: 1

Time spent in forums: 1 h 24 m 16 sec
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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


def primeNumbers(limit):
    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


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

print(primeNumbers(20))

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.

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
... 
>>> 
>>> def primeNumbers(limit):
...     primeList = [2]
...     n = next(nextPrime(primeList))
...     while n <= limit:
...         primeList.append(n)
...         print(primeList)  ###################### here
...     return primeList
... 

>>> import math
>>> primeNumbers(8)
[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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