#1
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    Devshed Newbie (0 - 499 posts)

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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.
  2. #2
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    Devshed Demi-God (4500 - 4999 posts)

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    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
    [code]Code tags[/code] are essential for python code and Makefiles!

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