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Feb 8th, 2010
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Help with loops and when they end? for a newbie?

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Hi, im a begginer to python, and i've been doing it a small while now, i've been attempting some puzzles of a website, and have met a small problem, and i can't find it. My code is below and i will explain my problem, ask how to sort it, and ask if i am missing something important.
Python Syntax (Toggle Plain Text)
  1. a = range(1,1000)
  2. b = []
  3. d = 0
  4. for n in a:
  5.     while n != 1:
  6.         if n % 2 == 0:
  7.             b.append(n)
  8.             n = n/2           
  9.         if n % 2 != 0:
  10.             b.append(n)
  11.             n = n*3 + 1
  12.     else: b.append(n)
  13.     if len(b) > d:
  14.         d = len(b)
  15.         b = []
Now the aspects of this code is to find out how long it takes to get to 1, using the 'Collatz Problem' i take the value of n to be inside the range, and if its not 1, i want it to go through the number, decide if its even or odd, then perform a calculation, firstl, im recording the number in a list, so i know the starting number, and then it will perform the calculation, i want it to continue this loop until it reaches 1, im not sure how to do this, i have the else statement at ehd end because i believe that is what i need, i then will take the length of b which is my list, and see if it is higher than my previous list, i then want it to record if so and erase the list, so i can start over at 0, with my next number, but i seem to not be able to make my list work, and i get memory failure of B several times, if someone can help, it would really be apprechiated.
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sammyboy289 is offline Offline
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Feb 8th, 2010
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Re: Help with loops and when they end? for a newbie?
So in summary (for lazy to read like me), what is the issue? Doesn't work as intended or throws exception? What is the error?
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evstevemd is offline Offline
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Re: Help with loops and when they end? for a newbie?
The error is, my b statement doesn't keep resetting which it should, and my list continues even once its reached 1, which i dont understand why also, im not sure if my else: statement is in the right place. Sorry i write a lot
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sammyboy289 is offline Offline
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Re: Help with loops and when they end? for a newbie?
Let's see what happens when n is 2. Your first if condition is true, you divide n by 2 and it becomes 1. Now your second if condition is also true! Welcome to the infinite loop.
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nezachem is offline Offline
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Re: Help with loops and when they end? for a newbie?
should it loop, i didn't think it would, because thats while n != 1, so it should stop that loop and move onto the else: statement, or is that incorrect? i can't quite work out, how to make 2 loops, and when one ends, go to the next statement. That's the main problem im dealing with, sorry if im explaining wrong or being really useless
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sammyboy289 is offline Offline
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Re: Help with loops and when they end? for a newbie?
Shorten the list and add a temporary print for testing. See what that will do for you:
Python Syntax (Toggle Plain Text)
  1. # shorten list for test
  2. a = range(1, 10)
  3. b = []
  4. d = 0
  5. for n in a:
  6.     while n != 1:
  7.         if n % 2 == 0:
  8.             b.append(n)
  9.             n = n/2
  10.         elif n % 2 != 0:
  11.             b.append(n)
  12.             n = n*3 + 1
  13.         else: b.append(n)
  14.     print b, d, len(b)  # test
  15.     if len(b) > d:
  16.         d = len(b)
  17.         b = []
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Ene Uran is offline Offline
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Re: Help with loops and when they end? for a newbie?
At run time, things happen one step at a time. Let's trace the execution of your program with n being 2

while n != 1:
    # Python checks that n is not 1, and enters a loop
    if n % 2 == 0:
        # Python checks that n is even and enters the if-clause
        ...
        n = n/2 # n becomes 1
    if n % 2 != 0:
        # Remember, n is 1, an odd number, so python enters the if-clause
        ...
        n = n*3 + 1 # n becomes 4
    # while loop goes into next iteration, n is 4
while n != 1:
    # n is 4, enter the loop
    if n % 2 == 0:
        # n is even, enter the if-clause
        ...
        n = n/2 # n becomes 2
    if n % 2 != 0: # n is even, if-clause is not entered.
# Next iteration of while loop. n is 2. Etc, etc, etc
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nezachem is offline Offline
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Re: Help with loops and when they end? for a newbie?
wow, that worked, thank you so much, if its not too much to ask, do you mind saying how you changed it, and what it does?, because i notice the elif: statement which im not good with, and one question about the last part, as i will only want one resut to show, will i need to change
Python Syntax (Toggle Plain Text)
  1. #
  2. print b, d, len(b) 
  3. if len(b) > d:
  4. d = len(b)
  5. b = []
to something like
Python Syntax (Toggle Plain Text)
  1. print b, d, len(b)
  2. if len(b) > d:
  3. d = len(b)
  4. b = []
  5. if len(b) < d:
  6. b = []
and thank you very much
Last edited by sammyboy289; Feb 8th, 2010 at 5:53 pm.
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sammyboy289 is offline Offline
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Re: Help with loops and when they end? for a newbie?
ok, i want to thank nezachem for showing why it went wrong, and i want to thankEne Uran as to showing me what was the way to fix it. I have finally sorted out my code, and i am very thankful to all of you for helping me. You have helped me understand and hopefully i can become better at python
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sammyboy289 is offline Offline
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