# [Tutor] syracuse sequence (collatz or hailstone)

Kent Johnson kent37 at tds.net
Sun Oct 30 15:24:11 CET 2005

```andrade1 at umbc.edu wrote:
> would this be a possible use of a list and appending even though I recieve
> an error from it:
>
> def main():
>      x = [1]
>      x[0] = input('enter an int to start your syracuse sequence\n')
>      while not isinstance(x[0], int):
>          x[0] = input('no, please enter an int to start your syracuse
> sequence\n')
>      while x[-1] != 1:
>          if ((x[-1] % 2) == 0):
>              x.append(x[-1] / 2)
>          else:
>              x.append((3 * x) + 1)

This should be
x.append((3 * x[-1]) + 1)

The error is caused by the expression ((3*x) + 1). When x is a list, 3*x is a valid operation; it creates a new list with the elements of the original list repeated three times. Then you do newlist+1. For a list, + means concatenate - it puts two lists together. Since the right-hand value is not a list, you get an error.

Other than that your code is OK but it would be clearer (and avoid errors like the above!) if you used separate names for the current value of x and for the list.

Kent

>      print len(x), x
>
>     print "The Syracuse sequence of your starting value is:", x
>
> main()
>
> line 10, in main
>     x.append((3 * x) + 1)
> TypeError: can only concatenate list (not "int") to list
>
>
>
>
>>
>>>hello,
>>>
>>>Could I gather all of the values from print x into a string or a range?
>>>Since, I am not familiar with lists yet.
>>
>>Here is a simple example of gathering values into a list and making a
>>string:
>> >>> for x in range(3): # x will be 0, 1, 2 in sequence
>> ...   r.append(str(x*x)) # Put x*x (as a string) onto r
>> ...
>> >>> r
>>['0', '1', '4']
>> >>> ', '.join(r) # make a single string by joining the elements of r with
>>', '
>>'0, 1, 4'
>>
>>Kent
>>
>>
>>>
>>>def main():
>>>        x = input("Please enter a positive starting value: ")
>>>         while x != 1:
>>>             if x%2 == 0:
>>>                 x = x/2
>>>            else:
>>>                x = x*3+1
>>>            print x
>>>    print "The Syracuse sequence of your starting value is:", x
>>>
>>>main()
>>>
>>>
>>>
>>>
>>>----- Original Message -----
>>>From: "Frank Bloeink" <frankbloeink at nerdshack.com>
>>>Sent: Friday, October 28, 2005 5:06 AM
>>>Subject: Re: [Tutor] syracuse sequence (collatz or hailstone)
>>>
>>>
>>>
>>>
>>>>Hey,
>>>>
>>>>your code seems almost alright to me, except that in your case it's only
>>>>printing the last number of your sequence, which obviously is not what
>>>>you want. Quick fix would be to insert a line "print x" just below else
>>>>statement:
>>>>---snip--
>>>>else:
>>>>  x=x*3+1
>>>>print x
>>>>---snip
>>>>This should make clear where the error is: You are just calculating, but
>>>>not printing the sequence!
>>>>If you want to leave the output to the end of the program you could as
>>>>well gather all the calculated values in a list or similar structure and
>>>>then print the contents of the list..
>>>>
>>>>hth Frank
>>>>
>>>>On Fri, 2005-10-28 at 01:22 -0400, andrade1 at umbc.edu wrote:
>>>>
>>>>
>>>>>Hello
>>>>>
>>>>>I am trying to create a program that will calculate the syracuse
>>>>>sequence
>>>>>which is also known as collatz or hailstone. the number that is input
>>>>>by
>>>>>the user may be either even or odd. the number goes through a series of
>>>>>functions which are x/2 if the number is even and 3x+1 if the number is
>>>>>odd. it keeps doing so until the number reaches 1. An example would be
>>>>>if
>>>>>the user inputed 5 they should recieve: 5, 16, 8, 4, 2, 1 as the
>>>>>sequence
>>>>>for the value that they started with. My code currently just prints a 1
>>>>>and none of the numbers that would have preceded it. any ideas on how I
>>>>>could get the program to not do this would be greatly appreciated.
>>>>>
>>>>>
>>>>>def main():
>>>>>   try:
>>>>>       x = input("Please enter a starting value: ")
>>>>>       while x != 1:
>>>>>
>>>>>           if x%2 == 0:
>>>>>               x = x/2
>>>>>           else:
>>>>>               x = x*3+1
>>>>>
>>>>>   except ValueError, excObj:
>>>>>       msg = str(excobj)
>>>>>       if msg == "math domain error":
>>>>>           print "No negatives or decimals."
>>>>>       else:
>>>>>           print "Something went wrong."
>>>>>
>>>>>
>>>>>
>>>>>   print "The Syracuse sequence of your starting value is:", x
>>>>>
>>>>>main()
>>>>>
>>>>>
>>>>>_______________________________________________
>>>>>Tutor maillist  -  Tutor at python.org
>>>>>http://mail.python.org/mailman/listinfo/tutor
>>>>
>>>>
>>>>
>>>_______________________________________________
>>>Tutor maillist  -  Tutor at python.org
>>>http://mail.python.org/mailman/listinfo/tutor
>>>
>>>
>>
>>--
>>http://www.kentsjohnson.com
>>
>>_______________________________________________
>>Tutor maillist  -  Tutor at python.org
>>http://mail.python.org/mailman/listinfo/tutor
>>
>
>
>
>
>

--
http://www.kentsjohnson.com

```