# [Tutor] syracuse sequence (collatz or hailstone)

Kent Johnson kent37 at tds.net
Fri Oct 28 12:02:29 CEST 2005

```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

Above you have a loop. Each time through the loop you change the value of x. You don't retain the old value of x so it is lost. The simplest thing would be to print x in the loop so each successive value is shown to the user.

Kent
>
>     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()
>
>
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> http://mail.python.org/mailman/listinfo/tutor
>
>

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