On Tue, 27 Sep 2011 14:32:00 -0400
c smith <illusiontechniq...@gmail.com> wrote:
> hi list,
> i understand the general idea of recursion and if I am following well
> written code I can understand how it works, but when I try to write
> it for myself I get a bit confused with the flow.
> I was trying to turn an ackerman function into python code for
> practice and I tried writing it like this:
> #!/usr/bin/env python
>
> import sys, os
> def ack(m,n):
>
> if m == 0:
> return n+1
> elif m > 0 and n == 0:
> ack(m-1,1)
> elif m > 0 and n > 0:
> ack(m-1,ack(m,n-1))
>
> if __name__=='__main__':
> sys.argv[1] = int(sys.argv[1])
> sys.argv[2] = int(sys.argv[2])
>
> print ack(sys.argv[1], sys.argv[2])
>
> The major problem, I think, is that I cant figure out where to turn
> the args into ints.
You did it right, just when you loaded them. What is not correct is to
try to assign them to a property inside a standard library module. You
should assign them to local variables (``m`` and ``n`` for example).
> Also, does the second 'elif' make sense?
I have am not familiar with the ackerman functions, but since you
mentioned recursion, I suppose you wanted to recurse in those elif`s
and return the result of a new call to the funtion.
If that is the case, you could happily skip the "else" part of "elif"
because if the ``if`` code will run, the interpreter will never reach
the ``elif`` line.
> I am not sure if the function can take itself as an argument.
> thanks for any suggestions, this list has been a big help.
Here's the code readapted (but verify it still does what is intended
to!):
#!/usr/bin/env python
import sys, os
def ack(m,n):
if m == 0:
return n+1
if m > 0:
if n == 0:
return ack(m-1,1)
if n > 0:
return ack(m-1,ack(m,n-1))
raise BaseException('Something is wrong here!')
if __name__=='__main__':
m = int(sys.argv[1])
n = int(sys.argv[2])
print ack(m, n)
HTH!
/mac
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