Re: [Tutor] Control flow

[Orri Ganel]

Gilbert Tsang wrote:
Hi there, I have this logic that I cannot wrap my mind it:
def go_jogging():
   # go out and jog
   return
if ( bad_weather =='y' ):
   # ask user only if weather is bad.
   b = input ( "Weather is really bad, still go out to jog?[y/n]" )
   if b == 'y':
      go_jogging()
   else:
       # program should exit now
else:
   go_jogging()
 
####################################################
I can't get the program to stop processing further in the middle 
(apparently neither exit nor goto-label exist in Python, sorry for the 
C++ mindset) so I used exception to achieve what I want. I know in 
that example you could probably manipulate the logic so that program 
ends at the bottom of the if-tree. My question is then how to exit in 
the middle of a if-then-else tree? Thanks, Gilbert.

try:
   if ( bad_weather =='y' ):
       b = input ( "Weather is really bad, still go out to jog?[y/n]" )
       if b == 'y':
           go_jogging()
       else:
           raise Exception( "quit" )
   else:
       go_jogging()
except Exception, inst:
   print "Program exits now"
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Well, if its in a function you can just do return instead of  raise 
Exception("quit") since the function exits once it returns something.  
It's sort of the same as the following:

Say you have a recursive function  fact(n) which takes in a number and 
returns the factorial of that number.  One way to code this would be:

def fact(n):
   if n > 3:
      return n*fact(n-1)
   else:
      return 2*n
However, since the function 'exits' after a  return statement, it saves 
space in general to do the following:

def fact(n):
   if n > 3:
      return n*fact(n-1)
   return 2*n
Because the only way  return 2*n will be reached is if n is smaller than 
or equal to three, which is what we wanted anyway.

BTW, the way this function works is like this:  Let's say you want to 
calculate fact(10).  10 is greater than 3, so the program calculates 
10*fact(9) and so on until fact(3) is reached, at which point it will 
return 10*(9*(8*(7*(6*(5*(4*(6))))))) (6 is 2 * 3). or the factorial of 10.

I probably didn't do too good a job explaining that, and that's probably 
not the most efficient implementation (it breaks @ around fact(1000) by 
default, and around  fact(11462) when you use  
sys.setrecursionlimit(2000) (11462 is as high as it'll go on my 
machine)), so please feel free to ask questions.

HTH,
Orri
--
Email: singingxduck AT gmail DOT com
AIM: singingxduck
Programming Python for the fun of it.
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