New user needs help

omission9 omission9 at invalid.email.info
Sat Feb 21 06:32:50 CET 2004


John wrote:

> I am new to using Python. Everytime I run this program it prints "The lowest
> common factor is 0", no matter what numbers I use. Can anybody see anything
> wrong with my program? Thanks in advance.
> 
> 
> 
> def getnum1(a):
>     a = input("What is the first number?")
>     if a == 0:
>         getnum1(a)
> def getnum2(b):
>     b = input("What is the second number?")
>     if b == 0:
>         getnum2(b)
> def euclid(num1, num2, num3, num4):
>     if num1 < num2:
>         num3, num4 = num1, num2
>         num1, num2 = num4, num3
>         euclid(num1, num2, num3, num4)
>     elif num2 != 0:
>         num3, num4 = num1, num2
>         num1 = num4
>         num2 = num3 % num4
>         euclid(num1, num2, num3, num4)
>     else:
>         print "The lowest common factor is: ", num1
> a = 0
> getnum1(a)
> b = 0
> getnum2(b)
> x, y = 2, 100
> euclid(a, b, x, y)

This is a nice simple example of what is called "scope" and how it can 
trick a beginner.
Put as simply as possible,"a" and "b" are actually defined twice. Once 
"locally" in the getnum functions and another time "globally" . When you 
set a and b to the user input in the functions the other a and b have no 
idea about this assignment. Below is a slight revision which return the 
a and b set by the user back to where they are called. By putting a 
simple print statement in the script you can see where the values are 
set and that can help you.
I hope that helps. There is probbaly a couple of other changes you could 
make as well but nothing too major, in my opinion.


def getnum1(a):
     a = input("What is the first number?")
     if a == 0:
         getnum1(a)
     return a
def getnum2(b):
     b = input("What is the second number?")
     if b == 0:
         getnum2(b)
     return b
def euclid(num1, num2, num3, num4):
     print num1, num2, num3, num4
     if num1 < num2:
         num3, num4 = num1, num2
         num1, num2 = num4, num3
         euclid(num1, num2, num3, num4)
     elif num2 != 0:
         num3, num4 = num1, num2
         num1 = num4
         num2 = num3 % num4
         euclid(num1, num2, num3, num4)
     else:
         print "The lowest common factor is: ", num1
a = 0
b = 0
a=getnum1(a)
b=getnum2(b)
x, y = 2, 100
euclid(a, b, x, y)




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