[Tutor] finding factorials - hmm..., gcd

[glingl]

--- Payal Rathod <payal-python@staticky.com> schrieb:
> 
> Sorry Danny I am still not getting it. Now I am so confused that I think
> I don't know what exactly a function does and what is it it returns.
> 
> I have posted my code below with some comments regarding issues I don't
> get.
> 
> #!/usr/local/bin/python
> 
> def euclid(a,b):
>         while b != 0:
>                 c = a
>                 a = b
>                 b = c % a
>                 print 'A = ', a
>                 print 'B = ', b
>                 return euclid(a,b)
> # What will this return exactly?
>         else:
>                 return a
> 
> x = 100
> y = 20
> 
> result = euclid(x, y)
> 
> # Why do we define x and y here seperately?
> # Can't we have result = euclid(100,20)
> # This apparently is not working. 
> 
> print x
> print y
> 
> # This always prints 100 and 20. I think it should print the present values
> # of x and y which are not 100 and 20 always.
> 
> print result
> 
> # The result is given as 20 which I think is wrong. Is there anything 
> # wrong with my mathematical logic or my function?
> 
> With warm regards,
> -Payal
> 

Your code is ok and gives the right result!
e.g change:  x, y = 25, 15 and you will get:

>>> 
A =  15
B =  10
A =  10
B =  5
A =  5
B =  0
25
15
5

hmmm..., o.k., it's not perfektly ok, because ...

def euclid(a,b):
        while b != 0:
                c = a
                a = b
                b = c % a
# (1) normally it's not considered good practice to pu
#     print-statements in a function which aims at calculating
#     a numerical rsult. - 
#     But here let's use it for getting insight in the working 
#     of euclid
                print 'A = ', a
                print 'B = ', b
# (2) here you use an uncoditional return - so the body of
#     the while-loop will be exited after the first run. 
#     Consequently it will never work as a loop and you could
#     (a)  use an if instead of while or
#     (b)  drop the return and recursive call of euclid - 
#          recursion also accomplishes some sort of iteration
#          and should/need/(must?) not be done in addition 
#          to the while-iteration  
                return euclid(a,b)
        else:
                return a

x = 25
y = 15

result = euclid(x, y)

print x
print y

print result

################################
# If you decide for the use of while you arrive at:

def euclid(a,b):
        while b != 0:
                c = a
                a = b
                b = c % a
                print 'A = ', a
                print 'B = ', b
#               return euclid(a,b)
#       else:
        return a

x = 25
y = 15

result = euclid(x, y)
print x
print y
print result

########################################
an alternative is to decide for the use of recursion,
in which case you arrive at:

def euclid(a,b):
        if b != 0:
                c = a
                a = b
                b = c % a
                print 'A = ', a
                print 'B = ', b
                return euclid(a,b)
        else:
               return a

x = 198
y = 94

result = euclid(x, y)

print x
print y

print result

# (3) here you can leave out the three statements
#
#               c = a
#               a = b
#               b = c % a
#
# if think carefully about which values are to be
# inserted as arguments in the recursive function
# call.
# What are the values of a and b respectively after
# execution of these 3 statements. (Remember: ist is
# perfectly right to insert arithmetic expressions like
# a / b, a+b etc as arguments - in which case they
# will be computed (evaluated( before the results are 
# "bound" to the parameters.)

Finally you should delete (or comment out) the 
print statements!

Hmmm, will this help?

Regards, Gregor

P.S.: Maybe http://www.ibiblio.org/obp/thinkCSpy/ ,
especially chapters 4 to 6,
contains material, which could be helpful for you.