calling functions

[bruno modulix]

anthonyberet wrote:
> This is the first time I have tried out functions (is that the main way
> of making subroutines in Python?)

A function is allowed to change it's arguments and to return None, so
yes, you can consider it as a 'subroutine'.

> 
> Anyway, my function, mutate, below
> 
> #make a child string by randomly changing one character of the parent
> 
> Def mutate():
s/Def/def/

<meta>
please copy-paste code - retyping it increases the risk of typos.
</meta>

>     newnum=random.randrange(27)
>     if newnum==0:
>             gene=' '
>     else:
>         gene=chr(newnum+96)

> position=random.randrange(len(target))
> child=parent[:position-1]+gene+parent[position+1:]

Where does this 'gene' come from ?-)

> mutate()
> 
> 
> The trouble is when I later (as in further down the code) attempt to
> retrieve the value of gene I get an error saying that gene is undefined.

Of course it is.

> It works fine when I don't have the routine defined as a function. - the
> IF- Else structure means gene must have a value of ' ' or 'a' to 'z'.

This 'gene' only lives in the function body - as with almost any other
programming language.

> It seems that the line:
> 
> mutate()
> 
> is not invoking the function, 

It is. But this function does not return anything (well, it returns
None, which is the Python representation of exactly nothing) - and you'd
loose it if it did anyway.

<non-pythonic-explanation>
A variable created in a function is local to the function. It disappears
as soon as the function returns - unless you keep a reference to it one
way or another. The usual way to do so is to return the variable to the
caller :
</non-pythonic-explanation>


def mutate():
  newnum = random.randrange(27)
    if newnum == 0:
      gene=' '
    else:
      gene = chr(newnum + 96)
  return gene

gene = mutate()
# target and parent where undefined...
# please post working code
parent = "0123456789"
#position = random.randrange(len(target))
position = random.randrange(len(parent))
child=parent[:position-1]+gene+parent[position+1:]

Now you may want to check your algorithm, since it doesn't perform as
described - but this is another problem !-)

hints:
 import string
 string.ascii_lowercase
 help(random.choice)

 astring = "abcd"
 alist = list(astring)
 alist[0] = 'z'
 astring2 = ''.join(alist)

Also note that a function can take arguments:
 def fun_with_args(arg1, arg2):
   print "in func_with_name : arg1 = %s - arg2 = %s" % (arg1, arg2)

 fun_with_args('toto', 'titi')

so you can have the whole algorithm in the function body:

def createChild(parent):
  # code here to create child
  return child

parent = "0123456789"
child = createChild(parent)
print "parent : %s\nchild : %s" % (parent, child)

> Thanks again - this group is great. I despair of ever being able to
> contribute though :-(

You did. There would be no answer if there were no questions !-)
BTW, may I suggest you to spend some time on a good Python tutorial ?
(there are many good ones freely available on the net).

HTH
-- 
bruno desthuilliers
python -c "print '@'.join(['.'.join([w[::-1] for w in p.split('.')]) for
p in 'onurb at xiludom.gro'.split('@')])"