Yes, I knew there was something I wasn't getting. This is the explanation I
was looking for - and I'm sorry about the incorrect indentation. And I
agree with Alan that 'foreach' would have been a good name for this type of
loop. I need to sort out 'loop structures' in my notes and read up on
these for sure.
Thanks,
Patty
----- Original Message -----
From: "Dave Angel" <da...@ieee.org>
To: "Patty" <pa...@cruzio.com>
Cc: <tutor@python.org>
Sent: Monday, January 03, 2011 9:52 AM
Subject: Re: [Tutor] How does it work?
On 01/-10/-28163 02:59 PM, Patty wrote:
<snip>
for c in 'abcd':
......
When I first looked at this - I thought that the variable 'c' would
have had to be initialized
first earlier in the program. And I thought that the programmer would
input a single char or a single space. I wasn't thinking counting
variable for some reason. So is 'for c in', one or more of key words
that Python understands that a loop is here and to actually trigger
counting in a loop? Does it start with a 1 or 0 internally to do this?
There's not necessarily any integer indexing going on. for xxx in yyy
is a specific syntax that specifies a loop based on a sequence. In this
case the string 'abcd' is a sequence of chars, so you get c to have a
value of 'a', 'b', 'c', and 'd'. But other sequences might not have any
index meaning at all, so it doesn't make sense to ask if it's 0 or 1
based. For example, you can iterate over the keys of a map, where the
order is irrelevant. All you really know is you'll get them all if you
finish the loop.
I also realized a mistake I may have made - maybe I confused 'for c
in' with 'while c in'.
r=''
c="d"
while c in 'abcd':
r=c+r
You messed up the indentation. But if the r=c+r line is indented, then
this is an infinite loop. It'll quit when memory is exhausted. Why?
Because nothing in the loop changes c. So since it's in the sequence at
the beginning, it will always be.
Or
r=''
c="d"
while c in ['a', 'b', 'c', 'd']:
r=c+r
Similarly here.
Also for myself I think I would have used different descriptive names
for variables so that would have been less likely to throw myself off.
Good idea.
And also doing this fast to see if I have learned.
I really feel comfortable with Python now after six months and my
small application is completed and looks really good. I don't know how
I would have been able to make the simplest, smallest GUI part of my
application - using Tkinter and PIL- fast without the references and
explanations of Wayne Werner and Alan - Thanks to those on the list
who are so helpful!
Patty
HTH
DaveA
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