[Tutor] Newbie Anxiety

[John Fouhy]

On 11/11/05, Terry Kemmerer <wildcard2005 at comcast.net> wrote:
>  I'm working on Ch. 5, "Fruitful Functions", or "How To Think Like A
> Computer Scientist" and I still can't count.
>  (Don't laugh! I can't play the violin either...)
>
>  In Basic, I would have said:
>
>  10  x = x + 1 :  print x : goto 10
>
>  run
>  1
>  2
>  3
>  etc....
>
>  How is this done in Python? (So I can stop holding my breath as I study
> this great language....and relax.)

Hi Terry,

There's a couple of options.

First, we could do it with a while loop.  This is not the best or the
most idiomatic way, but it's probably most similar to what you've seen
before.

#### count forever
i = 0
while True:
    print i
    i = i + 1
####

Of course, we generally don't want to keep counting forever.  Maybe
we'll count up to 9.

#### count to 9
i = 0
while i < 10::
    print i
    i = i + 1
####

A while loop contains an implicit "GOTO start" at the end.  At the
start, it checks the condition, and breaks out of the loop if the
condition is false.

Like i said, though, this is not idiomatic Python.  Python has for
loops which are based around the idea of iterating over a sequence. 
So, we could count to 9 like this:

#### count to 9
for i in [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]:
    print i
####

The loop will go through the list, assigning each item to i in turn,
until the list is exhausted.  The range() function is useful for
building lists like that, so we don't have to type it out manually.

#### count to 9
for i in range(10):
    print i
####

And, of course, once we've got range(), we can give it a variable
limit (eg, n = 10; range(n)).  Iteration is the key to doing all kinds
of funky stuff in python (including new ideas like geneartor
functions), so it's good to get the hang of :-)

--
John.