[Tutor] Language truce

[Danny Yoo]

On Thu, 28 Jun 2001, Michael Powe wrote:

> >>>>> "Danny" == Danny Yoo <dyoo@hkn.eecs.berkeley.edu> writes:
> 
>     Danny> On Thu, 28 Jun 2001, Michael Powe wrote:
> 
>     >> A couple things I mentioned, like you can't increment through
>     >> loops.
> 
>     Danny> I'm a little confused by what you mean by "incrementing
>     Danny> through loops".  Can you give an example of what you mean?
> 
> The standard C (for i=0; i < n; i++), which uses the increment
> operator (i++) which is not present in python.  I'm sure there must be
> some discussion somewhere about why the decision was made to leave
> this out, I just have not come across it.
> 
> Yes, as someone else points out, you can use range(), but obviously
> that is not the same thing. (In some cases, it may be functionally

You're right that converting C's:

    for (i = 0; i < n ; i++) { ... }

isn't quite the same as Python's

    for i in range(n): ...

but it comes pretty close --- the extra thing that Python is doing is
actually constructing a list 'n' elements wide before going looping.

One reason that 'for/in' might be preferred is because we are guaranteed
that the loop will always terminate: there can only be a finite number of
elements in a list.  That's one big advantage of a loop construct that
goes along elements --- theoretically, we'll never get into infinite
loops.

On the other hand, there's a little bit of inefficiency due to actually
constructing that n-element list.  If we're just constructing the list,
and then just picking out the numbers in order "1...2...3...", it seems a
little wasteful to make that whole list.

In this case, Python provides an "xrange" function ("extended" range).  
It looks very much like range(), but it actually doesn't construct a
list.  So:

    for i in xrange(n):

works exactly like the C equivalent, without the messy syntax... *grin*



> equivalent and maybe that is the point.)  Also, maybe it's just me,
> but I consider
> 
> k = k+1
> 
> as a loop counter to just be unacceptably clumsy.  Forcing me to use
> that construct tells me that I should not be designing my program that
> way. 

Very true, but there are many ways to avoid the explicit maintenance of a
loop counter.  range/xrange is one way to avoid loop counters.  Another
function that Python provides is called the 'map' function.  map is very
neat, but we'll need an example to shows why:


###
>>> messy_strings = ['    this string has leading spaces     ',
...                  'and trailing spaces too        ',
...                  '    we can use map to strip all the strings   ',
...                  'in one shot!']              
>>> import string
>>> clean_strings = map(string.strip, messy_strings)
>>> clean_strings
['this string has leading spaces', 'and trailing spaces too',
 'we can use map to strip all the strings', 'in one shot!']
###

The cleanest code is that which doesn't exist.  *grin*



> My 'teach-yourself-python' project is an address book.  I happen to
> need one and I don't like anything that's already available.
> Unfortunately, the python books just aren't making it as instructional
> tools.  I know quite a bit about python syntax &c and practically
> nothing about actually writing programs in python.  Anyway, I have the
> data entry part and the write-to-file parts completed -- that was
> easy.  But, the file-reading and data storage parts are more
> interesting.  In C, I would just use a struct and a linked list and
> write/read the structs to/from a binary file -- I've done all that
> type of stuff before.

It's nice to see a fellow C programmer here!  Yeah, it will take a while
to get used to Python's way of doing things, but it's very much worth it
--- I've found that my own C programs look a lot better after learning
Python.


To save your address book to disk, you might be interested in the 'pickle'
module: it handles writing structures directly to disk and back, so it
makes saving most things ridiculously easy.

    http://python.org/doc/lib/module-pickle.html


We're here, so if you have any questions, feel free to ask them.  If you
want to express your questions in C terms, that's ok too.


Good luck!