[Tutor] Write array to Status text

Danny Yoo dyoo at hkn.eecs.berkeley.edu
Tue Nov 18 15:05:09 EST 2003

> > A Python 'List' is a collection that can dynamically grow in size, and
> > that's probably what you want to use for general use.  We can
> > construct lists by using '[]'.
> >
> > An 'array' is meant to be a statically-sized homogenous collection of
> > primitive objects --- like numbers --- and it has a very very
> > specialized purpose that most folks won't need to touch.  I guess I'm
> > trying to say politely: don't use them now!  *grin*
> Funny thing though, my application is just that. There are exactly 4
> entries and all are hexadecimal values.

Hi Vicki,

Hmmm!  Out of curiosity, what are some examples of StatusText that the
code wants to display to the user?  I just want to understand the context
of this problem better.  You've mentioned that you're getting hexadecimal
values back, so can I assume you're getting some character string from a
hardware device?

> The 10 in my array initialization was just to make sure I didn't exceed
> my array boundaries. I changed it to 4 later. It is a far more
> comfortable thing for me since I used them in C, and it allows me the
> convenience of the tostring functionality.

Sure.  We can also pre-size a Python list:

>>> l = [0]*4
>>> l
[0, 0, 0, 0]
>>> l[2] = 0x34
>>> l
[0, 0, 52, 0]

and if we never do an append() on a list, then it behaves almost exactly
like a standard C array.  Indicing and index assignment should work
exactly how you'd expect in C.

(If you've used C++ before, you can think of a Python list as a "vector",
where we can push back (pushback()) elements at the end of the container
with relative ease.  Doing the same thing with an array alone won't work,
because arrays are fixed-sized containers.)

Lists don't have tostring().  On the other hand, they do have quite a few
things that arrays don't have:

>>> dir([])
['__add__', '__class__', '__contains__', '__delattr__', '__delitem__',
'__delslice__', '__doc__', '__eq__', '__ge__', '__getattribute__',
'__getitem__', '__getslice__', '__gt__', '__hash__', '__iadd__',
'__imul__', '__init__', '__le__', '__len__', '__lt__', '__mul__',
'__ne__', '__new__', '__reduce__', '__repr__', '__rmul__', '__setattr__',
'__setitem__', '__setslice__', '__str__', 'append', 'count', 'extend',
'index', 'insert', 'pop', 'remove', 'reverse', 'sort']

As Python's generic container class, a list can do a lot of stuff,
including sort()s and arbitrary insert()ion.  And a key advantage of a
list is that we don't have to hardcode its size:

>>> l = []
>>> while 1:
...     text = raw_input('next? ')
...     if not text: break
...     l.append(int(text, 16))
next? 0x32
next? 0x16
next? 0x42
next? 0x7
next? 0xdeadbeef
>>> l
[50, 22, 66, 7, 3735928559L]
>>> l2 = [None] * 2
>>> l2[0] = 'hello'
>>> l2[1] = 'world'
>>> l2
['hello', 'world']
>>> ':'.join(l2)

The last example shows that if our list contains only strings, then it's
fairly easy to join them all together into a single string.

> But having said that, I will use either if I can make the StatusText
> thing work. It is such a minimal part of the project that I really don't
> care which I use. It seems really klunky to have to do a join each time
> I add a value to my array (list), but if that is what it takes, I'll do
> it.

Without knowing more about the context of the program, I can't say for
certain that the 'array' module is not what you need.  You mentioned that
you needed the array's tostring() method, so perhaps arrays will be a win
here.  I won't push too hard on forcing you to use lists.  *grin*

But when you have the time, I'd recommend looking more into Python lists,
because the majority of the Python code you'll see will use lists as a
fundamental type.  If you'd like more information about them, you can take
a look here:


Good luck to you!

More information about the Tutor mailing list