On Mon, 6 Nov 2006, Chris Hengge wrote:
> I may have just missed the point to your attempt to derail this
> conversation =P
Hi Chris,
Ah! Are we talking about regular expressions anymore?
No?
Good. *grin*
> Why do all that when you can just
>
> str = "Hello World"
> print str * 2
Let me do a more substantial example. As you know, Python's for loop
works on "iterables":
#####################
>>> for x in list("hello"):
... print x
...
h
e
l
l
o
#####################
We're running a for loop across a list of characters. In this case, our
list is iterable. (We could iterate directly on the string itself, but
let's stick with lists for the moment.) An "iterable" in Python is
something that can give us an iterator.
###############################
>>> my_iterator = iter(list("hello"))
###############################
An iterator is something that responses to next() requests until we
consume everything:
#################################
>>> my_iterator.next()
'h'
>>> my_iterator.next()
'e'
>>> my_iterator.next()
'l'
>>> my_iterator.next()
'l'
>>> my_iterator.next()
'o'
>>> my_iterator.next()
Traceback (most recent call last):
File "<stdin>", line 1, in ?
StopIteration
##################################
So there's a few layers of indirection here.
iterable -> iterator -> elements
Ruby takes a different approach: they also have an iterator protocol, but
what they require is something that provides an each() method. For
example:
####################################################
irb(main):002:0> 'hello'.split(//).each {|c| puts c}
h
e
l
l
o
####################################################
This each method takes in a code block, and runs it on every element in
the thing we're iterating across. The iterable itself provides the
looping mechanism instead of the client code.
The concept sorta looks like this in Python:
#######################################################
class MyString:
def __init__(self, s):
self.s = s
def each(self, f):
i = 0
while i < len(self.s):
f(self.s[i])
i = i + 1
MyString("hello").each(lambda c: sys.stdout.write(c + '\n'))
########################################################
This, too, "iterates" across our iterable.
The one handling the control flow is the 'each' method, which is a little
wacky to think about at first. The Ruby folks make this work because
their syntax is custom tailored for this pattern, so it's easier to read
and write.
I'm probably not doing it much justice with my ugly examples. *grin*
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