[Tutor] skipping ahead within a loop
dkuhlman at rexx.com
Thu Mar 15 17:32:14 CET 2007
On Thu, Mar 15, 2007 at 03:35:27PM +0100, Rikard Bosnjakovic wrote:
> On 3/15/07, Clay Wiedemann <clay.wiedemann at gmail.com> wrote:
> > If doing a loop, how can one skip forward a specific amount randomly
> > determined within the loop?
> y = 0
> while y < HEIGHT:
> linewidth = random(3, 9)
> # drawlines etc
> y += linewidth
> The reason why you cannot alter the for-variable beats me, though.
The while-statement looks like a good solution to me.
Two additional points:
1. You can alter the for-variable, *but* each time you go to the
top of the for-loop, that variable is set to the next item from
the iterator, wiping out any alteration. Because of this, you
can modify and use that variable within the loop body without
having to worry about messing up the loop sequence. What Clay
wants to do is mess with the sequencing of items in the loop.
2. The reason that modifying the for-variable does not change the
sequence of objects processed by the loop is that the
for-statement is generating a sequence of objects from an
iterator. In this case the range function creates a list, which
the for-statement turns into an iterator.
You don't even *want* to give Clay what he is asking for. He asked
about being able to jump forward in the loop or sequence. If you
were able to give him that one, he would come back and ask whether
he could jump *backward* in the loop.
For more on iterators, see "Iterator types" at
http://docs.python.org/lib/typeiter.html. Also see the "iter"
function in "Built-in functions":
You can think of the following:
for item in range(4):
as syntactic sugar for this:
myrange = range(4)
myiter = iter(myrange)
item = myiter.next()
except StopIteration, e:
Hoping this is not TMI (too much information), but sometimes it
helps to understand what is going on underneath, because the for
statement and iterators are very general, powerful,
and elegant features of Python.
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