"Joey Wong" <······@ses.curtin.edu.au> writes:
> I'm just starting LISP and was wondering if nested for loops are ok
You can nest loops.
Wait 'til you start nesting functions, though!
Joe Marshall <·········@alum.mit.edu> writes:
> "Joey Wong" <······@ses.curtin.edu.au> writes:
>
> > I'm just starting LISP and was wondering if nested for loops are ok
> You can nest loops.
>
> Wait 'til you start nesting functions, though!
yeah. I was wondering if there's a language that _doesn't_ let you
nest loops. Of course, he could have meant "recursion", and just
expressed it a bit oddly.
dave
In article <··············@alum.mit.edu>,
David Bakhash <·····@alum.mit.edu> wrote:
>Joe Marshall <·········@alum.mit.edu> writes:
>> "Joey Wong" <······@ses.curtin.edu.au> writes:
>>
>> > I'm just starting LISP and was wondering if nested for loops are ok
>> You can nest loops.
>>
>> Wait 'til you start nesting functions, though!
>
>yeah. I was wondering if there's a language that _doesn't_ let you
>nest loops. Of course, he could have meant "recursion", and just
>expressed it a bit oddly.
FORTH is a little weird with regard to its loops; you can nest them as
deep as you like, but you can typically only access the loop counters
of the top two or three loops on the stack. (Well, actually, you can
write nonportable code to grovel through the rstack and find the other
loop counters, but that's beside the point.)
x86 assembler has a LOOP instruction that implicitly decrements CX
(ECX? I don't know 386 asm). You can only nest loops built with these
guys if you go to extra trouble to save and restore CX.
I can't think of any other languages that have restrictions on nesting
loops.
--
<······@pobox.com> Kragen Sitaker <http://www.pobox.com/~kragen/>
The Internet stock bubble didn't burst on 1999-11-08. Hurrah!
<URL:http://www.pobox.com/~kragen/bubble.html>
The power didn't go out on 2000-01-01 either. :)
> I'm just starting LISP and was wondering if nested for loops are ok
What is it that lead you to believe you needed to ask this
question? Do you find something strange or suspect about
nested loops? And which lisp forms are you referring to
specifically?
C/
Centuries ago, Nostradamus foresaw a time when Joey Wong would say:
>I'm just starting LISP and was wondering if nested for loops are ok
Several answers:
a) FOR is neither a macro nor a function, so there's no such thing as
a "for loop."
b) Using a nested LOOP _may_ be indicative that you didn't use LOOP
properly, as you can put almost infinite complexity into a single
LOOP.
c) A nested loop _may_ be indicative of questionable factoring.
Functions that nest loops are pretty much guaranteed to be complex,
and thus may indicate that you should have created a "helper
function."
[To the picky, I said "may indicate" as opposed to "always
indicates".]
d) Aside from those other issues, nothing technical prevents you from
having control structures nested 45 levels deep.
--
Have you heard of the new Macsyma processor? It has three instructions --
LOAD, STORE, and SKIP IF INTEGRABLE.
········@ntlug.org- <http://www.ntlug.org/~cbbrowne/lsf.html>
In article <·····················@news5.giganews.com>,
Christopher Browne <········@hex.net> wrote:
>b) Using a nested LOOP _may_ be indicative that you didn't use LOOP
>properly, as you can put almost infinite complexity into a single
>LOOP.
I suppose this is possible, but I don't think it's likely. Nested loops
are extremely common whenever you're traversing multi-dimensional
structures, e.g. a 2-d array with
(loop for i upto length
do
(loop for j upto width
do
(do-something-with (aref a i j))))
or a list of lists with:
(loop for inner in outer
do
(loop for item in inner
do
(do-something-with item)))
LOOP has quite a bit of complexity, but it doesn't actually have any way to
do either of these easily in a single loop (well, in the first case you
could do it with ROW-MAJOR-AREF, but that's not really a LOOP feature,
that's a special array feature).
--
Barry Margolin, ······@genuity.net
Genuity, Burlington, MA
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