| Author |
Message |
Suresh Jeevanandam
Guest
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 Posted:
Thu Aug 19, 2004 9:57 am Post subject:
Getting the first n elements from a list |
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Hi all,
Is there a skill function to get the first n elements of a list.
Thanks in advance,
--
Suresh
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Suresh Jeevanandam
Guest
|
| Posted:
Thu Aug 19, 2004 10:57 am Post subject:
Re: Getting the first n elements from a list |
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Suresh Jeevanandam wrote:
| Quote: | Hi all,
Is there a skill function to get the first n elements of a list.
Thanks in advance,
--
Suresh
|
I could come up with this. Your suggestion to make it more efficient is
most welcome.
procedure( nElements(theList, n)
let( (x)
x = nil
for(i 1 n
x = tconc(x car(theList))
theList = cdr(theList)
)
car(x)
);let
);procedure
regards,
Suresh |
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Andrew Beckett
Guest
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| Posted:
Thu Aug 19, 2004 2:51 pm Post subject:
Re: Getting the first n elements from a list |
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Suresh,
If you don't mind it being destructive, you can use:
procedure(nElements(theList n)
rplacd(nthcdr(sub1(n) theList) nil)
theList
)
You could use a little trick to destructively modify it, copy, and then
modify back again:
procedure(nElements(theList n)
let((tail tailcdr)
tail=nthcdr(sub1(n) theList)
tailcdr=cdr(tail)
rplacd(tail nil)
prog1(
copy(theList)
rplacd(tail tailcdr)
)
)
)
However, I don't think this is any more efficient than what you're doing. It
may even be a little less efficient because it traverses the list once in the
nthcdr, and again in the copy (just the first n elements).
The function below may be a little more efficient, because you're
using a built-in loop (forall) to do the loop traversal:
procedure(nElements(theList n)
let((new)
forall(elem theList n-->0 && (new=tconc(new elem)))
car(new)
)
)
Note - in your code, there's no need to initialise x to nil, since that is the
default value for anything in a let().
Probably I'd do some profiling to be certain... just wanted to give a few
options for the benefit of you and the audience ;-)
Finally, why are you getting the first n elements in a list? Are you approaching
the bigger problem in the right way?
Andrew.
On Thu, 19 Aug 2004 12:15:50 +0530, Suresh Jeevanandam
<sureshj@DELETETHISti.com> wrote:
| Quote: | Suresh Jeevanandam wrote:
Hi all,
Is there a skill function to get the first n elements of a list.
Thanks in advance,
--
Suresh
I could come up with this. Your suggestion to make it more efficient is
most welcome.
procedure( nElements(theList, n)
let( (x)
x = nil
for(i 1 n
x = tconc(x car(theList))
theList = cdr(theList)
)
car(x)
);let
);procedure
regards,
Suresh
|
--
Andrew Beckett
Senior Technical Leader
Custom IC Solutions
Cadence Design Systems Ltd
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Suresh Jeevanandam
Guest
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| Posted:
Fri Aug 20, 2004 7:52 pm Post subject:
Re: Getting the first n elements from a list |
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Andrew Beckett wrote:
[....]
| Quote: |
procedure(nElements(theList n)
let((tail tailcdr)
tail=nthcdr(sub1(n) theList)
tailcdr=cdr(tail)
rplacd(tail nil)
prog1(
copy(theList)
rplacd(tail tailcdr)
)
)
)
However, I don't think this is any more efficient than what you're doing. It
may even be a little less efficient because it traverses the list once in the
nthcdr, and again in the copy (just the first n elements).
Andrew, |
I think the code snippet above would be the most efficient:
Reasons:
1)Locating the tail is done using 'nthcdr'. Mine uses for loop which
would be less efficient.
2) Copy is done by the standard copy function. Mine does copying by calling
| Quote: | x = tconc(x car(theList))
theList = cdr(theList)
n times, which would be less efficient. |
thanks,
Suresh
----
A little experiment on this,
procedure(lrange(num)
;return a list of numbers from 1 to num
let((rlist)
rlist = nil
while(num > 0
rlist = cons(num rlist)
num = num-1
)
rlist
))
a = lrange(100000)
procedure( nElements(theList, n)
let( (x)
x = nil
for(i 1 n
x = tconc(x car(theList))
theList = cdr(theList)
)
car(x)
);let
);procedure
procedure(nElements2(theList n)
let((tail tailcdr)
tail=nthcdr(sub1(n) theList)
tailcdr=cdr(tail)
rplacd(tail nil)
prog1(
copy(theList)
rplacd(tail tailcdr)
)
)
)
x = measureTime(nElements(a 50000))
y = measureTime(nElements2(a 50000)) |
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