| Re: dynamic array workaround? [message #45197 is a reply to message #45098] |
Sun, 14 August 2005 19:09  |
Robert Moss
Messages: 74
Registered: February 1996
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Member |
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David Fanning wrote:
>
>
> I'd probably use a pointer to do this:
>
> IF Ptr_Valid(ptr) THEN ptr = Ptr_New([mylist]) ELSE $
> *ptr = [Temporary(*ptr), mynewList]
>
> finalList = Temporary(*ptr)
>
>
You should be careful if your number of concatenations become large.
Using the method David suggests is fine for small n (I use it
frequently), but if (say) you wanted to do it 90,000 times, the overhead
becomes prohibitive. For example, the test program at the end of this
message produced the following results:
% Compiled module: DELETEME.
IDL> deleteme
method 1 time elapsed = 17.2340002060
method 2 time elapsed = 0.0159997940
Method 1 was using David's method (again, perfectly fine for small n),
and method 2 simply guesses at a large number of elements and counts the
ones being used. This is much, much faster. If you really have no idea
how big to guess, you could use CATCH to extend the array by another
large chunk if it fills up.
Robert Moss, Ph.D.
pro deleteme
compile_opt idl2
t0 = systime( /sec )
for i = 0, 90000 do begin
IF ~Ptr_Valid(ptr) THEN ptr = Ptr_New([ i ]) ELSE $
*ptr = [ Temporary(*ptr), i ]
endfor
finalList = Temporary(*ptr)
print, "method 1 time elapsed = ", systime( /sec ) - t0, format =
'(%"%s",D13.10)'
PTR_FREE, ptr
t1 = systime( /sec )
finalList = intarr( 200000, /nozero )
count = 0
for i = 0, 90000 do begin
finalList[ i ] = i
count++
endfor
finaList = finalList[ 0:count-1 ]
print, "method 2 time elapsed = ", systime( /sec ) - t1, format =
'(%"%s",D13.10)'
end
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