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Re: [Q]: ID analog to FORTRAN "sign" function
Phillip David wrote:
> Dick Jackson wrote:
> > Do I dare offer one more? Subscript lookups seem faster than arithmetic
> > operations, making this one faster, more compact and no less cryptic!
> > Return, Abs(a) * ([-1, 1])[b GE 0]
> to which I reply:
> If this one really works, then why not go even one step further?
> Return, ([-Abs(a), Abs(a)])[b GE 0]
> Return, ([a, -a])[(a*b) LT 0]
Right, these would work fine for scalar a and b, with negligible time taken
in any case. I was looking for the most efficient way when we need this to
work on large arrays a and b.
About the 1 vs 1.0 debate, Mark Hadfield wrote:
> I think that's a little *too* clever. I just tried multiplying a float
> with 10^7 elements by 1 and then by 1.0. Time taken = 0.52 seconds in both
This is getting interesting. For reference, here are a couple of handy timer
routines I use:
PRO TStart ; Timer Start
; Save current time for use by TReport
COMMON Timer, t0
t0 = SysTime(1)
PRO TReport ; Timer Report
; Print elapsed time since last TStart
COMMON Timer, t0
Print, Format='(D10.3," seconds.")',SysTime(1)-t0
Here's some testing runs from my Win2000 PC:
IDL> tstart & for i=1,10 do c=Abs(a) * ([-1, 1])[b GE 0] & treport
IDL> tstart & for i=1,10 do c1=Abs(a) * ([-1.0, 1.0])[b GE 0] & treport
I think the time saving here is not in the multiplying itself, but in the
time building an integer array, then converting it to float. In this case
it's 10^6 ones/minus-ones, perhaps in your case it was converting only a
single 1 to 1.0, then multiplying it.
Fascinating, isn't it? I'd be happy to hear further refinements!
Dick Jackson / firstname.lastname@example.org
D-Jackson Software Consulting / http://www.d-jackson.com
Calgary, Alberta, Canada / Voice/Fax: +1-403-242-7398