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How is the performance of Position[] ..


I've got a problem, see anyone can shed some light.
I have an array (a list of sublists) with the following structure,

ARRAY = { { {a11a,a11b}, a12, ...},
	  { {a21a,a21b}, a22, ...},
	  { {a31a,a31b}, a32, ...},
		:
		:
	  { {am1a,am2b}, am2, ...}}
          |<-----   n elements  ----->|

Thus I have a m by n matrix with the first element in each sublist as 
a 2-element list.  My question is if I know a particular pair of this
first element eg. {1000,2000}, how will be the computation of Position[]
if I want to use this as a key to extract the row number where
{1000,2000} situates in ARRAY.

If anyone know of any formal literature (maybe Mathematica technical
report), I would be very appreciated.

Thanks
Joseph



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