Counting number of numbers in a large list between two valus
- To: mathgroup at smc.vnet.net
- Subject: [mg114499] Counting number of numbers in a large list between two valus
- From: Lyle <lgordon at gmail.com>
- Date: Sun, 5 Dec 2010 21:57:05 -0500 (EST)
Dear Listers,
I have a large (5-20million) one dimensional list of real numbers and
I want to count the number of entries in the list that lie between 2
specific values (x1, x2). I need to run the function for a number of
different ranges.
ie. number of list entries (l), where x1 <= l <= x2
I've tried:
tallydata[{x1_, x2_}] := Count[data, x_ /; x1 <= x <= x2]
that takes about 3-4 seconds
and
tallydata[{x1_, x2_}] := Length[Select[data, x1 <= # <= x2 &]]
which takes a little bit longer.
The best I've managed is (this last one might be off by 1 or 2 but
this doesn't really matter to me):
sorteddata = Sort[data];
nf = Nearest[sorteddata];
tallyrange[{x1_, x2_}] :=
First[Position[sorteddata, First[nf[x2]]]] -
First[Position[sorteddata, First[nf[x1]]]]
which takes between 1 and 2 seconds but I was hoping there might be a
faster way to do this?
Any help would be great!
Thanks,
Lyle Gordon
Northwestern University