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Need to Speed up Position[]
*To*: mathgroup at smc.vnet.net
*Subject*: [mg110671] Need to Speed up Position[]
*From*: Garapata <warsaw95826 at mypacks.net>
*Date*: Thu, 1 Jul 2010 08:28:20 -0400 (EDT)
I have a large nested list, "myList"
It has 3 sublists with the following dimensions:
Dimensions/@ myList
{{19808, 5}, {7952, 5}, {7952, 5}}
The 5th position (i.e., column) in each of the sublists has
SQLDateTime[]s
(This may or may not affect what I need, but I thought everyone should
know).
myIntersection = Intersection @@ (myList[[All, All, 5]]);
gives me the SQLDateTimes[]s common to all sublists. I get 3954
common elements.
Length[myIntersection]
3954
All of the above works great and runs very fast.
I then find the positions in myList where all the common
SQLDateTimes[]s occur and then use Extract pull them out into a new
list
myPositions = Drop[(Position[data, #] & /@ myIntersection), None,
None, -1];
myOutput = Extract[myList, #] & /@ myPositions;
I end up with just what I need, which in this case gives me 3954 rows
of {9, 5} sublists. This occurs because myList[[1]] has 5 occurrences
of each common date element and sublists myList[[2]] and myList[[3]]
each have 2 occurrences of each common date element.
The Extract[] runs very fast.
My problem =85. the Position[] runs very very slow (over 90 seconds on a
dual core iMac).
All the code together:
myIntersection = Intersection @@ (myList[[All, All, 5]]);
myPositions = Drop[(Position[data, #] & /@ myIntersection), None,
None, -1];
myOutput = Extract[myList, #] & /@ myPositions;
So, does anyone know a way to speed up:
myPositions = Drop[(Position[data, #] & /@ myIntersection), None,
None, -1]; ?
Or can anyone suggest another approach to doing this that could run
faster.
Patterns?
ParallelMap?
Parallelize?
Sorting?
Changing SQLDateTimes to DateList[]s before calculating myPositions?
Not clear what to try.
Please advise.
Thanks.
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