I have a matrix with thousands of rows and want the submatrix comprising the rows of the original matrix that have, e.g. a negative element in column 3. How to do that?
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I think this question is reasonably a duplicate of: How to find rows that have maximum value? (or several similar questions) and I will delete this answer if it is closed as such. Nevertheless, again for reference:
SeedRandom[1];
a = RandomInteger[{-3, 5}, {20, 5}]
Pick[a, Negative @ a[[All, 3]]]
{{-2, 1, -3, 4, -3}, {-2, 0, -1, -2, 3}, {1, 0, -3, -2, 0}, {2, 0, -3, 0, -1}, {0, 2, -2, 2, -1}, {0, -2, -3, 1, 1}, {-2, 2, -1, 4, 5}, {0, -1, -2, -2, 3}, {2, 3, -3, 4, -2}, {0, -2, -1, 2, 5}, {1, 4, -3, 4, 4}}
Faster in versions 8+ should be to use UnitStep, due to packed array optimizations:
Pick[a, UnitStep @ a[[All, 3]], 0]
In version 7 optimal speed may be had with:
a[[SparseArray[BitXor[UnitStep @ a[[All, 3]], 1]]["AdjacencyLists"]]]
Timings compared to other methods proposed, in version 7:
a = RandomInteger[{-3, 5}, {1500000, 5}];
Pick[a, Negative @ a[[All, 3]]] // Timing // First
a[[SparseArray[BitXor[UnitStep @ a[[All, 3]], 1]]["AdjacencyLists"]]] // Timing // First
(col3 = a[[All, 3]]; rowsToGet = Flatten[Position[col3, _?((# < 0) &)]]; a[[rowsToGet]]) //
Timing // First
Select[a, #[[3]] < 0 &] // Timing // First
0.234
0.031
0.905
1.17
answered May 20 '13 at 13:03
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$\begingroup$ I'm curious, what computer do you run? For
Pickpicks a second almost on my i7 (+ Win7 and Mma 9). Can I tweak my system any towards the performance of yours? Another slower method, by a factor of 3.5 with my timing, is to:Scanwith levelspec{-2}andSowrows with negative third element. $\endgroup$ – BoLe May 20 '13 at 14:07 -
$\begingroup$ @BoLe I use an i5-2500K running at a maximum (single core) of 4.6 GHz. I know there have been changes to
Picksince version 7 but I thought they would make things faster rather than slower. $\endgroup$ – Mr.Wizard♦ May 20 '13 at 14:50 -
$\begingroup$ @BoLe I added two new methods optimized for speed. Please try both and let me know how they perform. I expect
Pick[a, UnitStep @ a[[All, 3]], 0]to be best on v9. $\endgroup$ – Mr.Wizard♦ May 20 '13 at 14:58 -
$\begingroup$ @Mr.Wizard I get (on v9.0.0) {1.896415, 0.146656, 3.255081, 3.815207} for your 4 tested values and 0.051483 for Pick[a, UnitStep @ a[[All, 3]]] $\endgroup$ – Jacob Akkerboom May 20 '13 at 16:57
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$\begingroup$ @Mr.Wizard It must be clocking and a newer model I guess, mine is a bit old already (2009) and running at 2.93 GHz. My times are
{0.952, 0.0780, 1.84, 2.04}which make approximately same ratios as yours, somewhere 10 : 1 : 25 : 30. P.S.PickwithUnitStepis the fastest here with 0.047. $\endgroup$ – BoLe May 20 '13 at 17:51
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m = RandomInteger[{-1, 1}, {10, 10}];
m1 = Select[m, #[[3]] < 0 &];
Show it:
ArrayPlot[#, ColorRules -> {1 -> Red, 0 -> Blue, -1 -> Yellow, _ -> Gray}] & /@ {m, m1}
answered May 20 '13 at 12:08
