6
$\begingroup$

Delete is not suitable to delete the rows of a matrix, as illustrated below:

  SeedRandom[0];
; r = 100
; rows = RandomReal[{0, 1}, {r, 2}]
; toDrop = Select[Range[r], PrimeQ]
; reducedRows = Delete[rows, toDrop]

Mathematica graphics

I know that I can always do something like

  toKeep = Complement[Range[r], toDrop]
; reducedRows = rows[[toKeep]]

...but this strikes me as inefficient, at least whenever toKeep is large (IOW, whenever r is much greater than Length[toDrop]).

Is there any other built-in that achieves what Delete[rows, toDrop] aspires to?

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7
  • 1
    $\begingroup$ toDrop = Transpose[{Select[Range[r], PrimeQ]}] $\endgroup$
    – ciao
    Commented May 20, 2016 at 19:04
  • 6
    $\begingroup$ Have you tried Delete[rows, List /@ toDrop]? $\endgroup$ Commented May 20, 2016 at 19:10
  • $\begingroup$ Despite acknowledgement of Complement I feel that this problem is already described in other questions: (17002), (20228), (43785), (108336). Note that the answers below largely duplicate methods already given in answer to these. I shall probably mark this as a duplicate unless someone makes a compelling argument against that. $\endgroup$
    – Mr.Wizard
    Commented May 20, 2016 at 20:26
  • $\begingroup$ Also related (StackOverflow): Efficient way to pick/delete a list of rows/columns in a matrix in Mathematica $\endgroup$
    – user1066
    Commented May 21, 2016 at 14:29
  • $\begingroup$ @J.M.: Would you kindly post your comment as an answer? $\endgroup$
    – kjo
    Commented May 21, 2016 at 17:19

3 Answers 3

8
$\begingroup$

Make the to-be-dropped rows vanish:

rows[[toDrop]] = ##&[]

or as a function:

f = Module[{m = #, d = #2}, m[[d]] = ##&[]; m] &;

f[rows, toDrop]

Some timings

ClearAll[f0, f1, f2, f3, f4, f5]
f0 = ReplacePart[#, Transpose[{#2}] -> Nothing[]]&; (* ciao's answer *)
f1 = ReplacePart[#, Thread[#2 -> Sequence[]]] &; (* v9 version of ciao's answer*)
f2 = Module[{m = #, d = #2}, m[[d]] = ## &[]; m] &;
f3 = #[[Complement[Range@Length@#, #2]]] &; (* from george2079's deleted answer*)
f4 = Delete[#, Transpose[{#2}]] &; (* from ciao's comment *)
f5 = Delete[#, List /@ #2] &; (* from J.M.'s comment *)


SeedRandom[0];
r = 1000000;
rows = RandomReal[{0, 1}, {r, 2}];
toDrop = Select[Range[r], PrimeQ];
{HoldForm[#], First[AbsoluteTiming[#[rows, toDrop]]]} & /@ {f0, f1, f2, f3, f4, f5} // Grid

Mathematica graphics

Equal @@ (#[rows, toDrop] & /@ {f0, f1, f2, f3, f4, f5})

True

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0
10
$\begingroup$

Probably the simplest way will be:

reducedRows = ReplacePart[rows, Transpose[{toDrop}] -> Nothing[]]
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5
  • $\begingroup$ I believe you need to do List /@ toDrop -> Nothing[]. (Cant verify with 10.1 though) $\endgroup$
    – george2079
    Commented May 20, 2016 at 20:01
  • 1
    $\begingroup$ @george2079 - oops, copied wrong - fixed (Transpose faster than map List) $\endgroup$
    – ciao
    Commented May 20, 2016 at 20:05
  • 3
    $\begingroup$ (+1) because I've learned Nothing. $\endgroup$
    – Jens
    Commented May 20, 2016 at 21:25
  • $\begingroup$ I didn't know Nothing as well, I was using ##&[] instead. I'll do a find and replace in all my code ! $\endgroup$
    – faysou
    Commented May 21, 2016 at 13:45
  • 2
    $\begingroup$ I fixed the Transpose correction. Also you don't need Nothing[], Nothing only works. $\endgroup$
    – faysou
    Commented May 21, 2016 at 13:54
2
$\begingroup$

A solution that keeps instead of drops.

toKeep = Select[Range[r], PrimeQ /* Not];
reducedRows = rows[[toKeep]];

Hope this helps.

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