Efficient way to pick/delete a list of rows/columns in a matrix in Mathematica

Question

This question is in a way a continuation of the question I asked here:Simple way to delete a matrix column in Mathematica to which @belisarius and @Daniel provided very help

Answer 1

You can also use explicit ranges in a way that is fairly efficient. They may provide some more flexibility. Here is your example.

a = RandomInteger[1000, {5000, 5000}];

Timing[b = Drop[a, {101}, {101}];]

Out[66]= {0.041993, Null}

Timing[
  c = a[[Join[Range[100], Range[102, 5000]], 
   Join[Range[100], Range[102, 5000]]]];]

Out[67]= {0.061991, Null}

c == b

Out[62]= True

I would also suggest use of Span except offhand I do not see how to get it to work in this setting.

Daniel Lichtblau Wolfram Research