Re: how to pick out alternating columns and rows in a matrix?

• To: mathgroup at christensen.cybernetics.net
• Subject: [mg588] Re: [mg551] how to pick out alternating columns and rows in a matrix?
• From: Count Dracula <lk3a at kelvin.seas.virginia.edu>
• Date: Wed, 22 Mar 1995 08:49:22 -0500

```Here's a definition for a function to select odd(default) or even (with
sel = EvenQ) rows and columns of a matrix:

altrowcol[(m_)?MatrixQ, sel_:OddQ] := m[[Apply[Sequence, (Select[Range[#1], sel] & ) /@ Dimensions[u]]]]

Example usage:

In[1]:= u = Array[a, {6, 7}];

In[2]:= altrowcol[u]//MatrixForm

Out[2]//MatrixForm= a[1, 1]   a[1, 3]   a[1, 5]   a[1, 7]

a[3, 1]   a[3, 3]   a[3, 5]   a[3, 7]

a[5, 1]   a[5, 3]   a[5, 5]   a[5, 7]

In[3]:= altrowcol[u, EvenQ] //MatrixForm

Out[3]//MatrixForm= a[2, 2]   a[2, 4]   a[2, 6]

a[4, 2]   a[4, 4]   a[4, 6]

a[6, 2]   a[6, 4]   a[6, 6]

___________________________________________________________________________________
Levent Kitis           lk3a at cars.mech.virginia.edu    lk3a at kelvin.seas.virginia.edu
University of Virginia  Department of Mechanical, Aerospace and Nuclear Engineering
___________________________________________________________________________________

```

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