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From: Jacob Sparre Andersen <sparre@nbi.dk>
Newsgroups: comp.lang.ada
Subject: Re: Any easy/build-in construct to extract submatrices from larger
 matrix?
Date: Tue, 26 Jun 2012 09:15:39 +0200
Organization: Jacob Sparre Andersen Research & Innovation
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Date: 2012-06-26T09:15:39+02:00
List-Id: <comp.lang.ada>

Nasser M. Abbasi wrote:
> J-P. Rosen wrote:

>>   function Exclude (A : Complex_Matrix; I, J : Integer)
>>       return Complex_Matrix is
>>       AI, AJ : Integer := A'First (1);

I suspect a compiler would advise that the above line is changed to:

         BI, BJ : Integer;

>>   begin
>>       return B : Complex_Matrix (1..A'Length (1)-1, 1..A'Length (2)-1) do
>>          BI := 1;
>>          for AI in A'First (1) .. I-1 loop
>>             BJ := 1;
>>             for AJ in A'First (2) .. J-1 loop
>>                B (BI, BJ) := A (AI, AJ);
>>                BJ := BJ + 1;
>>             end loop;
>>
>>             for AJ in J+1 .. A'Last (2) loop
>>                B (BI, BJ) := A (AI, AJ);
>>                BJ := BJ + 1;
>>             end loop;
>>             BI := BI + 1;
>>          end loop;
>>
>>          for AI in I+1 .. A'Last (1) loop
>>             BJ := 1;
>>             for AJ in A'First (2) .. J-1 loop
>>                B (BI, BJ) := A (AI, AJ);
>>                BJ := BJ + 1;
>>             end loop;
>>
>>             for AJ in J+1 .. A'Last (2) loop
>>                B (BI, BJ) := A (AI, AJ);
>>                BJ := BJ + 1;
>>             end loop;
>>             BI := BI + 1;
>>          end loop;
>>       end return;
>>   end Exclude;

> In Mathematica for example, I do this whole operation in one line:
>
> ---------------------------
> Table[ReplacePart[A,{{i},{i,_},{_,j}}:>Sequence[]],{i,nRow},{j,nCol}];
> ---------------------------

I'm not sure if Mathematica has sufficient syntax highlighting to make
it easy to keep track of the grouping of the various parentheses, but as
it appears here, I would definitely want to break it up a bit, to make
it more readable. - My Mathematica so rusty that even when I do that,
I'm still not quite certain that your function works correctly.

Yes, the Ada solution Jean-Pierre presented is more verbose.  (And yes,
there was an error in it. ;-)

But to an Ada programmer with a bit of experience, the challenging part
is to make sure the indexes aren't mixed up anywhere.  This takes some
time. - But (to me) counting parenteses and making sure they match up
also takes time.  (But probably not quite as much.)

Greetings,

Jacob
-- 
"It ain't rocket science!"