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Re: Deleting rows in matrices

From: CdeMills
Subject: Re: Deleting rows in matrices
Date: Tue, 13 Sep 2011 05:49:08 -0700 (PDT)

MMolloy wrote:
> Hi,
> I am having (another!) problem on a seemingly innocuous task. Say I
> have many (le x 1) matrices, A, B, C, ......, all of the same length
> "le". I want to check each of the elements in "A", and if an element
> exceeds a limit I want to delete that row, plus the corresponding rows
> in B, C, ...., . Here's my try..
> le = length(A)
> for i = 1:le
> if (A(i,1) > 5)
> A(i,:) = [];
> B(i,:) = [];
> C(i,:) = [];
> .....
> endif
> endfor
>  But I keep getting errors that say the index exceeds the matrix
> dimension - I understand why, it's obvious, but I can't think of
> another method to do this!

The problem is rather simple. You initialise le with the length of A, then
delete rows. This change A size, but this is not reflected in le. Apart from
the obvious solution given, you can rewrite your loop as

le = 1; while le <= length(A)
if (A(le, 1) > 5)
  A(le, :) = [];
  le = le + 1;

This way:
- if you delete a row, you don't touch le, because it's the row number of
the NEXT untested line after deletion
- otherwise, you use standard increment
In both cases, le is tested against the length of A at each iteration. This
way, you can't access unexistent lines.



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