How to delete a submatrix from a matrix using the ZEROS command instead of [ ] square brackets?

I have a 3x3 matrix i.e. A=[5,2,6; -1,3,4; 0,5,3] and if I want to delete my middle row, how do I delete that without using the square brackets? I realized zeros=(0) is the equivalent of a square bracket. But when I tried to substitute the second script i.e.

>> B=zeros(0)
B =
     []

>> A(2,1:3) = B

and I get an error message of "Subscripted assignment dimension mismatch."

Answer 1

[Matt J]

   A=A([1,3],:)

[Matt J]

this is the question for our class: Q1: Delete a submatrix from a matrix using the command zeros (instead of [ ]).

Is this a smart-aleck answer? :)

   B=A(1:length(zeros(1,2)):3,:)

[James Tursa]

A = A([1 3],cumsum(zeros(1,3).^zeros(1,3)))

Answer 2

[Kirby Fears]

Hi Kevin,

Using the square bracket assignment is the standard way to delete a row or column. I'm curious why you're looking for another method.

Anyway, you can avoid the square brackets this way:

 m=magic(3);
 m=m([1 3:end],:);

With a larger matrix m, you can delete the kth row (where k is not the first or last row) as follows:

 m=m([1:k-1 k+1:end],:);

The typical square brackets method is easier to write:

 m=magic(3);
 m(2,:)=[];

Hope this helps.

[Kevin Lai]

Hi Kirby Fears, I'm really specifically looking for a way to use the ZEROS command because it is posed to our class and no one has any idea on how to solve it

[Kirby Fears]

As you pointed out, zeros(0) does not work properly. Could you post the exact question?

[Kevin Lai]

this is the question for our class: Q1: Delete a submatrix from a matrix using the command zeros (instead of [ ]).

[arich82]

The following is a stupid, stupid way of doing things, but if you insist on interpreting the question literally, you could wrap a call to zeros inside of a call to logical to create a mask:

 >> A = [...
          5, 2, 6; ...
         -1, 3, 4; ...
          0, 5, 3; ...
         ];

 >> A(logical([1, zeros(1), 1]), :)

 ans =

       5     2     6
       0     5     3

My guess, however, is that James Tursa's interpretation is the correct one, and that "delete" really implies "set to zero":

>> A(2, :) = zeros(size(A(2, :)))

   A =

       5     2     6
       0     0     0
       0     5     3

Note that, in general, you cannot "delete" an arbitrary submatrix from a matrix and still be left with a matrix; it only works if you delete (multiples of) full rows xor full columns. You can, however, set arbitrary submatrices to zero:

>> B = reshape(1:9, 3, 3)

B =

       1     4     7
       2     5     8
       3     6     9

>> B(1:2, 1:2) = []
Subscripted assignment dimension mismatch.

>> B(1:2, 1:2) = zeros(2, 2)

B =

       0     0     7
       0     0     8
       3     6     9

You can apply a mask to delete an arbitrary submatrix, but the result is a linear-indexed vector:

>> C = reshape(1:9, 3, 3)

C =

       1     4     7
       2     5     8
       3     6     9

>> (logical([zeros(2, 2), ones(2, 1); ones(1, 3)]))

ans =

       0     0     1
       0     0     1
       1     1     1

>> C(logical([zeros(2, 2), ones(2, 1); ones(1, 3)]))

ans =

       3
       6
       7
       8
       9

Answer 3

[Matt J]

I'm guessing you've been asked not to remove the row, but rather to set all the row's elements to zero. Otherwise, I can't see what motivates the use of the ZEROS command specifically. It is possible to set the middle row to zero using the ZEROS command as follows,

   A(2,:)=zeros(1,3);

but even that is very artificial and unnecessary. You could simply do,

   A(2,:)=0;

Answer 4

[James Tursa]

Using explicit brackets [ ] on the right hand side of an equation combined with using indexing on the left hand side variable is a special MATLAB syntax that means "remove elements". It is NOT the same thing as an assignment where you have a variable on the right hand side. E.g.,

A = [5,2,6; -1,3,4; 0,5,3];
A(2,1:3) = zeros(0); % <-- This is an assignment with a variable on the rhs (generates an error)
A(2,1:3) = []; % <-- This is a special syntax that tells MATLAB you want to remove elements

In the last code line above, the [ ] on the right hand side combined with indexing on the left hand side is an instruction to the MATLAB parser that you are using a special syntax to remove elements from the left hand side variable. It is NOT treated the same as using zeros(0) on the right hand side (or using a variable that was set to zeros(0)). You need to type the brackets [ ] explicitly to get the special treatment.

If you are simply looking for another coding way to eliminate some elements that mimics the use of [ ] on the rhs, then see Matt J's first answer.