Getting error while deleting every row of the matrix M that contains the variable x

I'm trying to write a function N = Deletevar(x,M) that returns a matrix N by deleting every row of the matrix M that contains the variable x.

My code is :

function N = Delete_var(x,M) 
    N = [];
    [a,b] = size(M);
    for i = 1:a
        for j = 1:b
            if M(i,j)== x
                M(i,:)=[];
                N = M;
            end
        end
    end
end

However when i call Delete_var(3,[1,2,3;3,4,5;7,8,9]), I get:

Index in position 1 exceeds array bounds (must not exceed 2).

Error in Delete_var (line 9)

if M(i,j)== x

Why is that? How to solve the problem? Please help!

[David Fletcher]

The inherent problem you have with the logic of your code is that you are basing the for loops on the starting size of the array (M). Consider what happens when you delete a row in M - the row dimension becomes smaller, but this is not reflected in the for loop which is still based on the starting size of the array. The code will always ultimately throw an error (unless no rows are deleted) since there will be a mismatch between the starting size of M and its size after rows are deleted.

[Ningze Xia]

Ohhhh I understand what you mean! Combined your comment and per isakson's answer I successfully solve the problem! Thank you for your help!

Answer 1

[per isakson]

The trick is to iterate in reverse order, i.e a:-1:1 instead of 1:a. Try this

%%
M = magic(5);
x = 13;
N = Delete_var(x,M)

N = 4×5

17 24 1 8 15 23 5 7 14 16 10 12 19 21 3 11 18 25 2 9

%%
function  N = Delete_var(x,M) 
    N = [];
    [a,b] = size(M);
    for i = a:-1:1
        for j = b:-1:1
            if M(i,j)== x
                M(i,:)=[];
                N = M;
            end
        end
    end
end

[Ningze Xia]

Perfectly solve my problem! Thanks a lot!

Answer 2

[Turlough Hughes]

No need for any loops. Here's an example of data to work with

 M = randi(10,[10 3]);  % sample data
 x = 3;                 % delete rows with this value

You can delete any rows in M containing x as follows:

M(any(M==x,2),:)=[];

[Ningze Xia]

Thank you for your answer! This is a new techinique for me and it's very useful! Really appreciate!