How know the size of matrix after delete raws

What function do you use to know the size of the array after one or more rows are removed after a condition is achieved within a loop

[dpb]

size, maybe???

Answer 1

[dpb]

"I want to delete all rows that start with an element greater than 5 in the matrix"

In Matlab, use "logical addressing" for such problems...

>> s(s(:,1)>5,:)=[]
s =
   2    98    52    41    63
   4     5     2     6     9
>>

Answer 2

[Shweta Singh]

You can use the 'size' function as follows:

matrix_example = rand(7,8); %matrix of size 7x8
[matrix_x, matrix_y] = size(matrix_example); 

% Delete a row
matrix_example(3,:) = [];

% Get the updated dimensions
[matrix_x, matrix_y] = size(matrix_example);

Hope this helps!

Answer 3

[asma khaled]

But if I place this code inside a loop for 1 to the number of rows of matrix, a error message will appear because the size of the array has changed.

[Shweta Singh]

That should be a warning and not an error. Can you share your code?

Answer 4

[asma khaled]

 s=[10 20 30 50 60;80 90 40 10 60;2 98 52 41 63;4 5 2 6 9];
[count,count1]=size(s);
for i=1:count
   if (s(i,1)>5)
    s(i,:)=[];
     [count,count1]=size(s); 
end

    end

error:Index exceeds matrix dimensions.

[dpb]

Not because of size, though, it's because you've removed too many elements and the loop index i is greater than the number of elements left in the first dimension...

>> s(i,1)
Index exceeds matrix dimensions. 
>> i
i =
   4
>> size(s)
ans =
   3     5
>>

Are trying to actually do something specific here or just learning and "playing around"?

[dpb]

Answer moved to comment -- Asma; use the Comment button to add to Q?, not Answer -- dpb

I want to delete all rows that start with an element greater than 5 in the matrix, s(i,:)=[]; to delete the raw. then the size of the matrix changes so I placed [count,count1]=size(s); But why this code does not work.I do not understand.

[dpb]

Because when you got to the loop index with i = 4, you've already removed more than that number of rows so when you write

s(i,1)

where i==4 and there are only three rows left, the array addressing is outside the array dimensions and that causes the error.

To do something like this in a loop, you would want to start at the end of the array and work backwards so that the loop index is decreasing at at least the same rate as the size of the array--

nr=size(s,1);                    % initial number rows in s
for i=nr:-1:1                    % start from end, work to beginning...
  if s(i,1)>5, s(i,:)=[]; end    % eliminate if desired.
end

By going backwards, you always have at least as many rows left in the array as the index value because you've removed only those of higher initial position previously for the next iteration.

BTW, don't feel badly; this is a general precept that every beginning programmer has had to learn the hard way first... :)

[dpb]

Again, move to Comment...-dpb

Thank you so much for your help