Delete row from Matrix

I have a Matrix of 400 rows and 40 columns. I want to completely remove rows 3 and 9 to get a matrix with 398 rows. How can I do it in MATLAB.

[Walter Roberson]

See also http://www.mathworks.com/matlabcentral/answers/105768-how-can-i-delete-certain-rows-of-a-matrix-based-on-specific-column-values#answer_226615

[Yuvraj Praveen Soni]

if assume A is your matrix with 400 rows and 40 column,

To remove 3rd row

a(3,:)=[];

now your 9th row become 8th row;

a(8,:)=[];

and with this your 3rd and 9th row will be deleted, you can also check the size by

size(a);

Thank You

Answer 1

[Jan]

If you have a matrix A and want to delete the 3rd and 9th rows, you can use the command:

A([3,9],:) = [];

[Srikanth]

Does this use the same technique as a row deletion in a database. Because if we say we delete more than 1000 entries recursively, then should we be worried about any [primary keys.

[Walter Roberson]

databases have their own implementations and their own tradeoffs. For example,

deletes = [3, 9];
q = ['(', strjoin(sprintfc('%d', deletes), ','), ')'];
sqlquery = ['DELETE * FROM inventoryTable WHERE productNumber in ' q];

curs = exec(conn,sqlquery);

How the database implements this is up to the database. As is typical with databases, the efficiency of this can depend upon whether productNumber is an indexed key. But that has nothing to do with MATLAB.

[Katarina]

I used this in MATLAB R2017a and e.g.

A(3,:) = [] 

removes a row, but

A([3,5],:) = []
A([3,3],:) = []
A([k,k],:) = []

also removes only a row, but not a column as said above.

Is it that it doesn't work in my version of matlab of this answer needs updating?

[Walter Roberson]

Katarina, I am not clear as to where you are seeing anything about deleting columns in this Answer?

If you want to delete a column then name it in the second index:

A(:,k) = [];

Answer 2

[Peter]

"I have a Matrix of 400 rows and 40 columns.I want to completely remove rows 3 and 9 to get a matrix with 398 rows. How can I do it in MATLAB."

Matrix_2 = Matrix_1( [1:2,4:8,10:end] , : )

Best,

Pete

Answer 3

[Dan W]

I'm not sure if this is new syntax or not, but it works with R2012a and it's fast and simple.

x = rand(100);
tic;
x([3,9],:) = [];
toc; % Elapsed time is 0.000230 seconds.

[Mehul Agrawal]

Thanks, this works pretty easy. But I have a problem where the elements to remove are decided dynamically. So, I have a matrix m1 of size 100 X 100. And another matrix m2 of size 10X1. m2 has the row number to remove from m1 (they are not in any order). What is the best way to do this ?

Eg: m1 = rand(100); m2 = [1,6,4,8,10]; (this is the output of another function call).

[Andrei Bobrov]

Hi Mehul! It new question.

[Andrei Bobrov]

out = m1;
out(m2,:) = [];

Answer 4

[Andrei Bobrov]

m = m(setdiff(1:size(m,1),[3,9]),:);

[Sean de Wolski]

mmm glass half full

[Ryan]

Anyone care to explain this one?

[Walter Roberson]

It copies the rows of m that are _not_ row 3 or 9.

[Jan]

SETDIFF has a remarkable overhead. ISMEMBER is smarter and twice as fast for a 100x100 matrix:

m = m(~ismember(1:size(m, 1), [3,9]), :);

[pradeep kumar]

@ Andrei Bobrov , @ Walter Roberson,@ Jan Simson . how delete a particular row and column of a matrix by using "setdiff" . Say m= [1 2 3 4 ; 5 6 7 8; 9 10 11 12 ; 13 14 15 16 ]. i want to delete 1st row and 2nd column to obtain m=[5 7 8; 9 11 12;13 15 16]

Answer 5

[Alireza Rezvani]

sry, how i can Deleting individual columns of a matrix, any body know?

[Muhammad Usman Saleem]

Assume out is your matrix and you want to delete its first column, try this code,

out(:,1) = [];

[surendra bala]

Yes. This is the easiest way you can do.

Answer 6

[Drew Closner]

So I have to make a function that is able to delete a row in a matrix [I have 3 by 3]. I am not sure how to do this, please leave any help you can!

Answer 7

[LISSA DUVVU]

i want to delete all columns data where the 4th row contains number 7

[Jan]

Please do not attach a new (and very vague) question in the section for answers of another questions. Such thread-hijacking is confusing only, because it is not longer clear, to which question an answer belong. Please open your own question and delete this pseudo-answer. Thanks.