Remove a column from a matrix in GNU Octave

Question

In GNU Octave, I want to be able to remove specific columns from a matrix. In the interest of generality. I also want to be able to remove specific rows from a matrix.

Suppose I have this:

mymatrix = eye(5)

mymatrix =

Diagonal Matrix

   1   0   0   0   0
   0   1   0   0   0
   0   0   1   0   0
   0   0   0   1   0
   0   0   0   0   1

I want to remove columns 2 and 4, but when I remove column 2, the position of column 4 has moved to column 3, and that makes my head hurt. There has to be a better way!

score 78 · Answer 1 · edited Jul 23 '19 at 05:36

78

In case you don't know the exact number of columns or rows you can use the magic "end" index, e.g.:

mymatrix(:,2:end)  % all but first column

mymatrix(2:end,:)  % all but first row

This also allows you to slice rows or columns out of a matrix without having to reassign it to a new variable.

edited Jul 23 '19 at 05:36

mpr

3,250
26
44

answered Nov 12 '14 at 15:00

pors

3,856
36
33

2

so much cleaner :D – Kaolin Fire Jun 30 '17 at 03:17
Exactly what I was looking for! – Vivek Shankar Jun 14 '20 at 11:48

score 63 · Accepted Answer · edited May 22 '18 at 15:36

GNU Octave delete Columns 2 and 4 from a Matrix

mymatrix = eye(5); 
mymatrix(:,[2,4]) = []; 
disp(mymatrix)

Prints:

GNU Octave delete Rows 2 and 4 from a Matrix:

mymatrix = eye(5); 
mymatrix([2,4],:) = [];
disp(mymatrix)

Prints:

1   0   0   0   0
0   0   1   0   0
0   0   0   0   1

Time complexity

GNU Octave's CPU complexity for slicing and broadcasting here is a fast linear time O(n * c) where n is number of rows and c a constant number of rows that remain. It's C level single-core vectorized but not parallel.

Memory complexity

Working memory complexity is linear: O(n * 2) C makes a clone of the two objects, iterates over every element, then deletes the original.

The only time speed will be a problem is if your matrices are unrealistically wide, tall, or have a number of dimensions that blow out your fast memory, and speed is limited by the transfer speed between disk and memory.

score 14 · Answer 3 · answered Nov 25 '13 at 11:37

14

The reverse method of doing this:

columns_you_want_to_keep = [1, 3, 5]
new_matrix = my_matrix(:,columns_you_want_to_keep)

answered Nov 25 '13 at 11:37

thethakuri

509
5
16

Eric Leschinski · Answer 4 · 2012-11-27T20:17:30.193

How to remove multiple columns in octave:

How to remove columns 2 and 4:

columns_to_remove = [2 4];
matrix(:,columns_to_remove)=[]

Illustrated:

mymatrix = eye(5)
mymatrix =

   1   0   0   0   0
   0   1   0   0   0
   0   0   1   0   0
   0   0   0   1   0
   0   0   0   0   1



columns_to_remove = [2 4];

mymatrix(:,columns_to_remove)=[]


mymatrix =

   1   0   0
   0   0   0
   0   1   0
   0   0   0
   0   0   1

Remove a column from a matrix in GNU Octave

4 Answers4

How to remove multiple columns in octave:

Linked