Remove a column from a matrix in GNU Octave
MatrixOctaveMatrix Problem Overview
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!
Matrix Solutions
Solution 1 - Matrix
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.
Solution 2 - Matrix
GNU Octave delete Columns 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
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.
Solution 3 - Matrix
The reverse method of doing this:
columns_you_want_to_keep = [1, 3, 5]
new_matrix = my_matrix(:,columns_you_want_to_keep)
Solution 4 - Matrix
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