How can I add n columns to a matrix?

Dear experts,

I have a Matrix X(4082x2). Now I want to add for example two columns to the matrix with every cell = 0.

That´s how I´ve done it so far:

amount_rows = numel(X(:,1));
randomdata = rand(amount_rows,1);
added_column = 0*randomdata;
X = [X added_column added_column];

Until now, that worked completely fine but my problem is that I now have a dataset where I need to add more than 100 columns. It would be pretty annoying to add those 100 times the added_column into the bracket (I hope you know what I mean). On top of that, I want the script to work for every dataset so that it automatically adds the "added_collumn" e.g. n-times. Somehow like that:

X = [X (added_column)*n]

(in the case described above n =2)

I know that this is not correct but I hope you get the idea.

Thanks in advance.

Kind regards, TG

Answer 1

[Stephen23]

A simpler, more versatile, and much more efficient approach is to use ZEROS:

R  = size(X,1);
C = number_of_new_columns;
X = [X,zeros(R,C)];

Another simple approach is to use indexing (which implicitly fills the other new elements with zeros):

X(1, end+C) = 0 % Thank you Image Analyst

[Image Analyst]

Or, more simply

X(end, end+C) = 0

[Steven Lord]

In the general case where you want to augment a matrix with copies of a vector (not necessarily the zero vector) you can use repmat.

A = magic(3)

A = 3×3

8 1 6 3 5 7 4 9 2

v = [42; -99; NaN]

v = 3×1

42 -99 NaN

B = [A, repmat(v, 1, 5)]

B = 3×8

8 1 6 42 42 42 42 42 3 5 7 -99 -99 -99 -99 -99 4 9 2 NaN NaN NaN NaN NaN

Or if you need to augment with a series of vectors that can be created by arithmetic operations you can use implicit expansion.

c = [1; 2; 3]

c = 3×1

1 2 3

d = 1:5

d = 1×5

1 2 3 4 5

E = [A, v, c+d] % c+d makes a 3-by-5 matrix

E = 3×9

8 1 6 42 2 3 4 5 6 3 5 7 -99 3 4 5 6 7 4 9 2 NaN 4 5 6 7 8