How can I add n columns to a matrix?

Question

Tom 2021-9-27

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https://ww2.mathworks.cn/matlabcentral/answers/1461394-how-can-i-add-n-columns-to-a-matrix

编辑： Stephen23 2021-9-28

采纳的回答： Stephen23

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

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Answer 1

Stephen23 2021-9-27

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https://ww2.mathworks.cn/matlabcentral/answers/1461394-how-can-i-add-n-columns-to-a-matrix#answer_795929

编辑：Stephen23 2021-9-28

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

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Image Analyst 2021-9-27

编辑：Image Analyst 2021-9-27

Or, more simply

X(end, end+C) = 0

Steven Lord 2021-9-27

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