Adding a column every nth column without replacing existing one

Hey all,

I am looking for a solution to the problem I mentioned in the title.

FOr example, I have matrix A

A = [1 2 3 4; 5 6 7 8; 9 10 11 12];

I now want to add every 2nd (and later 3th) column a new column containing zeros.

The result should be like this:

result = [1 0 2 0 3 0 4 0; 5 0 6 0 7 0 8 0; 9 0 10 0 11 0 12 0];

I know how to create a new column containg only zeros

B = zeros(number_of_rows,1);

Previously, I used

C = [A(:,1:2) B A(:,2+1:end)];

But this only works for a specific column and not every nth column.

Thanks for your answers in advance!

Answer 1

[Adam Danz]

The variable n indicates every n-columns that should be 0s. It must be an integer greater than 1.

See inline comments for details.

% INPUTS
A = [1 2 3 4; 5 6 7 8; 9 10 11 12];
n = 2;  % for n=2, every 2nd col will be 0s


% Determine the number of columns of resultant matrix
nCol = floor(size(A,2)/(n-1)*n); 

% matrix of all 0s 
result = zeros(size(A,1),nCol); 

% Determine column indices of non-zeros
colIdx = 1:nCol; 
colIdx(n:n:end) = []; 

% Insert values from A into result matrix
result(:,colIdx) = A

[Adam Danz]

Here's the function-version of that (inspired by Max' answer).

function result = insertZeroColumns(A,n)
% A is a matrix
% n is a scalar that inserts a column of 0s into every n-th column

% Determine the number of columns of resultant matrix
nCol = floor(size(A,2)/(n-1)*n) 

% matrix of all 0s 
result = zeros(size(A,1),nCol); 

% Determine column indices of non-zeros
colIdx = 1:nCol; 
colIdx(n:n:end) = []; 

% Insert values from A into result matrix
result(:,colIdx) = A;
end

Example

A = [1 2 3 4; 5 6 7 8; 9 10 11 12];
result = insertZeroColumns(A,2); 

result = insertZeroColumns(A,3); 

[Enthusiasten]

Thanks for your answer! I chose the non-function code because I wanted to implement it into an existing script with other things and realized, that in my version it is not possible to insert a function in a script.

[Adam Danz]

Glad I could help out!

Answer 2

[Max Murphy]

Test Data

A = [1 2 3 4; 5 6 7 8; 9 10 11 12];

Function

function A = addDataNthColumn(A,B,n)
% ADDDATANTHCOLUMN  Add data in column vector or matrix B to every nth 
%                   column of A

if nargin < 3
    n = 3;
end

if size(A,1) ~= size(B,1)
    error('A and B must have same number of rows');
end

if size(A,2) >= n
    % Use recursion to iterate on A
    A = [A(:,1:(n-1)), B, addDataNthColumn(A(:,n:end),B,n)];
else
    A = [A, B]; % Last "set" just concatenate them    
end

end

Usage

>> test = addDataNthColumn(A,zeros(3,2),2)

test =

     1     0     0     2     0     0     3     0     0     4     0     0
     5     0     0     6     0     0     7     0     0     8     0     0
     9     0     0    10     0     0    11     0     0    12     0     0

[Chuguang Pan]

rowA=size(A,1);
colA=size(A,2);
B=zeros(rowA,colA*2);
B(:,1:2:2*colA)=A;

It is every 2th column.

[Adam Danz]

Nice profile picture. Is that a well isolated neuron I see? :)

[Max Murphy]

Nice profile picture. Is that a well isolated neuron I see? :)

Haha it's from a sparse extracellular array, so I don't know about well-isolated, but otherwise yes :)

[Adam Danz]

Well there must have been good thresholding on your instrumentation, then. It's such a good feeling to work with an isolatable single neuron but mult-iunit activity is also fun. Now you have me all nostalgic.

[Enthusiasten]

Thanks for your answer!

Answer 3

[Chuguang Pan]

It is every 2th column. Now I put it in Answer this question

rowA=size(A,1);
colA=size(A,2);
B=zeros(rowA,colA*2);
B(:,1:2:2*colA)=A;

[Enthusiasten]

Thanks for your answer!