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matlabmatrixshift
Young Ikey
Young Ikey

Reputation: 7

How to shift non circularly in Matlab

I am trying to shift non circularly in MATLAB so even if I shift outside of the index it will add 0s to correct it. I tried following the answer in How do I shift columns (left or right) in a matrix? but had no success.

data = [1 2 3 4 5; 11 12 13 14 15; 21 22 23 24 25; 31 32 33 34 35]
d = 3; % shift; positive/negative for right/left
result = zeros(size(data), 'like', data); % preallocate with zeros
result(:,max(1,1+d):min(end,end+d)) = data(:,max(1,1-d):min(end,end-d)); % write values

In my output results is nothing but the same size but all zeroes

Desired output:

 0     0     0     1     2    3      4      5  
 0     0     0     11    12   13     14     15 
 0     0     0     21    22   23     24     25
 0     0     0     31    32   33     34     35

Upvotes: 0

Views: 407

Answers (1)

Hoki
Hoki

Reputation: 11812

You can do it by creating a matrix result, the final size, filled with zeros, then copying the original data into the final result, making sure you place the data at the right indices.

What you have in your example code is not right for what you ask. If I run it,the final result is padded fine but truncated at the size of the original data matrix. This is how some matrix are shifted (with the shifted columns dropped altogether), but that's not what you asked.

A simple way to do it, is to create a padding matrix of the proper size, then simply concatenate it with your original data matrix. This can be done as below:

%% Initial data
data = [1 2 3 4 5; 11 12 13 14 15; 21 22 23 24 25; 31 32 33 34 35] ;
d = 3 ;

%% shift and pad with zeros
nrows   = size(data,1) ;            % Number of rows in [data]
pad     = zeros( nrows , abs(d) ) ; % create padding matrix
if d>0
    result = [pad data] ; % Concatenate the filler matrix on the left
else
    result = [data pad] ; % Concatenate the filler matrix on the right
end

And just to be sure:

>> result
result =
     0     0     0     1     2     3     4     5
     0     0     0    11    12    13    14    15
     0     0     0    21    22    23    24    25
     0     0     0    31    32    33    34    35

If you want to reuse the same way than in your example code, you have to adjust it a bit to allow for the new columns:

%% create result and copy data
result = zeros( size(data,1) , size(data,2)+abs(d) ) ;
colStart = max(1,1+d) ;
result(:,colStart:colStart+size(data,2)-1) = data ;

This will create the same result matrix as above.

Upvotes: 1

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