# How do I shift columns (left or right) in a matrix

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NA on 8 Aug 2020
Commented: NA on 8 Aug 2020
Hi All,
I would like to non-circularly shift my matrix and then have zeros padded to either the left or right (depending on the shift) i.e. if the matrix shifts to the right, zeros would be padded to the left.
My code so far looks like this:
%dummy data
data = rand(5, 16);
channelSink = 9; %this variable will either be >layerIV, <layerIV or =layerIV
layerIV = 7;
chDiff = layerIV - channelSink;
for channel = 1:16
if channelSink > layerIV
%shift columns to the left by ab(chDiff)
%and
%set columns shifted by ab(chDiff) to zero
elseif channelSink < layerIV
%shift columns to the right by chDiff
%and
%set columns shifted by chDiff to zero
else %if chDiff = 0, don't shift
chDiff = 0;
disp('Sink at channel 7; not necessary to re-align');
end
end

Walter Roberson on 8 Aug 2020
shift = abs(diff)
%left
data = [data(:, shift+1:end), zeros(size(data,1),shift)]
%right
data = [zeros(size(data,1),shift), data(:, end-shift:shift)];
Note: we recommend against using diff() as a variable name, as it conflicts with the common function diff() . If nothing else, it will tend to confuse other people who read your code.
NA on 8 Aug 2020
Thank you, Walter. I just made a tiny adjustment as the shifted columns were off by 1.
data = [zeros(size(data,1),shift), data(:, shift-1:end-shift)];
But, it works perfectly now. Thanks again!

Abdolkarim Mohammadi on 8 Aug 2020
Edited: Abdolkarim Mohammadi on 8 Aug 2020
I used the vectorization capability of MATLAB, so it is faster and there is no need for a for loop.
data = rand (5,16);
channelSink = 9; % this variable will either be >layerIV, <layerIV or =layerIV
layerIV = 7;
diff = layerIV - channelSink;
dataFirstCol = 1;
dataLastCol = size (data,2);
dataShiftedFirstCol = 1;
dataShiftedLastCol = size (data,2);
if diff < 0
dataFirstCol = dataFirstCol + abs(diff);
dataShiftedLastCol = dataShiftedLastCol - abs(diff);
elseif diff > 0
dataLastCol = dataLastCol - abs(diff);
dataShiftedFirstCol = dataShiftedFirstCol + abs(diff);
end
dataShifted = zeros (size(data));
dataShifted (:,dataShiftedFirstCol:dataShiftedLastCol) = data (:,dataFirstCol:dataLastCol);
NA on 8 Aug 2020
Thanks for this. The reason I wrote my code in a for loop was because I need to run it on multiple datasets (n x 16 matrix). But I will try and adapt what you have done to suit what I want. I do apprecaite your help.

Bruno Luong on 8 Aug 2020
k > 0 shift right
k < 0 shift left
A*diag(ones(1,size(A,2)-abs(k)),k)

R2019b

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