# delete dimension based on user input

6 views (last 30 days)
Joe_Z on 22 Oct 2019
Commented: Adam Danz on 23 Oct 2019
Dear Matlab users,
I have a 4D array and I want to delete a dimension based on user input. I have written the below code for this but it uses eval statements (which I have seen people warn against) and feels like this could be done in a more optimum manner. Any suggestions would be much appreciated.
%find input file and take user input (time dimension)
vtc = xff('73_zk18w7_036.RUN1_RS_undist_SCCTBL_3DMCTS_THPGLMF2c_IDENTITY_ALIGNED.vtc');
disp(['VTC dimensions are: ' num2str(size(vtc.VTCData))]);
timeDim = input('What dimension is time? ');
%delete user input time dimension
switch timeDim
case 1
index = '(1,:,:,:)';
case 2
index = '(:,1,:,:)';
eval(['vtc.VTCData=vtc.VTCData' index])
case 3
index = '(:,:,1,:)';
case 4
index = '(:,:,:,1)';
otherwise
disp('Input a dimension between 1 and 4');
end
eval(['vtc.VTCData=vtc.VTCData' index]);
vtc.VTCData = squeeze(vtc.VTCData);
Best,
Joe

Daniel M on 22 Oct 2019
Edited: Daniel M on 22 Oct 2019
This should work.
a = rand(4,3,2,5); % random 4D matrix
dim = 3; % let's remove 3rd dimension
numDims = 1:length(size(a)); % [1 2 3 4]
[~,permInd] = sort(dim == numDims); % [1 2 4 3]
b = permute(a,permInd); % rearranges a
b = b(:,:,:,1); % only takes first value of last dimension
We can test it:
c = squeeze(a(:,:,1,:));
isequal(b,c)
% ans = 1
Daniel M on 23 Oct 2019
Edited: Daniel M on 23 Oct 2019
Note: my test case above isn't completely robust since squeeze will remove ALL singleton dimensions, not just the specified one. (But the solution itself will not do that).

Adam Danz on 22 Oct 2019
Edited: Adam Danz on 22 Oct 2019
You can use classes overload subsref to implement custom indexing.
% Set up default index
S.type = '()';
S.subs = {':',':',':',':'};
% Choose dimension and replace that value with 1
timeDim = 3
S.subs{timeDim } = 1;
% Do indexing
vtc.VTCData = subsref(vtc.VTCData,S)
vtc.VTCData = squeeze(vtc.VTCData);
If you'd rather use the switch-case method, just do the indexing within the cases.
switch timeDim
case 1
vtc.VTCData = vtc.VTCData(1,:,:,:);
case 2
vtc.VTCData = vtc.VTCData(:,1,:,:);
case 3
vtc.VTCData = vtc.VTCData(:,:,1,:);
case 4
vtc.VTCData =vtc.VTCData(:,:,:,1);
otherwise
disp('Input a dimension between 1 and 4');
end
vtc.VTCData = squeeze(vtc.VTCData);
Adam Danz on 23 Oct 2019
Good point. I'd personally avoid using squeeze here and let the singlton dimensions be. I only added it because the OP included it in his code at the end. But it's good you're bringing that to his attention.

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