dynamic subindices <-> linear indices conversion, data crop /trim

Is there a way to dynamically crop N-dimensional data into an N-1 set? I have no knowledge of N before runtime, nor of the dimension on which to crop.

I need to write a 'trimming' function of the form:

>> data_NDm1 = one_dim_trim(ndata, 1, x);

% which will allow me to replace any of the

>> data_NDm1 = data_ND(x,:);

>> data_NDm1 = data_ND(x,:,...,:);

% or replace

>> data_NDm1 = ndata(:,x,:,...,:);% with

>> data_NDm1 = one_dim_trim(ndata, 2, x);

I have already written a 'cropping' function like

>> data_1D = one_dim_crop(ndata, [3, NaN, 4, 7]);

% which takes variable size arguments and replaces:

>> data_1D = ndata(3, :, 4, 7);

...but I'm kind of stuck here :-)

Any hints would be greatly appreciated.

[tudor dima]

corrected format below ---------------------

Is there a way to dynamically trim an N-dimensional matrix into an N-1 matrix given a dimension and the wanted subindex?

I have no knowledge of N before runtime, nor of the dimension on which to trim.

Details:

I need to write a 'trimming' function of the form:

>> data_NDm1 = one_dim_trim(data_ND, 1, a);

% which will allow me to replace any of the

>> data_NDm1 = data_ND(a,:);

>> data_NDm1 = data_ND(a,:,...,:);

% or replace

>> data_NDm1 = data_ND(:,b,:,...,:);% with

>> data_NDm1 = one_dim_trim(data_ND, 2, b);

I have already written a 'cropping' function of the form:

>> data_1D = one_dim_crop(data_ND, [a, NaN, c, ..., z]);

% which takes variable size arguments and replaces:

>> data_1D = data_ND(a, :, c, ..., z);

...but I'm kind of stuck here :-)

Any hints would be greatly appreciated.

Answer 1

[Stephen23]

Method one: comma-separated list: many MATLAB users don't realize that indices can be provided as a comma-separated list, and that the colon can then be the char ':'. This makes the task simply:

>> A = rand(3,4,5,6);
>> C = repmat({':'},1,ndims(A));
>> C{3} = [1,3]; % C{d} = x
>> Z = A(C{:});
>> size(Z)
ans =
     3     4     2     6

Method two: alternatively call subsref directly: subsref allows overloading of indexing... e.g. useful for flexibility in defining the indices:

>> A = rand(3,4,5,6);
>> C = repmat({':'},1,ndims(A));
>> C{3} = [1,3];
>> Z = subsref(A,substruct('()',C));
>> size(Z)
ans =
   3   4   2   6

Bonus these can easily be put these into a function, as requested:

function B = myidx(A,d,x)
n = max(ndims(A),d);
C = repmat({':'},1,n);
C{d} = x;
B = A(C{:});
end

And then to use it:

>> A = rand(3,4,5,6);
>> B = myidx(A,3,[1,3]);
>> size(B)
ans =
     3     4     2     6

All of these work dynamically, as one can specify the dimension d and required index x.

Use permute to remove singleton dimensions, if required.

[tudor dima]

Brilliant, it works like a charm, thanks !

I didn't know about subsref, as it doesn't show up under "see also" .

Meanwhile I had written something that works, will check for speed just in case, although I don't really expect it to be faster than subsref.

Thanks again.