How to gracefully truncate a matrix of unknown dimensions?

Suppose here comes a

function Truncated=MatrixTruncate(Input,Dimension,a,b)

Input is a matrix, whose dimension is unknown and depends on the caller. This function will truncate the matrix along the dimension Dimension, from index a to b. Now I use eval to generate code at runtime:

Truncated=eval(['Input(' repmat(':,',1,Dimension-1) num2str(a) ':' num2str(b) repmat(',:',1,ndims(Input)-Dimension) ')']);

Personally I feel the code somewhat unnatural, though it did work. Are there any more graceful solutions?

Answer 1

[Bruno Luong]

Input = rand(3,4,5)
Dimension = 2;
a=2; b=4;

commaidx = cell(size(size(Input)));
commaidx(:) = {':'};
commaidx{Dimension} = a:b;
Truncated = Input(commaidx{:})

[Guillaume]

size(size(Input))

is simply:

ndims(Imput)

[Guillaume]

Another option would be to permute the dimension so that the dimension to truncate is first. reshape into a 2D matrix size(dimensiontotruncate) x prod(size(ofotherdimensions)), do the truncation in the ifrst dimension, reshape and permute back.

Bruno solution is probably faster as it rearranges the elements of the matrix only once.

[Bruno Luong]

size(size(...)) returns array of 2 elements, so the cell created is vector cell, using ndims it creates a matrix

>> ndims(rand(3,4,5))

ans =

     3

>> size(size(rand(3,4,5)))

ans =

     1     3

>> 

[Guillaume]

cell(1, ndims(Input))

then. I just think the intent is clearer.

[Bruno Luong]

Debatable. What is in the comma list must come from a cell having the same size of the size of the Input.

[Tmfu Vh]

Splendid solution. What if I want to use the "end" index here?

commaidx{Dimension} = a:end;

This code is not allowed.

[Bruno Luong]

No you can't use 'end' in commalist, you have to replace it with

size(Input,Dimension)