How can I return only part of a matrix from a function, inline?

I want to return part of a matrix from a function without saving the entire matrix. The way I currently do this is to define the matrix, then call the portion in a second instance.

a = rand(3);
a = a(:,1);

But I would like to be able to do something like,

a = rand(3)(:,1);

where I only save the portion of interest and disgard the rest. This is something I do in Python frequently but have never figured out how to do it in MATLAB, and I have no idea what search terms to use when trying to find a solution.

Note: This should work on any function that returns a matrix; rand is just an example.

Answer 1

[Walter Roberson]

There is no nice built-in syntax for this: it is not permitted to syntactically index into the results of a function call or expression.

The official solution if you need expression syntax would be to use subsref() to do the indexing.

subsref() is pretty ugly, so I find it must better to use an auxillary function to do the work.

NthCol = @(M, N) M(:,N);

after which you can

a = NthCol(rand(3), 1);

[Stephen23]

While this does provide a work-around, I doubt that this would allow the JIT engine to optimize the code efficiently. Quite possibly simply allocating to a temporary variable would provide the most efficient solution. It would be interesting to do some tests.

For code clarity and portability reasons I would also avoid doing this: better to learn and use standard MATLAB practices, so that your colleagues do not rue all all of those arcane "shortcuts" when they have to work with your code.

[Walter Roberson]

A real function rather than an anonymous function would almost certainly be faster:

function selected = NthCol(M, N)
selected = M(:,N);
end