How can I index a MATLAB array returned by a function without first assigning it to a local variable?

Question

For example, if I want to read the middle value from magic(5), I can do so like this:

M = magic(5); value = M(3,3); 

to get value == 13. I'd like to be able to do something like one of these:

value = magic(5)(3,3); value = (magic(5))(3,3); 

to dispense with the intermediate variable. However, MATLAB complains about Unbalanced or unexpected parenthesis or bracket on the first parenthesis before the 3.

Is it possible to read values from an array/matrix without first assigning it to a variable?

Answer 1

It actually is possible to do what you want, but you have to use the functional form of the indexing operator. When you perform an indexing operation using (), you are actually making a call to the subsref function. So, even though you can't do this:

value = magic(5)(3, 3); 

You can do this:

value = subsref(magic(5), struct('type', '()', 'subs', {{3, 3}})); 

Ugly, but possible. ;)

In general, you just have to change the indexing step to a function call so you don't have two sets of parentheses immediately following one another. Another way to do this would be to define your own anonymous function to do the subscripted indexing. For example:

subindex = @(A, r, c) A(r, c);     % An anonymous function for 2-D indexing value = subindex(magic(5), 3, 3);  % Use the function to index the matrix 

However, when all is said and done the temporary local variable solution is much more readable, and definitely what I would suggest.

Answer 2

There was just good blog post on Loren on the Art of Matlab a couple days ago with a couple gems that might help. In particular, using helper functions like:

paren = @(x, varargin) x(varargin{:}); curly = @(x, varargin) x{varargin{:}}; 

where paren() can be used like

paren(magic(5), 3, 3); 

would return

ans = 16 

I would also surmise that this will be faster than gnovice's answer, but I haven't checked (Use the profiler!!!). That being said, you also have to include these function definitions somewhere. I personally have made them independent functions in my path, because they are super useful.

These functions and others are now available in the Functional Programming Constructs add-on which is available through the MATLAB Add-On Explorer or on the File Exchange.