matlab array index without assigning to a local variable

Question

How can you read from an array/matrix without first assigning it to a variable?

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 with the intermediate variable. Matlab complains about Unbalanced or unexpected parenthesis or bracket on the first bracket before the 3.

Is this possible?

Answer 1

unfortunately syntax like magic(5)(3,3) is not supported by matlab. you need to use temporary intermediate variables. you can free up the memory after use, e.g.

tmp = magic(3);
myVar = tmp(3,3);
clear tmp

Answer 2

Think about the case of the magic(n) function... it's contents depends on its size, correct? It doesn't seem to make sense to expect to read from a given row/column without first defining its size.

More on magic from Matlab. This shows you can to do things like:

sum(Magic(5));

Answer 3

It actually is possible to do what you want, but only if you 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:

subindex = @(A,r,c) A(r,c);      %# An anonymous function to index a matrix
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.