MATLAB: collect from array of structs

Question

The output of eg >>w = whos; returns an array of structs. I would like to construct an array whose elements are the scalars from a particular field name in each struct.

The most obvious way of doing this doesn't return an array as I want, but each answer separately.

>> w(1:2).bytes
ans =
    64
ans =
   128

I could do it with a loop, but was wondering if there's a nicer way.

Answer 1

Put square brackets around the expression, i.e.

[w(1:2).bytes]

Answer 2

In situations like these, using cat is more general purpose. Suppose you wanted to do the same with a bunch of strings, then the [ ] method wouldn't work, and you'd have to use:

cat(1,w(1:2).class)

And in the case above,

cat(1,w(1:2).bytes)

Additionally, you'd want to keep things as columns in MATLAB for better performance.

Answer 3

Accessing a field for an array of structures will return as an output a comma-separated list (or CSL). In other words, the output from your example:

w(1:2).bytes

is equivalent to typing:

64, 128

As such, you can use the output in any place where a CSL could be used. Here are some examples:

a = [w(1:2).bytes];         % Horizontal concatenation = [64, 128]
a = horzcat(w(1:2).bytes);  % The same as the above
a = vertcat(w(1:2).bytes);  % Vertical concatenation = [64; 128]
a = {w(1:2).bytes};         % Collects values in a cell array = {64, 128}
a = zeros(w(1:2).bytes);    % Creates a 64-by-128 matrix of zeroes
b = strcat(w.name);         % Horizontal concatenation of strings
b = strvcat(w.name);        % Vertical concatenation of strings