Vectorization of multi-level indexing of structs in MATLAB

Question

Say I have the following in MATLAB:

a(1).b.c = 4;
a(2).b.c = 5;
a(3).b.c = 7;
....

I would like to collect the values [4 5 7 ...] in a single array, without looping and in a vectorized manner.

I have tried:

>> a(:).b.c 
# Error: Scalar index required for this type of multi-level indexing.

and

>> a.b.c
# Error: Dot name reference on non-scalar structure.

but they didn't work. The best I could come up with was:

arrayfun(@(x) x.b.c, a);

but as far as I understand arrayfun is not vectorized, or is it?

Answer 1

Your call to arrayfun seems idiomatic enough to me in Matlab. I don't think this is vectorized but it's well optimized and maybe the fastest way. You should also try to benchmark with a loop to see if the JIT compiler performs well here. It's hard to know without testing.

Answer 2

You can do it in two lines:

>> s = [a.b];
>> y = [s.c]
y =
     4     5     7

Another possible one-liner (less readable!):

>> y = squeeze(cell2mat( struct2cell([a.b]) ))
y =
     4
     5
     7