Got confused with a vector indexed by a matrix, in Matlab

Question

The following codes runs in Matlab:

a = [1 2 3 4]
b = [ 1 2 3; 1 2 3; 1 2 3]
a(b)

The result of a(b) is a matrix:

[ 1 2 3;         


        

Answer 1

Notice that the return value of a(b) is the same size as b.

a(b) simply takes each element of b, call it b(i,j), as an index and returns the outputs a(b(i,j)) as a matrix the same size as b. You should play around with other examples to get a more intuitive feel for this:

b = [4 4 4; 4 4 4];
a(b) % Will return [4 4 4; 4 4 4]
c = [5; 5];
a(c) % Will error as 5 is out of a's index range