Matlab - Accumarray confusion?

Question

I'm just not understanding how exactly accumarray works. I checked the official matlab documentation , but I'm still unable to comepletely understand.

If it's something like this, where:

subs = [1
    3
    4
    3
    4] 

val = 101:105';

accumarray(subs,val) = 
[101
 0
 206
 208]

I get that it was A(1)= 101, A(2)= 0, A(3)= 102+104, and A(4)= 103+105

But if it was something like:

subs = [1 1
        2 2
        3 2 
        1 1 
        2 2 
        4 1]

val = 101:106';

accumarray(subs,val) = 
[205 0 
 0 207
 0 203 
 106 0]

I don't understand how the method works.... I kinda get the A(1) = 101+104, and then that A(2) = 102+105.

But why is that displayed in the second column? Also, why is the [3 2] line not included in the calculation?

I know this is a really simple question, but this is the first time I'm using Matlab.. any help on this would be greatly appreciated, thank you!!

Answer 1

As described in the docs for accumarray:

Considering a call to the function like the following:

A = accumarray(subs,val)

The values in each row of the m-by-n matrix subs define an n-dimensional subscript into the output, A.

Therefore, in your case since subs is a Something x 2 array, each of its row is considered a subscript pointing to the output A, which is of size 4x2 because the maximal values in each columns are respectively 4 and 2.

Therefore, if we decompose subs into similar rows, ie similar subscripts, we see that there are 2 rows pointing to the same coordinates (1,1) as well as (2,2). There is only 1 pointing to (4,1) and 1 pointing to (3,2). Therefore, we would expect output A to have accumulated values only at those coordinates:

(1,1)
(2,2)
(3,2)
(4,1)

which is exactly the case:

A =

   205     0
     0   207
     0   103
   106     0

Is that clearer?