on the use of the accumarray function

Could someone please explain to me how the accumarray function works. For example:

val = 101:105;
subs = [1; 2; 4; 2; 4];
A = accumarray(subs,val)

A =

     101
     206
       0
     208

I cant understand why it performs the following calculations:

     101       % A(1) = val(1) = 101
     206       % A(2) = val(2)+val(4) = 102+104 = 206
       0       % A(3) = 0
     208       % A(4) = val(3)+val(5) = 103+105 = 208

Answer 1

[Sean de Wolski]

Accumarray uses each sub as an index into the val vector. It then takes all values with this index and performs the operation on it (the dedault being sum)

Above:

1. The index 1 is used onces corresponding to 101. 101 plus nothing else is 101.
2. The index 2 is used twice corresponding to 102 and 104, these two numbers are summed.
3. There is no occurence of index 3 so it is zero. (See note below)
4. The index 4 corresponds to to 103 and 105, the rest is history...

Note

• accumarray uses fillval to fill in elements who have no subs. You can set it using the 5th input to accumarray

-An accumarray fan

[Richard]

thank you

Answer 2

[Jakob Hannibal]

When doing this:

vals=101:106';

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

A=accumarray(subs,vals)

A =

   205     0
     0   207
     0   103
   106     0

I don't get this? Shouldn't it be:

A=

205 311

207 310

104 0

106 0

[Sean de Wolski]

No, each row of subs is an index combination. [3 1] is not one so A(3,1) should equal fill value. Consider it with just one rather than vals

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

A=accumarray(subs,1)

A =
   2     0
   0     2
   0     1
   1     0

Which is exactly what we see for subs: 2 sets of [1, 1], two sets of [2,2], one [3,2], and one [4,1]