Problem with multiple accesses in array indexing

Hi everybody,

I have problems to make calculations on arrays with multiple accesses to the same indexes. For instance, if I have a simple vector

A = [7,8,9]

and I would like to make calculations several times on the same index

A([1,1]) = A([1,1]) + 1

the results shoud be equal to 9 and not to 8 (in tis case, I would like to make the computation twice). The only way I found to do this is with a simple statement :

b = [1,1];
for k = 1:length(b)
   A(b(k)) = A(b(k)) +1;
end

But that is not cheap with big arrays. So, I would be pleased to know what is the solution of this.

Thank you guys.

[AC]

Hi,

I think you need to give a more complicated example, because in this case, you could just do:

A(b)=A(b)+length(b);

But I'm guessing that may not generalize to your real problem...

Answer 1

[F.]

Like AC, I think that your example is too easy to give you a solution.

But I'm going to try with some supoositions.

For me

   b = [ 1 1 2 2 3 1 2 3 ];
   A = [ 7 8 9 ];

and you add "a" and not 1.

So (I can't make the test but ...)

   [ c, ib,ic ] = unique( b )
   V = arrayfun( @(D) sum( ic == D ), [ 1 : max(ic) ] );
   A = A + V * a ;

but I'm not sure for the creation of V.

Answer 2

[BIASI Valentin]

I found a solution here

http://stackoverflow.com/questions/3955109/incrementing-one-value-of-a-matlab-array-multiple-times-in-one-line

It uses the accumarray function.

Thank you for your contributions