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Problem with multiple accesses in array indexing

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BIASI Valentin
BIASI Valentin 2012-7-6
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.
  1 个评论
AC
AC 2012-7-6
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...

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回答（2 个）

F.
F. 2012-7-6
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.
BIASI Valentin
BIASI Valentin 2012-7-6
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

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