# Why does accumarray require vals and subs to be of the same size?

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Jonathan on 23 Jun 2014
Commented: Cedric Wannaz on 23 Jun 2014
Hi,
I understand this is a built-in function and that "legacy constraints" might be the answer, but still I'm really wondering why the inputs `subs` and `vals` need to be of the same size in `accumarray`.
IMO, a sufficient input check on subs (forgetting about input cells..) would be something like
subs = floor(subs);
assert( all(subs>0) && all(subs <= numel(vals)), 'Index out ouf bounds.' );
I really don't understand this constraint, can someone explain the logic behind this?

Jan on 23 Jun 2014
The documentation of accumarray is one of the most complicated instructions I've read. It cannot compete with the cute clarity of the other parts of Matlab's excellent docs.
The job of accumarray is "accumulating elements of the vector val using the elements of subs as indices". Therefore the number of values and the corresponding indices must be equal. The i.th value is accumulated accoring to the i.th subs element.
Cedric Wannaz on 23 Jun 2014
Yep, understand each element of subs as the "address" where to stack the corresponding element of vals before applying the accumulation function.