Re: lookback function? or instlookback?

[Francesco Potorti`]

>-Take two specific vectors of data, say I and J
>-Start two counters at the first element. One increments by one every row
>(until reset) and the other increments by 1 when either
>       --there is a non-zero difference between the current non-zero
>value of I and the most recent previous non-zero value of I
>       or
>       --the same is true for J
>The other challenge is that both I and J change their non-zero values at
>the same time, but because only I or J is non-zero in a given row, I also
>have to figure out how to tell the counters to reset & increment for the
>first of those two changes but not for the second.

It seems to me that the two statements
- both I and J change their non-zero values at the same time
- only I or J is non-zero in a given row
are contradictory, so I don't understand what you want to do.

Anyway, if I have understood anything, this kind of things can be done
using combinations of the functions cumsum, diff and find, possibly with
the help of prepad and postpad.

-- 
Francesco Potortì (ricercatore)        Voice: +39 050 315 3058 (op.2111)
ISTI - Area della ricerca CNR          Fax:   +39 050 313 8091
via G. Moruzzi 1, I-56124 Pisa         Email: address@hidden
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