R help - Jan 2008 - Grouping data via an index

Hello r-help,

I have a lengthy vector of data (with values anywhere from 1-200), and
another index vector of 'groups' representing values 0-2, 3-5, 6-8, ...
of length 67.  The index vector has the structure (1, 4, 7, ... , 196,
199), where each value is the midpoint of each respective group.

I'm trying to convert the data vector such that values falling into each
group are changed to the 'number' of that group.  That is, the position
of the midpoint of that group in the index vector.  For example, 4 or 5
would become a 2; 6 would become 3; 9 or 10 would become 4, and so on.

Haven't had any success thus far -- does anyone know of a simple method
offhand?

I've started by converting the data vector to a vector of the midpoints
of each group, via 
> round(data.vector/3)*3 + 1
But haven't been able to accomplish much past that.  I'm guessing it can
be accomplished via a simple loop or otherwise.

Thanks,

--

jared tobin, student research assistant
fisheries and oceans canada
tobinjr at dfo-mpo.gc.ca

Tobin, Jared wrote:
> Hello r-help,
>
> I have a lengthy vector of data (with values anywhere from 1-200), and
> another index vector of 'groups' representing values 0-2, 3-5, 6-8,
...
> of length 67.  The index vector has the structure (1, 4, 7, ... , 196,
> 199), where each value is the midpoint of each respective group.
>
> I'm trying to convert the data vector such that values falling into
each
> group are changed to the 'number' of that group.  That is, the
position
> of the midpoint of that group in the index vector.  For example, 4 or 5
> would become a 2; 6 would become 3; 9 or 10 would become 4, and so on.
>
> Haven't had any success thus far -- does anyone know of a simple method
> offhand?
>
> I've started by converting the data vector to a vector of the midpoints
> of each group, via 
>   
>> round(data.vector/3)*3 + 1
>>     
>
> But haven't been able to accomplish much past that.  I'm guessing
it can
> be accomplished via a simple loop or otherwise.
>
>   
You could convert midpoints to breakpoints and use cut().
Do you know that each group contains 3 consecutive values? Otherwise it
gets a bit sticky.
If it does, use

as.integer(cut(data.vector, breaks=seq(-.5,200.5,3)))

(which also works for other sets of breakpoints) or just use integer
division

data.vector %/% 3 + 1
> Thanks,
>
> --
>
> jared tobin, student research assistant
> fisheries and oceans canada
> tobinjr at dfo-mpo.gc.ca
>
> ______________________________________________
> R-help at r-project.org mailing list
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> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>   

-- 
   O__  ---- Peter Dalgaard             ?ster Farimagsgade 5, Entr.B
  c/ /'_ --- Dept. of Biostatistics     PO Box 2099, 1014 Cph. K
 (*) \(*) -- University of Copenhagen   Denmark          Ph:  (+45) 35327918
~~~~~~~~~~ - (p.dalgaard at biostat.ku.dk)                  FAX: (+45) 35327907