[R] Summarizing counts by multiple factors

[David Winsemius]

On May 11, 2010, at 8:38 PM, Noah Silverman wrote:

> Hi,
>
> An example data set is:
>
> group    level    color
> A        1        "blue"
> A        1        "Red"
> B        1        "blue"
> B        2        "Red"
> A        2        "Red"
> B        2        "Red"
> B        2        "blue"
> B        2        "blue"
> A        2        "blue"
> A        2        "Red"
>
>
> I'd like to compute a summary of counts for each combination of  
> group, level, color.
>
> An example output would be something like this:
> group    level    color        count
> A        1        "blue"        1
> A        2        "red"        2
> B        2        "red"        2
> etc..
>
>
> The tapply function seems to do this for a single variable, but I  
> can figure out to to do it for combinations of more than one factor.

This doesn't work?

tapply(example$color, c(example$group, example$level), length)


> I also see the ave function, but can't figure out how to apply it to  
> multiple factors.

It would give you as man lines as in the orignal which is not what you  
want.

-- 
David,
>
> Any suggestions?
>
> Thanks
>
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David Winsemius, MD
West Hartford, CT