[R] Calculating the mean of values in tables using formulae

[Daniel Malter]

Check the *apply() series of functions. tapply() will do what you want.

attach(mtcars)
tapply(hp,list(cyl,gear),mean)

HTH,
Daniel





Mark Alen wrote:
> 
> I know commands like xtabs and table allow a user to do cross-tabulation
> For example the following command generates a pivot table that shows the
> number of cars that have the same number of gears and cylinders.
> 
>>xtabs(~cyl+gear,data =mtcars)
>    gear
> cyl  3 4 5
>   4 1 8 2
>   6 2 4 1
>   812 0 2
>>
> 
> We can extend the formula so it could show the sum of the horse power for
> the cars in each bin
> 
>>xtabs(hp~cyl+gear,data =mtcars)
>    gear
> cyl    3   4   5
>   4  97 608 204
>   6 215 466 175
>   82330   0 599
>>
> 
> I am now wondering, is it possible to calculate the mean of horse powers
> for cars in each bin? for example something like
> this xtabs(mean(hp)~cyl+gear, data = mtcars)
> 
> Thank you 
> Mark
> 
> 
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