Dear Dan,
Try this:

with(dummy,tapply(as.numeric(len),as.factor(V1),summary))

See ?with, ?tapply and ?summary for more information.

HTH,

Jorge



On Fri, Mar 27, 2009 at 10:15 PM, Dan Kortschak <
dan.kortschak@adelaide.edu.au> wrote:

> Hello R users,
>
> I have a data set which is a set of lengths and types of objects. I want
> to calculate the mean length for each type of object as opposed to the
> mean of all the objects in the set.
>
> This is in order to make a comparison between the lengths of each type
> of objects and the number of those objects.
>
> > x
>       Chromosome Begin   End      Type   Class     Norm Length
> 458327          Y     1   318 L2_Plat1b LINE/L2 5.758902    317
> 458330          Y   439   673 L2_Plat1i LINE/L2 5.455321    234
> 458331          Y     2   309 L2_Plat1i LINE/L2 5.726848    307
> 458332          Y  1746  2232 L2_Plat1d LINE/L2 6.186209    486
> 458333          Y   948  1132 L2_Plat1e LINE/L2 5.214936    184
> 458335          Y  1511  2043 L2_Plat1f LINE/L2 6.276643    532
> 458336          Y     1   908 L2_Plat1f LINE/L2 6.810142    907
> 458337          Y   907  1037 L2_Plat1g LINE/L2 4.867534    130
>
> So a toy set for the relevant parts of the data would be e.g.:
>
> type<-sample(c("L2_Plat1a","L2_Plat1b","L2_Plat1c"),1000,replace=TRUE)
> len<-rnorm(1000)
> dummy<-as.data.frame(cbind(as.character(type),len))
>
> so looking for
>
> as.data.frame(summary(dummy$V1)) ~ /*average of each type's length*/
>
> as my final goal.
>
> I apologise for the syntax I use (I've come only recently from a perl
> background, so there is a certain messiness and lack of consideration
> for style to my coding) - I'm still having a really difficult time
> figuring out how various data types are used and manipulated in R, but I
> think I'm slowly getting the hang of it, but any suggestions of a good
> reference for that (other than the R Introduction which didn't help all
> that much), would be greatly appreciated.
>
> thanks for any help
> Dan
>
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