identify duplicate entries in data frame and calculate mean

Using dplyr

$ library(dplyr)
$ x<-data.frame(Length=c(321,350,340,180,198),
                        ID=c(rep('A234',3),'B123','B225') )
$ x %>% group_by(ID) %>% summarise(m=mean(Length))



On Tue, May 24, 2016 at 3:46 PM, Matthew 
<mccormack at molbio.mgh.harvard.edu 
<mailto:mccormack at molbio.mgh.harvard.edu>> wrote:

    I have a data frame with 10 columns.
    In the last column is an alphaneumaric identifier.
    For most rows, this alphaneumaric identifier is unique to the
    file, however some of these alphanemeric idenitifiers occur in
    duplicate, triplicate or more. When they do occur more than once
    they are in consecutive rows, so when there is a duplicate or
    triplicate or quadruplicate (let's call them multiplicates), they
    are in consecutive rows.

    In column 7 there is an integer number (may or may not be unique.
    does not matter).

    I want to identify each multiple entries (multiplicates) occurring
    in column 10 and then for each multiplicate calculate the mean of
    the integers column 7.

    As an example, I will show just two columns:
    Length  Identifier
    321     A234
    350     A234
    340     A234
    180     B123
    198     B225

    What I want to do (in the above example) is collapse all the
    A234's and report the mean to get this:
    Length  Identifier
    337     A234
    180     B123
    198     B225


    Matthew