identify duplicate entries in data frame and calculate mean

Don't see that as being a big problem. If your data grows then dplyr 
supports connections to external databases. Alternately if you just 
want a mean, most databases can do that directly in SQL.

On Tue, May 24, 2016 at 4:17 PM, Matthew 
<mccormack at molbio.mgh.harvard.edu 
<mailto:mccormack at molbio.mgh.harvard.edu>> wrote:

    Thank you very much, Tom.
    This gets me thinking in the right direction.
    One thing I should have mentioned that I did not is that the
    number of rows in the data frame will be a little over 40,000 rows.


    On 5/24/2016 4:08 PM, Tom Wright wrote:

    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