identify duplicate entries in data frame and calculate mean

You have several  options.

1.  You could use the aggregate function.  If your data frame is called DF, you could do something like

with(DF, aggregate(Length, list(Identifier), mean))

2.  You could use the dplyr package like this

library(dplyr)
summarize(group_by(DF, Identifier), mean(Length))


Hope this is helpful,

Dan

Daniel Nordlund, PhD
Research and Data Analysis Division
Services & Enterprise Support Administration
Washington State Department of Social and Health Services

-----Original Message-----
From: R-help [mailto:r-help-bounces at r-project.org] On Behalf Of Matthew
Sent: Tuesday, May 24, 2016 12:47 PM
To: r-help at r-project.org
Subject: [R] identify duplicate entries in data frame and calculate mean

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