Creating new data with max values for each subject

[This article was first published on Stat Bandit » R, and kindly contributed to R-bloggers]. (You can report issue about the content on this page here)
Want to share your content on R-bloggers? click here if you have a blog, or here if you don't.

We have a data set dat with multiple observations per subject. We want to create a subset of this data such that each subject (with ID giving the unique identifier for the subject) contributes the observation where the variable X takes it’s maximum value for that subject.

An R solution

Using the excellent R package dplyr, we can do this using windowing functions included in dplyr. The following solution is available on StackOverflow, by junkka, and gets around the real possibility that multiple observations might have the same maximum value of X by choosing one of them.

library(dplyr)
dat_max <- dat %>% group_by(ID) %>% filter(row_number(X)==n())

To be a bit more explicit, row_number is a wrapper around rank, using the option ties.method=“first”. So you can use the rank function explicitly here as well.

A solution using the plyr package might be

library(plyr)
dat_max <- ddply(dat, .(ID), function(u) u[rank(u$X, ties.method=’first’)==nrow(u),])

A Python solution

In Python, the excellent pandas package allows us to do similar operations. The following example is from this thread on StackOverflow.

import pandas as pd
df = DataFrame({’Sp’:[‘a’,’b‘,’c’,’d‘,’e’,’f’], ‘Mt’:[‘s1’, ‘s1’, ‘s2’,’s2‘,’s2’,’s3’], ‘Value’:[1,2,3,4,5,6], ‘count’:[3,2,5,10,10,6]})
df.iloc[df.groupby([‘Mt’]).apply(lambda x: x[‘count’].idxmax())]

You could also do (from the same thread)

df.sort(‘count’, ascending=False).groupby(‘Mt’, as_index=False).first()

but it is about 50% slower.

To leave a comment for the author, please follow the link and comment on their blog: Stat Bandit » R.

R-bloggers.com offers daily e-mail updates about R news and tutorials about learning R and many other topics. Click here if you're looking to post or find an R/data-science job.
Want to share your content on R-bloggers? click here if you have a blog, or here if you don't.

Never miss an update!
Subscribe to R-bloggers to receive
e-mails with the latest R posts.
(You will not see this message again.)

Click here to close (This popup will not appear again)