· r-2

R: dplyr - squashing multiple rows per group into one

I spent a bit of the day working on my Wimbledon data set and the next thing I explored is all the people that have beaten Andy Murray in the tournament.

The following dplyr query gives us the names of those people and the year the match took place:


library(dplyr)

> main_matches %>% filter(loser == "Andy Murray") %>% select(winner, year)

            winner year
1  Grigor Dimitrov 2014
2    Roger Federer 2012
3     Rafael Nadal 2011
4     Rafael Nadal 2010
5     Andy Roddick 2009
6     Rafael Nadal 2008
7 Marcos Baghdatis 2006
8 David Nalbandian 2005

As you can see, Rafael Nadal shows up multiple times. I wanted to get one row per player and list all the years in a single column.

This was my initial attempt:


> main_matches %>% filter(loser == "Andy Murray") %>% 
     group_by(winner) %>% summarise(years = paste(year))
Source: local data frame [6 x 2]

            winner years
1     Andy Roddick  2009
2 David Nalbandian  2005
3  Grigor Dimitrov  2014
4 Marcos Baghdatis  2006
5     Rafael Nadal  2011
6    Roger Federer  2012

Unfortunately it just gives you the last matching row per group which isn't quite what we want.. I realised my mistake while trying to pass a vector into paste and noticing that a vector came back when I'd expected a string:


> paste(c(2008,2009,2010))
[1] "2008" "2009" "2010"

The missing argument was 'collapse' - something I'd come across when using plyr last year:


> paste(c(2008,2009,2010), collapse=", ")
[1] "2008, 2009, 2010"

Now, if we apply that to our original function:


> main_matches %>% filter(loser == "Andy Murray") %>% 
     group_by(winner) %>% summarise(years = paste(year, collapse=", "))
Source: local data frame [6 x 2]

            winner            years
1     Andy Roddick             2009
2 David Nalbandian             2005
3  Grigor Dimitrov             2014
4 Marcos Baghdatis             2006
5     Rafael Nadal 2011, 2010, 2008
6    Roger Federer             2012

That's exactly what we want. Let's tidy that up a bit:


> main_matches %>% filter(loser == "Andy Murray") %>% 
     group_by(winner) %>% arrange(year) %>%
     summarise(years  = paste(year, collapse =","), times = length(year))  %>%
     arrange(desc(times), years)
Source: local data frame [6 x 3]

            winner          years times
1     Rafael Nadal 2008,2010,2011     3
2 David Nalbandian           2005     1
3 Marcos Baghdatis           2006     1
4     Andy Roddick           2009     1
5    Roger Federer           2012     1
6  Grigor Dimitrov           2014     1
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