Felix S Felix S - 1 month ago 8
R Question

dplyr filter: Get rows with minimum of variable, but only the first if multiple minima

I want to make a grouped filter using

dplyr
, in a way that within each group only that row is returned which has the minimum value of variable
x
.

My problem is: As expected, in the case of multiple minima all rows with the minimum value are returned. But in my case, I only want the first row if multiple minima are present.

Here's an example:

df <- data.frame(
A=c("A", "A", "A", "B", "B", "B", "C", "C", "C"),
x=c(1, 1, 2, 2, 3, 4, 5, 5, 5),
y=rnorm(9)
)

library(dplyr)
df.g <- group_by(df, A)
filter(df.g, x == min(x))


As expected, all minima are returned:

Source: local data frame [6 x 3]
Groups: A

A x y
1 A 1 -1.04584335
2 A 1 0.97949399
3 B 2 0.79600971
4 C 5 -0.08655151
5 C 5 0.16649962
6 C 5 -0.05948012


With ddply, I would have approach the task that way:

library(plyr)
ddply(df, .(A), function(z) {
z[z$x == min(z$x), ][1, ]
})


... which works:

A x y
1 A 1 -1.04584335
2 B 2 0.79600971
3 C 5 -0.08655151


Q: Is there a way to approach this in dplyr? (For speed reasons)

Answer

Just for completeness: Here's the final dplyr solution, derived from the comments of @hadley and @Arun:

library(dplyr)
df.g <- group_by(df, A)
filter(df.g, rank(x, ties.method="first")==1)