sharoz sharoz - 1 month ago 28
R Question

Group by multiple columns in dplyr, using string vector input

I'm trying to transfer my understanding of plyr into dplyr, but I can't figure out how to group by multiple columns.

# make data with weird column names that can't be hard coded
data = data.frame(
asihckhdoydkhxiydfgfTgdsx = sample(LETTERS[1:3], 100, replace=TRUE),
a30mvxigxkghc5cdsvxvyv0ja = sample(LETTERS[1:3], 100, replace=TRUE),
value = rnorm(100)
)

# get the columns we want to average within
columns = names(data)[-3]

# plyr - works
ddply(data, columns, summarize, value=mean(value))

# dplyr - raises error
data %.%
group_by(columns) %.%
summarise(Value = mean(value))
#> Error in eval(expr, envir, enclos) : index out of bounds


What am my missing to translate the plyr example into a dplyr-esque syntax?

Answer

Just so as to write the code in full, here's an update on Hadley's answer with the new syntax:

library(dplyr)

df <-  data.frame(
    asihckhdoydk = sample(LETTERS[1:3], 100, replace=TRUE),
    a30mvxigxkgh = sample(LETTERS[1:3], 100, replace=TRUE),
    value = rnorm(100)
)

# Columns you want to group by
grp_cols <- names(df)[-3]

# Convert character vector to list of symbols
dots <- lapply(grp_cols, as.symbol)

# Perform frequency counts
df %>%
    group_by_(.dots=dots) %>%
    summarise(n = n())

output:

Source: local data frame [9 x 3]
Groups: asihckhdoydk

  asihckhdoydk a30mvxigxkgh  n
1            A            A 10
2            A            B 10
3            A            C 13
4            B            A 14
5            B            B 10
6            B            C 12
7            C            A  9
8            C            B 12
9            C            C 10