wdenton wdenton - 4 months ago 19
R Question

Filter data frame by character column name (in dplyr)

I have a data frame and want to filter it in one of two ways, by either column "this" or column "that". I would like to be able to refer to the column name as a variable. How (in

dplyr
, if that makes a difference) do I refer to a column name by a variable?

library(dplyr)
df <- data.frame(this = c(1, 2, 2), that = c(1, 1, 2))
df
# this that
# 1 1 1
# 2 2 1
# 3 2 2
df %>% filter(this == 1)
# this that
# 1 1 1


But say I want to use the variable
column
to hold either "this" or "that", and filter on whatever the value of
column
is. Both
as.symbol
and
get
work in other contexts, but not this:

column <- "this"
df %>% filter(as.symbol(column) == 1)
# [1] this that
# <0 rows> (or 0-length row.names)
df %>% filter(get(column) == 1)
# Error in get("this") : object 'this' not found


How can I turn the value of
column
into a column name?

Answer

I would steer clear of using get() all together. It seems like it would be quite dangerous in this situation, especially if you're programming. You could use either an unevaluated call or a pasted character string, but you'll need to use filter_() instead of filter().

df <- data.frame(this = c(1, 2, 2), that = c(1, 1, 2))
column <- "this"

Option 1 - using an unevaluated call:

You can hard-code y as 1, but here I show it as y to illustrate how you can change the expression values easily.

expr <- lazyeval::interp(quote(x == y), x = as.name(column), y = 1)
## or 
## expr <- substitute(x == y, list(x = as.name(column), y = 1))
df %>% filter_(expr)
#   this that
# 1    1    1

Option 2 - using paste() (and obviously easier):

df %>% filter_(paste(column, "==", 1))
#   this that
# 1    1    1

The main thing about these two options is that we need to use filter_() instead of filter(). In fact, from what I've read, if you're programming with dplyr you should always use the *_() functions.

I used this post as a helpful reference: character string as function argument r, and I'm using dplyr version 0.3.0.2.