Using column names as function arguments

Question

With a data frame, I'm using dplyr to aggregate some column like below.

> data <- data.frame(a=rep(1:2,3), b=c(6:11))
> data
  a  b
1 1  6
2 2  7
3 1  8
4 2  9
5 1 10
6 2 11
> data %>% group_by(a) %>% summarize(tot=sum(b))
# A tibble: 2 x 2
      a   tot
  <int> <int>
1     1    24
2     2    27

This is perfect. However I want to create a re-usable function for this such that a column name can be passed as argument.

Looking at answers to related questions like here, I tried the following.

sumByColumn <- function(df, colName) {
  df %>%
  group_by(a) %>%
  summarize(tot=sum(colName))
  df
}

However I'm not able to get it working.

> sumByColumn(data, "b")

 Error in summarise_impl(.data, dots) : 
  Evaluation error: invalid 'type' (character) of argument. 

> sumByColumn(data, b)

 Error in summarise_impl(.data, dots) : 
  Evaluation error: object 'b' not found. 
>

You should consult this blog post about programming with dplyr — bouncyball
– bouncyball, Commented Jan 2, 2018 at 13:54

LyzandeR · Accepted Answer · 2018-01-02 14:48:03Z

38

This can work using the latest dplyr syntax (as can be seen on github):

library(dplyr)
library(rlang)
sumByColumn <- function(df, colName) {
  df %>%
    group_by(a) %>%
    summarize(tot = sum(!! sym(colName)))
}

sumByColumn(data, "b")
## A tibble: 2 x 2
#      a   tot
#  <int> <int>
#1     1    24
#2     2    27

And an alternative way of specifying b as a variable:

library(dplyr)
sumByColumn <- function(df, colName) {
  myenc <- enquo(colName)
  df %>%
    group_by(a) %>%
    summarize(tot = sum(!!myenc))
}

sumByColumn(data, b)
## A tibble: 2 x 2
#      a   tot
#  <int> <int>
#1     1    24
#2     2    27

edited Jan 2, 2018 at 14:48

answered Jan 2, 2018 at 13:58

LyzandeR

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2 Comments

user3206440 Over a year ago

this works ... However if I were to add filter( !!myenc > 7 ) before group_by it doesn't return any rows. What would be the right way to specify the column name inside filter() ?

LyzandeR Over a year ago

This is part of the documentation. Instead of !! (which is a convenience function and which is not working with logical vectors), use UQ which is the proper function. i.e. filter(UQ(myenc) > 7). Then it works fine.

AndrewGB · Accepted Answer · 2022-04-04 21:19:11Z

15

We can use {{}}:

library(dplyr)

sumByColumn <- function(df, colName) {
  df %>%
    group_by(a) %>%
    summarize(tot=sum({{colName}}))
}

sumByColumn(data, b)

#      a   tot
#  <int> <int>
#1     1    24
#2     2    27

answered Apr 4, 2022 at 21:19

AndrewGB

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2 Comments

displayname123 Over a year ago

Where is {{ documented? I'd like to read more about it.

AndrewGB Over a year ago

@filups21 They talk a little about it in the 2019 release for rlang 0.4.0. You will sometimes see it called the curly curly operator.

Ronak Shah · Accepted Answer · 2021-02-09 13:28:34Z

8

We can use the .data pronoun.

library(dplyr)

sumByColumn <- function(df, colName) {
  df %>%
    group_by(a) %>%
    summarise(tot = sum(.data[[colName]]))
}

sumByColumn(data, "b")

#      a   tot
#* <int> <int>
#1     1    24
#2     2    27

answered Feb 9, 2021 at 13:28

Ronak Shah

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Comments

CPak · Accepted Answer · 2018-01-02 14:20:27Z

6

dplyr now also provides helper functions (summarise_at, which accepts arguments vars, funs) for this

sumByColumn <- function(df, colName) {
  df %>%
    group_by(a) %>%
    summarize_at(vars(colName), funs(tot = sum))
}

provides the same answer

# A tibble: 2 x 2
      # a   tot
  # <int> <int>
# 1     1    24
# 2     2    27

answered Jan 2, 2018 at 14:20

CPak

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3 Comments

G. Grothendieck Over a year ago

Note that the last line could be: summarize_at(colName, sum)

CPak Over a year ago

@G.Grothendieck, funs(tot = sum) in case OP wanted to rename the column

Evargalo Over a year ago

verbs with _at are now depleted, so the other answers are preferable for people who get to this question nowadays.

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