Count number of characters with column name in dynamic variable

Question

Provided that here there is written that I can pass {{colname}} to mutate, where colname is a variable that contains a column name, I cannot figure out why this code doesn't work:

testdf <- data.frame(a=c("Hello", "Hi", "Howy"))
varname <- "a"
testdf %>% mutate(b=nchar({{varname}}))

It returns the number of characters in the letter a:

      a b
1 Hello 1
2    Hi 1
3  Howy 1

How can I count the number of characters in a column and assign the value to another column, when the name of the first column is saved into a variable?

Does mutate(b=nchar(!!sym(varname))) give the expected result? — Andre Wildberg
– Andre Wildberg, Commented Nov 5, 2024 at 19:00

Nirav · Accepted Answer · 2024-11-05 19:05:38Z

1

The issue arises because {{varname}} doesn't evaluate to the content of the variable varname when you use it inside mutate().

In this case, {{varname}} is treated literally, which is why you're only seeing 1 for each row.

To solve this, you can simply use base R for a straightforward solution. Here’s how you can do it:

# Create a data frame
testdf <- data.frame(a = c("Hello", "Hi", "Howy"))

# Variable storing column name
varname <- "a"

# Use base R to add a column `b` with character counts
testdf$b <- nchar(testdf[[varname]])

OUTPUT

      a  b
1 Hello  5
2    Hi  2
3  Howy  4

testdf[[varname]] retrieves the column specified by varname from testdf.
nchar(testdf[[varname]]) counts the characters in each element of that column.
testdf$b <- ... assigns these counts to a new column b in testdf.

answered Nov 5, 2024 at 19:05

Nirav

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3 Comments

robertspierre Over a year ago

"The issue arises because {{varname}} doesn't evaluate to the content of the variable varname when you use it inside mutate().". That's not how I understood it. here there is written that I can pass {{colname}} to mutate, where colname is a variable that contains a column name.

Nirav Over a year ago

To make {{ }} work, you need to convert the column name string ("a") to an unquoted symbol. This can be done with rlang::sym():

Nirav Over a year ago

library(dplyr) library(rlang) testdf <- testdf %>% mutate(b = nchar({{ sym(varname) }}))

stefan · Accepted Answer · 2024-11-05 19:00:59Z

1

As varname is a character you can use the .data pro-noun:

testdf <- data.frame(a = c("Hello", "Hi", "Howy"))
varname <- "a"

library(dplyr, warn = FALSE)

testdf %>%
  mutate(b = nchar(.data[[varname]]))
#>       a b
#> 1 Hello 5
#> 2    Hi 2
#> 3  Howy 4

answered Nov 5, 2024 at 19:00

stefan

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2 Comments

robertspierre Over a year ago

Isn't my code supposed to work? here there is written that I can pass {{colname}} to mutate, where colname is a variable that contains a column name.

stefan Over a year ago

Nop. In the referenced post the function is called with an unquoted variable, i.e. meanofcol(iris, Petal.Width). But when you pass the column name as a character vector, e.g. meanofcol(iris, "Petal.Width") you should use the .data pronoun. See the Programming with dplyr vignette.

Friede · Accepted Answer · 2024-11-05 21:02:52Z

1

As it is designed like

library(dplyr)
f = \(dd, var) mutate(dd, b = nchar( {{var}} ))

giving

> f(testdf, a) # no quotes
      a b
1 Hello 5
2    Hi 2
3  Howy 4

As user @AndreWildberg points out in a comment, you will need to swap to

f2 = \(dd, var) mutate(dd, b = nchar( !!sym(var) ))

giving

> f2(testdf, "a") # quotes
      a b
1 Hello 5
2    Hi 2
3  Howy 4

answered Nov 5, 2024 at 21:02

Friede

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