How to use a variable value as column name in unnest_tokens

Question

I have the following code:

df <- tibble(c1 = 1:3, c2 = c("This is text1",
                              "This is text2",
                              "This is text3"))

#This works ok!
unnest_tokens(df, input=c2,
                  output=word)

#This works ok!
unnest_tokens(df, input="c2",
                 output=word)

#Why this one doesn't work?
a = "c2"
unnest_tokens(df, input=a,
                  output=word)

As can be seen above, unnest_tokens accepts both c2 per se (the column name as a variable) and "c2" (the column name as a string).

But I'd like to be able to use the third option. Pass "c2" as the value of a variable, let's say a and than use a value as the column name.

Is this possible to be done in R's tidytext package function unnest_tokens?

Answer 1

It has to do with quoting within the tidyverse. Try this with !!.

a = "c2"
unnest_tokens(df, input=!!a,
                  output=word)

# A tibble: 9 x 2
     c1 word 
  <int> <chr>
1     1 this 
2     1 is   
3     1 text1
4     2 this 
5     2 is   
6     2 text2
7     3 this 
8     3 is   
9     3 text3

A great resource for all of this is Hadley Wickham's "Advanced R".

19.4.1 Unquoting one argument

Use !! to unquote a single argument in a function call.