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Quotes

Charlotte Wickham May 8th 2018

library(tidyverse)
library(lobstr)
library(rlang)

Part I: Quotes

When do you need to surround something in quote marks?

Quiz

What’s the difference between

3

and

"3"

?

(For experienced users, how is this one different?)

`3`

Quotes tell R something is a character string

" and ' are used in R to form character strings.

3 is the number three,
"3" is the character string containing the digit 3.

str(3)
##  num 3

str("3")
##  chr "3"

Backticks are for non-syntatic names

Backticks ` are used for a object name (also known as symbol). You don’t need them unless the name is unusual and wouldn’t usually be considered a valid R object name, e.g. it begins with a number, contains spaces, or other reserved characters.

Here, we are asking for an object called 3,

`3`
## Error in eval(expr, envir, enclos): object '3' not found

but one doesn’t exist, so R returns an error. If we really wanted to confuse ourselves, we could create an object with the name 3:

`3` <- 4
`3`
## [1] 4

But, I wouldn’t advise it. More often, weird names occur as the result of inputting data where column names might involve spaces or special characters.

?Quotes          # Read about ",' and `
?make.names      # Read rules for names to be valid

Quiz

c <- 2

In the code below:

  • Which c is a character string?
  • Which c refers to a the name of an object?
  • Which c refers to a function name?
  • Which c refers to an argument name?
c(c = "c", c)

Quiz: solutions

The character string containing the letter c

c(c = "c", c)
       ^
       |

An object with the name c

c(c = "c", c)
           ^
           |

Function name

c(c = "c", c)
^
|

Argument name

c(c = "c", c)
  ^
  |

You don’t need quotes around function names or argument names

Function names and argument names very rarely need quotes. If you do put quotes around them, you probably won’t get an error, but it’s considered bad style.

These all work, and give the same result:

"mean"( x  =  c (1:5))
"mean"("x" =  c (1:5))
"mean"("x" = "c"(1:5))
 mean ( x  = "c"(1:5))
 mean ("x" = "c"(1:5))

But, you should write:

mean(x = c(1:5))

Except…if you need a backtick `

The exception is function names or argument names that aren’t considered valid R names, and you’ll need backticks (`).

E.g. the function with the name +

`+`(1, 2)
## [1] 3

OK that leaves argument values… to quote or not to quote?

Strategy #0: Trial and Error + Practice

Where most of us start!

The more you use a function the better you get at guessing/remembering what it is expecting.

But, it helps to have strategies to remove the guessing.

Strategy #1: What are you referring to?

You are referring to:
an object that exists in your workspace by name No quotes
a particular character string Quotes 
n_times <- 10

I want to repeat the string “A”, n_times times:

rep(x = "A", times = n_times)
##  [1] "A" "A" "A" "A" "A" "A" "A" "A" "A" "A"

A is the character string I want to repeat, so it needs quotes, n_times is the name of an object, so it doesn’t need quotes.

If I stored the string I wanted to repeat in str_to_repeat:

string_to_repeat <- "A"

Now, my x argument is the name of an object so it doesn’t need quotes:

rep(x = string_to_repeat, times = n_times)
##  [1] "A" "A" "A" "A" "A" "A" "A" "A" "A" "A"

Strategy #1: Your Turn

books <- c("The Art of R Programming", "R for Data Science",
  "Advanced R")
R <- R.version.string

I want to see where the character string R occurs in the titles in my books variable.

Which code is correct?

str_view(string = "books", pattern = "R")
str_view(string = "books", pattern =  R )
str_view(string =  books,  pattern = "R")
str_view(string =  books,  pattern =  R )

Strategy #1: Your Turn Solution

books is the name of an object, it doesn’t need quotes, R is a specific character string, it needs quotes, so the correct option is:

str_view(string = books, pattern = "R")
## PhantomJS not found. You can install it with webshot::install_phantomjs(). If it is installed, please make sure the phantomjs executable can be found via the PATH variable.
<script type="application/json" data-for="htmlwidget-37af9ee8853690aae1ee">{"x":{"html":"
    \n
  • The Art of R<\/span> Programming<\/li>\n
  • R<\/span> for Data Science<\/li>\n
  • Advanced R<\/span><\/li>\n<\/ul>"},"evals":[],"jsHooks":[]}</script>

    (Caveat: this strategy may not work if the function quotes its arguments, more in Quotation)

    Strategy #2: Read the documentation

    If the argument value is described as:

    • a character string, or string, use quotes,
    • an object, name, symbol, or expression don’t use quotes

    Strategy #2: Example

    The pull() function in dplyr extracts a column from a data frame.

    If I want the cyl column from mtcars, do I want pull(mtcars, cyl) or pull(mtcars, "cyl")?

    ?dplyr::pull
    Pull out a single variable

Arguments:

   .data: A table of data

     var: A variable specified as:

            • a literal variable name

            • a positive integer, giving the position counting from the
              left

            • a negative integer, giving the position counting from the
              right.

          The default returns the last column (on the assumption that's
          the column you've created most recently).

          This argument is taken by expression and supports
          quasiquotation (you can unquote column names and column
          positions).

    No quotes needed since this is a name. Also, look at examples section and see:

    mtcars %>% pull(cyl)
    ##  [1] 6 6 4 6 8 6 8 4 4 6 6 8 8 8 8 8 8 4 4 4 4 8 8 8 8 4 4 4 8 6 8 4

    (Aside, pull(mtcars, "cyl") works as well)

    When referring to an exsiting column inside a data frame, most (all?), tidyverse functions will not require quotes.

    Strategy #2: Your Turn

    The gather() function in tidyr takes multiple columns and collapses then into key and value columns.

    Do the values of the key and value arguments need quotes?

    ?tidyr::gather
    Gather columns into key-value pairs.

Arguments:

    data: A data frame.

key, value: Names of new key and value columns, as strings or symbols.

          This argument is passed by expression and supports
          quasiquotation (you can unquote strings and symbols). The
          name is captured from the expression with 'rlang::quo_name()'
          (note that this kind of interface where symbols do not
          represent actual objects is now discouraged in the tidyverse;
          we support it here for backward compatibility).

     ...: A selection of columns. If empty, all variables are selected.
          You can supply bare variable names, select all variables
          between x and z with 'x:z', exclude y with '-y'. For more
          options, see the 'dplyr::select()' documentation. See also
          the section on selection rules below.

   na.rm: If 'TRUE', will remove rows from output where the value
          column in 'NA'.

 convert: If 'TRUE' will automatically run 'type.convert()' on the key
          column. This is useful if the column names are actually
          numeric, integer, or logical.

factor_key: If 'FALSE', the default, the key values will be stored as a
          character vector. If 'TRUE', will be stored as a factor,
          which preserves the original ordering of the columns.

    Strategy #2: Your Turn Solution

    Do the values of the key and value arguments need quotes?

    “as strings” -> YES!

    gather(table4a, -country, key = "year", value = "count")

    “or symbols” -> NO!

    gather(table4a, -country, key =  year , value =  count )

    So, in this case it doesn’t matter.

    Strategy #3: <- Strategy #0

    Reading documentation isn’t always enlightening:

    ?purrr::pluck

    For the ... argument:

    Can be an integer position, a string name, or an accessor function.

    A “string name”, is that the name of a string, or name as a string?

    Revert to strategy #0…