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R Quote and Substitute

quote() holds code as data instead of running it. substitute() captures the code a caller wrote, and deparse() turns held code back into text.

Hold code with quote

quote() stores the expression total + tax as data. print() shows the code itself, and class() shows the held value is a call.

q <- quote(total + tax)
print(q)
print(class(q))

Turn held code back into text

deparse() takes a quoted expression and returns it as a character string. The string holds the same code that quote() stored.

held <- quote(score * 2)
print(held)
print(deparse(held))

Capture what a caller wrote

substitute() inside show_expr reads the expression passed for col. deparse() turns that captured expression into the string that prints.

show_expr <- function(col) {
  deparse(substitute(col))
}
print(show_expr(score + bonus))

Each call is captured as written

capture() runs twice with two different arguments. Each print shows the exact expression from its own call.

capture <- function(value) {
  deparse(substitute(value))
}
print(capture(total_score))
print(capture(price * count))

Example

report <- function(item) {
  deparse(substitute(item))
}
task <- quote(units * price + fee)
print(task)
print(class(task))
print(deparse(task))
print(report(units * price + fee))

Key points

  • quote() returns its code as data and does not run it.
  • A quoted expression prints as the code itself.
  • The class of a quoted expression is call.
  • substitute() inside a function captures the expression a caller passed for an argument.
  • deparse() turns held code back into a character string.
  • The captured text holds the caller's own names and operators.

In this example, quote() first holds units * price + fee as a call. deparse() turns the held call into text, and report() captures the same expression straight from its caller.

Try it Yourself

Edit the code, then press Run.

Running is simulated · rdrr.io R Snippets

Exercise

What does print(class(quote(score + bonus))) show?