# substitute

##### Substituting and Quoting Expressions

`substitute`

returns the parse tree for the (unevaluated)
expression `expr`

, substituting any variables bound in
`env`

.

`quote`

simply returns its argument. The argument is not evaluated
and can be any R expression.

`enquote`

is a simple one-line utility which transforms a call of
the form `Foo(....)`

into the call `quote(Foo(....))`

. This
is typically used to protect a `call`

from early evaluation.

- Keywords
- programming, data

##### Usage

```
substitute(expr, env)
quote(expr)
enquote(cl)
```

##### Arguments

##### Details

The typical use of `substitute`

is to create informative labels
for data sets and plots.
The `myplot`

example below shows a simple use of this facility.
It uses the functions `deparse`

and `substitute`

to create labels for a plot which are character string versions
of the actual arguments to the function `myplot`

.

Substitution takes place by examining each component of the parse tree
as follows: If it is not a bound symbol in `env`

, it is
unchanged. If it is a promise object, i.e., a formal argument to a
function or explicitly created using `delayedAssign()`

,
the expression slot of the promise replaces the symbol. If it is an
ordinary variable, its value is substituted, unless `env`

is
`.GlobalEnv`

in which case the symbol is left unchanged.

Both `quote`

and `substitute`

are ‘special’
primitive functions which do not evaluate their arguments.

##### Value

The `mode`

of the result is generally `"call"`

but
may in principle be any type. In particular, single-variable
expressions have mode `"name"`

and constants have the
appropriate base mode.

##### Note

`substitute`

works on a purely lexical basis. There is no
guarantee that the resulting expression makes any sense.

Substituting and quoting often cause confusion when the argument is
`expression(…)`

. The result is a call to the
`expression`

constructor function and needs to be evaluated
with `eval`

to give the actual expression object.

##### References

Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988)
*The New S Language*.
Wadsworth & Brooks/Cole.

##### See Also

`missing`

for argument ‘missingness’,
`bquote`

for partial substitution,
`sQuote`

and `dQuote`

for adding quotation
marks to strings,

`all.names`

to retrieve the symbol names from an expression
or call.

##### Examples

`library(base)`

```
# NOT RUN {
require(graphics)
(s.e <- substitute(expression(a + b), list(a = 1))) #> expression(1 + b)
(s.s <- substitute( a + b, list(a = 1))) #> 1 + b
c(mode(s.e), typeof(s.e)) # "call", "language"
c(mode(s.s), typeof(s.s)) # (the same)
# but:
(e.s.e <- eval(s.e)) #> expression(1 + b)
c(mode(e.s.e), typeof(e.s.e)) # "expression", "expression"
substitute(x <- x + 1, list(x = 1)) # nonsense
myplot <- function(x, y)
plot(x, y, xlab = deparse(substitute(x)),
ylab = deparse(substitute(y)))
## Simple examples about lazy evaluation, etc:
f1 <- function(x, y = x) { x <- x + 1; y }
s1 <- function(x, y = substitute(x)) { x <- x + 1; y }
s2 <- function(x, y) { if(missing(y)) y <- substitute(x); x <- x + 1; y }
a <- 10
f1(a) # 11
s1(a) # 11
s2(a) # a
typeof(s2(a)) # "symbol"
# }
```

*Documentation reproduced from package base, version 3.4.3, License: Part of R 3.4.3*