## 17.5 Evaluation runs code

Inspecting and modifying code gives you one set of powerful tools. You get another set of powerful tools when you evaluate, i.e. execute or run, an expression. Evaluating an expression requires an environment, which tells R what the symbols in the expression mean. You’ll learn the details of evaluation in Chapter 20.

The primary tool for evaluating expressions is base::eval(), which takes an expression and an environment:

eval(expr(x + y), env(x = 1, y = 10))
#> [1] 11
eval(expr(x + y), env(x = 2, y = 100))
#> [1] 102

If you omit the environment, eval uses the current environment:

x <- 10
y <- 100
eval(expr(x + y))
#> [1] 110

One of the big advantages of evaluating code manually is that you can tweak the environment. There are two main reasons to do this:

• To temporarily override functions to implement a domain specific language.
• To add a data mask so you can refer to variables in a data frame as if they are variables in an environment.