"Evaluating" "Expressions" in "Environments"?

"Evaluating" "Expressions" in "Environments"?

Expressions

The term "expression" is just a fancy way of describing any bit of (valid) code than can be parsed and evaluated (executed) by R. For example, the following bits of code are all valid R "expressions":

• 2 + 2

• sqrt(2)

• x <- (1:10)^2

• log(x, base = 10) |> mean(na.rm = TRUE)

• sum((x - mean(x))^2)

• {
    x <- 1:10
    b <- mean(x)
    z <- x * z
  }

Expressions are frequently built of other expressions: so 2 + 2 is an expression, and sqrt(2 + 2) is an expression. The { } operators are used to group any valid expressions into one bigger expression. You can also use ; to write two expressions on the same line, like x <- 2; log(x^2)

Evaluation

An expression like sum((x - mean(x))^2) is just a sequence of characters until we do something with it. We call this "evaluating" the expression. This exactly what the R "interpreter" does when you "run" some R code.

In R, an evaluated expression always "returns" a "result"—a value, like a number, character string, or some other data. Some expressions might "return" NULL as their result, but it's still a result! In a multi-line expression, like

{sqrt(2); 2 + 2}

or

{
  x <- 2
  x^2
}

the result of the overall expression is simply the result of the last expression—so the last two examples both return the result 4.

Environment

To evaluate an expression, R must look up any variable names in the expression in the current "environment"; For example, the expression sum((x - mean(x))^2) includes the variables sum, mean, and x. The variables sum and mean are base R functions, so R will find them no problem (unless you remove them). However, x is not generally going to be "defined" unless you've defined it. If you try to evaluate sum((x - mean(x))^2), you'll get an error if x is not defined.

There are many different "environments" in R, where R will search for variables: When you run R, you can save variables in the global environment; R-packages have their own environments; every time you call a function, the function has its own environment inside it—this is why a function can "see" it's own arguments, but variable you save inside a function isn't "visible" outside the function.

One of the greatest features of R is that we can often tell R to evaluate an expression using a specific data.frame as the environment, so we can use the column names of our data as variables. The humdrumR with(in) (and their tidyverse equivalents) use this functionality a lot!

Incomplete expressions

One of the most annoying things that can happen in R is if you try running something and it kind just hangs, getting stuck with nothing happening no matter how many times you press enter. This is usually because you have (accidentally) provided R an incomplete expression. For example, 2 + is an incomplete expression—that + needs a number after it! Failing to have properly paired parentheses will often result in incomplete expressions: For example, mean(sqrt(log(x)) is an incomplete expression!

See also

Other R lessons.: groupingFactors, partialMatching, recycling, vectorization