18.3: Conditional Statements

A second kind of flow control that programming languages provide is the ability to evaluate conditional statements. Unlike loops, which can repeat over and over again, a conditional statement only executes once, but it can switch between different possible commands depending on a CONDITION that is specified by the programmer. The power of these commands is that they allow the program itself to make choices, and in particular, to make different choices depending on the context in which the program is run. The most prominent of example of a conditional statement is the if statement, and the accompanying else statement. The basic format of an if statement in R is as follows:

if ( CONDITION ) {
        STATEMENT1
        STATEMENT2
        ETC
     }

And the execution of the statement is pretty straightforward. If the CONDITION is true, then R will execute the statements contained in the curly braces. If the CONDITION is false, then it dose not. If you want to, you can extend the if statement to include an else statement as well, leading to the following syntax:

 if ( CONDITION ) {
        STATEMENT1
        STATEMENT2
        ETC
     } else {
        STATEMENT3
        STATEMENT4
        ETC
     }     

As you’d expect, the interpretation of this version is similar. If the CONDITION is true, then the contents of the first block of code (i.e., STATEMENT1, STATEMENT2, ETC) are executed; but if it is false, then the contents of the second block of code (i.e., STATEMENT3, STATEMENT4, ETC) are executed instead.

To give you a feel for how you can use if and else to do something useful, the example that I’ll show you is a script that prints out a different message depending on what day of the week you run it. We can do this making use of some of the tools that we discussed in Section 7.11.3. Here’s the script:

## --- ifelseexample.R
# find out what day it is...
today <- Sys.Date()       # pull the date from the system clock
day <- weekdays( today )  # what day of the week it is_

# now make a choice depending on the day...
if ( day == "Monday" ) {
  print( "I don't like Mondays" )
} else {
  print( "I'm a happy little automaton" )
}

## [1] "I'm a happy little automaton"

Since today happens to be a Sunday, when I run the script here’s what happens:

source( "./rbook-master/scripts/ifelseexample.R" )

## [1] "I'm a happy little automaton"

There are other ways of making conditional statements in R. In particular, the ifelse() function and the switch() functions can be very useful in different contexts. However, my main aim in this chapter is to briefly cover the very basics, so I’ll move on.