8.1 If

The basic format of an ifstatement 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 does not. So the way R processes an ifstatement is illustrated by this schematic:

8.2 If-else

If you want to, you can extend the ifstatement to include an elsestatement 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. So the schematic illustration of an if-else construction looks like this:

In other words, when we use an if-else pair, we can define different behaviour for our script for both cases.

8.3 An example

To give you a feel for how you can use ifand else, the example that I’ll show you is a script (feelings.R) that prints out a different message depending the day of the week. Here’s the script:

if(today == "Monday") {
  print("I don’t like Mondays")
} else {
  print("I’m a happy little automaton")
}

So let’s set the value of todayto Mondayand source the script:

today <- "Monday"
source("./scripts/feelings.R")

## [1] "I don’t like Mondays"

That’s very sad. However, tomorrow should be better:

today <- "Tuesday"
source("./scripts/feelings.R")

## [1] "I’m a happy little automaton"

8.4 Another example

One useful feature of ifand elseis that you can chain several of them together to switch between several different possibilities. For example, the more_feelings.R script contains this code:

if(today == "Monday") {
  print("I don’t like Mondays")
  
} else if(today == "Tuesday") {
  print("I’m a happy little automaton")
  
} else if(today == "Wednesday") {
  print("Wednesday is beige")
  
} else {
  print("eh, I have no feelings")
}

This code allows our little 🤖 to have opinions about several different days. Imagine that we have a vector daysthat lists the name of every day of the week:

days <- c(
  "Monday", "Tuesday","Wednesday",
  "Thursday","Friday","Saturday",
  "Sunday"
)

Now let’s combine what we learned in the loops section with our new skills with ifand else. Let’s loop over all the daysand ask R to express some feelings on every day:

for(today in days) {
  source("./scripts/more_feelings.R")
}

## [1] "I don’t like Mondays"
## [1] "I’m a happy little automaton"
## [1] "Wednesday is beige"
## [1] "eh, I have no feelings"
## [1] "eh, I have no feelings"
## [1] "eh, I have no feelings"
## [1] "eh, I have no feelings"

8.5 Exercises

• Write your own version of the “feelings” script that expresses your opinions about summer, winter, autumn and spring. Test your script out.
• Expand your script so that it loops over vector of four seasonsand prints out your feelings about each of them

The solutions for these exercises are here.