6 Conditional Statements

The control flow of an R program is regulated by conditional statements and loops. 7 Conditional statements, like if statements, allow us to execute some code if a condition is fulfilled. Loops, like for and while, allow us to repeatedly run code.

This chapter 8 focuses on the conditional statement. They give us the ability to check conditions and change the behavior of the program accordingly.

6.1 What Ifs

if

The simplest form of control flow is if conditional statements. if takes a logical value (a logical vector of length one). If the condition is TRUE, the statement gets executed. If the condition is FALSE, if invisibly returns NULL.

How it works:

if(condition) {
  statement
}

Note: Use a pair of curly braces to wrap the statement(s).

Example:

x <- sample(1:15, 1) #sample() takes a sample of the specified size from the elements of x 
if (x > 5) {
  print(paste(x, "is larger than 5."))
}

This also works if we have only one statement.

How it works:

if(condition) statement

Example:

x <- sample(1:10, 1) 
if (x > 5) print(paste(x, "is larger than 5."))

if … else if

If there are other conditions to evaluate, include else if. else if is essentially saying “if the previous condition is not true, then try this condition”.

How it works:

if(condition) {
  statement
} else if (condition) {
  statement
}

Example:

x <- sample(1:15, 1)
if (x > 5) {
  print(paste(x, "is larger than 5."))
} else if (x < 5) {
  print(paste(x, "is smaller than 5."))
}
## [1] "4 is smaller than 5."

We can have more than one else if.

How it works:

if(condition) {
  statement
} else if (condition) {
  statement
} else if (condition) {
  statement
}

if … else if … else

else catches anything that is not caught by the preceding conditions. The else statement is only evaluated if the condition is FALSE.

How it works:

if(condition) {
  statement
} else if (condition) {
  statement
} else {
  statement
}

Example:

x <- sample(1:15, 1)
if (x > 5) {
  print(paste(x, "is larger than 5."))
} else if (x == 5) {
  print(paste(x, "is equal to 5."))
} else {
  print(paste(x, "is smaller than 5."))
}
## [1] "14 is larger than 5."

Note: The else statement must occur on the same line as the closing curly brace from the if clause.

if … else

We can have an else statement without the else if.

How it works:

if(condition) {
  true statement
}
else {
  false statement
}

Example:

x <- sample(1:15, 1)
if (x > 5) {
  print(paste(x, "is larger than 5."))
} else {
  print(paste(x, "is not larger than 5."))
}
## [1] "2 is not larger than 5."

Exercise

Write an R program to convert month name to a number of days. For instance, if we input “February”, then the output will be “No. of days: 28 or 29 days”.

Hint: Use readline(), which reads a line from the terminal in interactive use.

month <- as.character(readline("Input a month name: "))
month_30 <- c("April", "June", "September", "November")
month_31 <- c("January", "March", "May", "July", "August", "October", "December")

if (month %in% month_30){
  print("No. of days: 30 days") 
} else if (month %in% month_31){
  print("No. of days: 31 days")
} else if (month == "February") {
  print("No. of days: 28 or 29 days")
} else {
  print("Incorrect input")
}

6.2 Nested if

We can have if statements inside if statements. This is called nested if statements.

Example:

x <- sample(1:15, 1)

if (x > 5) {
  print(paste(x, "is larger than 5,"))
  if (x > 10) {
    print(paste("and 10."))
  } else {
    print("but not larger than 10.")
  }
} else {
  print(paste(x, "is not larger than 5"))
}
## [1] "9 is larger than 5,"
## [1] "but not larger than 10."

Exercise

Write an R program to find the median of three integers. Do not use the built-in function median().

a <- as.integer(readline("Input the 1st number: "))
b <- as.integer(readline("Input the 2nd number: "))
c <- as.integer(readline("Input the 3rd number: "))

if (a > b){
  if (b > c) { print(b) }
  else if (c > a) { print(a) } 
  else { print(c) }
} else {
  if (a > c){ print(a) }
  else if (c > b){ print(b) } 
  else{ print(c) }
}

6.3 Vectorized ifelse()

Remember that if only takes a logical vector of length one.

if(c(TRUE, FALSE))

If we pass a logical vector with a length of more than one, e.g. if(c(TRUE, FALSE)), only the first one will be used.

What if we have multiple logical values? There is a vectorized version of the if ... else construct, the ifelse() function.

How it works:

ifelse(condition, yes, no)

ifelse() takes three arguments. The first is a logical vector of conditions. The second contains values that are returned when the first vector is TRUE. The third contains values that are returned when the first vector is FALSE.

It returns a vector of the same length as condition, with elements yes[i] if condition[i] is TRUE, otherwise no[i]. yes and no are recycled where necessary.

Example:

x <- sample(1:15, 1)
yes <- paste(x, "is larger than 5")
no <- paste(x, "is not larger than 5")
ifelse(x > 5, yes, no)
## [1] "9 is larger than 5"

  1. https://adv-r.hadley.nz/control-flow.html↩︎

  2. This chapter provides a summary rather than a complete standalone resource. Please refer to the corresponding video for the complete tutorial.↩︎