Conditionals

Learning Objectives

• Understand how to use if and else statements to handle conditional programming.

Suggested Readings

• Chapters 9.2 - 9.3 of “Hands-On Programming with R”, by Garrett Grolemund

---

Like most programming languages, R can evaluate conditional statements. A conditional statement is a switch - it tells the code which command to execute depending on a condition that is specified by the programmer.

The most prominent examples of a conditional statement is the if statement, and the accompanying else statement.

---

if

The basic format of an if statement in R is as follows:

if ( CONDITION ) {
    STATEMENT1
    STATEMENT2
    ETC
}

If the condition is TRUE, then R will execute the statements contained in the curly braces, otherwise it will skip it. This schematic illustrates the idea:

Example 1

f <- function(x) {
    cat("A")
    if (x == 0) {
        cat("B")
        cat("C")
    }
    cat("D")
}

f(0)

#> ABCD

f(1)

#> AD

Example 2

Consider a simple absolute value function. Since abs() is a built-in function, we’ll call ours absValue():

absValue <- function(x) {
    if (x < 0) {
        x = -1*x
    }
    return(x)
}

absValue(7)  # Returns 7

#> [1] 7

absValue(-7) # Also returns 7

#> [1] 7

if else

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
}

The interpretation of this version is similar. If the condition is TRUE, then the contents of the first block of code are executed; but if it is FALSE, then the contents of the second block of code are executed instead. The schematic illustration of an if-else construction looks like this:

Example

f <- function(x) {
    cat("A")
    if (x == 0) {
        cat("B")
        cat("C")
    }
    else {
        cat("D")
        if (x == 1) {
            cat("E")
        } else {
            cat("F")
        }
    }
    cat("G")
}

f(0)

#> ABCG

f(1)

#> ADEG

f(2)

#> ADFG

else if

You can also chain multiple else if statements together for a more complex conditional statement. For example, if you’re trying to assign letter grades to a numeric test score, you can use a series of else if statements to search for the bracket the score lies in:

getLetterGrade <- function(score) {
    if (score >= 90) {
        grade = "A"
    } else if (score >= 80) {
        grade = "B"
    } else if (score >= 70) {
        grade = "C"
    } else if (score >= 60) {
        grade = "D"
    } else {
        grade = "F"
    }
    return(grade)
}

cat("103 -->", getLetterGrade(103))

#> 103 --> A

cat(" 88 -->", getLetterGrade(88))

#>  88 --> B

cat(" 70 -->", getLetterGrade(70))

#>  70 --> C

cat(" 61 -->", getLetterGrade(61))

#>  61 --> D

cat(" 22 -->", getLetterGrade(22))

#>  22 --> F

---

Page sources:

Some content on this page has been modified from other courses, including:

• CMU 15-112: Fundamentals of Programming, by David Kosbie & Kelly Rivers
• Danielle Navarro’s website “R for Psychological Science”

---

EMSE 4571: Intro to Programming for Analytics (Spring 2022)
Thursdays | 12:45 - 3:15 PM EST | Tompkins 208 | Dr. John Paul Helveston | jph@gwu.edu
LICENSE: CC-BY-SA