47 if, if else and case when

Written by Haoluan Chen and last updated on 7 October 2021.

47.1 Introduction

In this lesson, you will learn how to:

use the if() function.
use the if_else() function.
use the case_when() function.

Prerequisite skills include:

Setup RStudio.
Run R code in the console.
Install and load dplyr package.

Highlights:

Learn how to use if_else statement
Using case_when() to create new variables in your dataset

47.2 `if()`

The if() function is a tool for returning an output based on a logical condition. The syntax of the if() function is:

if (condition) {
  statement
}

If the condition is true, the {statement} will be executed. However, if the condition is false, then nothing will happen.

47.3 Example

x <- 5 
if (x > 0){
  print("A positive number")
}

Since x is set to 5, so our condition x > 0 is evaluated to true. Therefore, the statement print(“A positive number”) is executed.

x <- -3 
if (x > 0){
  print("A positive number")
}

However, if x is set to -3, our condition x > 0 is evaluated as false. Therefore, the statement print(“A positive number”) will not be executed, and nothing happens.

47.4 `if_else()`

The syntax of the if_else() function is:

if (condition) {
  statement1 
} else {
  statement2
}

If the condition is true, the {statement1} will be executed. However, if the condition is false, then {statement2} will be executed.

47.4.1 Examples

if (x > 0){
  print("A positive number")
} else {
  print("A negative number")
}

The above if_else statement will return “A positive number” if x is greater than 0 and “A negative number” if x is less than 0.

47.5 `case_when()` function

What if you have multiple condition? case_when() function is here to help.

The syntax of case_when() function with a single case is a follows:

case_when(condition ~ output_value)

If the condition is evaluated to true, then the case_when() function will return the output_value. With only a single case, this is exactly the same as an if statement.

The syntax of case_when() function with two cases is a follows:

case_when(condition ~ output_value_true, 
          TRUE ~ output_value_false)

If the condition is evaluates to true, then the case_when() function will return the output_value_true. However, if the condition is false, then the function will return the output_value_false.

The syntax of case_when() function with multiple cases is a follows:

case_when(condition_1 ~ output_1, 
          condition_2 ~ output_2,
          condition_3 ~ output_3, 
          ...)

Here, when condition_1 is evaluated to true, then the function returns output_1. When condition_2 is evaluated to true, then the function returns output_2 and so on.

47.5.1 Examples

Here is an example of using case_when() function with multiple cases.

We have seven students and their numeric grades for a course, and we want to assign the letter grade for each student. A grade between 80 and 100 is A grade, 70 to 79 is B grade, 60 to 69 is C grade, 50 to 59 is D grade, and below 50 is F grade.

test_score_df <- tribble(~student_id, ~grade
                  ,'1',  94
                  ,'2',  90
                  ,'3',  88
                  ,'4',  75
                  ,'5',  66
                  ,'6',  65
                  ,'7',  45 
                  )
test_score_df
#> # A tibble: 7 × 2
#>   student_id grade
#>   <chr>      <dbl>
#> 1 1             94
#> 2 2             90
#> 3 3             88
#> 4 4             75
#> 5 5             66
#> 6 6             65
#> 7 7             45

To assign the letter grade, we can use mutate and case_when() to create a new variable in our table.

mutate(test_score_df, letter_grade = 
                           case_when(grade <= 100 & grade >= 80 ~ 'A', 
                                     grade < 80 & grade >= 70 ~ 'B', 
                                     grade < 70 & grade >= 60 ~ 'C', 
                                     grade < 60 & grade >+ 50 ~ 'D',
                                     grade < 50 ~ 'F')) 
#> # A tibble: 7 × 3
#>   student_id grade letter_grade
#>   <chr>      <dbl> <chr>       
#> 1 1             94 A           
#> 2 2             90 A           
#> 3 3             88 A           
#> 4 4             75 B           
#> 5 5             66 C           
#> 6 6             65 C           
#> 7 7             45 F

47.6 Exercises

47.6.1 Exercises 1

Using if_else statement to see if x can be divided by 3 with remainder equal to zero.
Hint: %% is an operation to calculate the remainder

x <- 123456

x <- 123456
if (x %% 3 == 0){
  print(TRUE)
} else{
  print(FALSE)
}
#> [1] TRUE

47.6.2 Exercises 2

47.6.3 Exercises 3

test <- tribble(~student_id, ~pass
                  ,'1',  TRUE
                  ,'2',  TRUE
                  ,'3',  FALSE
                  ,'4',  TRUE
                  ,'5',  TRUE
                  ,'6',  TRUE
                  ,'7',  FALSE 
                  )
test
#> # A tibble: 7 × 2
#>   student_id pass 
#>   <chr>      <lgl>
#> 1 1          TRUE 
#> 2 2          TRUE 
#> 3 3          FALSE
#> 4 4          TRUE 
#> 5 5          TRUE 
#> 6 6          TRUE 
#> 7 7          FALSE

Here is a table called test. Replace the TRUE values in pass variable to 1 and FALSE values to 0 by using case_when()

47.7 Common mistakes & errors

Make sure you put the condition inside the ()
Make sure you put the statements in the {}
If you receive an unexpected output, you may test your if_else logic by testing some cases
Make sure you have the correct syntax for the case_when() function

47.8 Next steps

You can write a nested if_else for more complex situations. An example can be found on this website https://www.tutorialgateway.org/nested-if-else-in-r/

You can also combine for and while loop with if_else for other complex situations! For example


for (i in 1:6){
  if (i == 5){
    print(TRUE)
  }
  else{
    print(FALSE)
  }
}
#> [1] FALSE
#> [1] FALSE
#> [1] FALSE
#> [1] FALSE
#> [1] TRUE
#> [1] FALSE

47.9 Exercises

47.9.1 Question 1

if() function is a tool for returning an output based on a logical condition. a. True b. False

47.9.2 Question 2

When will the statement execute in if(condition){statement} a. Condition evaluate to False b. Condition evaluate to True c. statement will execute in any condition d. Condition evaluate to NULL

47.9.3 Question 3

else() should always be used with if() a. True b. False

47.9.4 Question 4

The if_else() function has two parts. The first part is exactly the same as the if statement, and the second part is the else statement. The else statement will be evaluated if the condition is FALSE. a. True b. False ### Question 5 ~ is used to connect the condition and the output when condition evaluated to TRUE in case_when(). a. True b. False

47.9.5 Question 6

x <- c(1,2,3,4,5,6,7,8,9,10) if (mean(x) > 5){ print(1) }else{ print(0) } Predict what is the output of this if_else statement? a. 0 b. 1 c. 5.5 d. 5

47.9.6 Question 7

x <- c(5, 4, 3, 2, 1) if (mean(x) > 5){ print(1) }else{ print(0) } Predict what is the output of this if_else statement? a. 0 b. 1 c. 5.5 d. 5

47.9.7 Question 8

case_when(mean(x) > 5 ~ 1, mean(x) >= 5 ~ 0) Using this case_when() is same as if_else in Question 6 and 7 a. True b. False

47.9.8 Question 9

case_when() can be used when you have multiple condition a. True b. False

47.9.9 Question 10

case_when() can be used when you have single condition a. True b. False

46 for and while

48 c, seq, seq along, and rep