The %in% Operator

Part of the Fundamentals section of Coddy's R journey — lesson 70 of 78.

Sometimes you need to check whether values exist within a vector. The %in% operator provides a clean way to test for membership, returning TRUE for each element that matches and FALSE otherwise.

The basic syntax checks if values on the left exist in the vector on the right:

fruits <- c("apple", "banana", "cherry")
print("banana" %in% fruits)

Output:

[1] TRUE

You can also check multiple values at once. The operator returns a logical vector with one result for each value being tested:

fruits <- c("apple", "banana", "cherry")
check <- c("banana", "grape", "apple")
print(check %in% fruits)

Output:

[1]  TRUE FALSE  TRUE

This makes %in% perfect for filtering. Combined with logical indexing, you can extract elements that belong to a specific set:

numbers <- c(1, 5, 3, 8, 2, 9)
allowed <- c(1, 2, 3)
print(numbers[numbers %in% allowed])

Output:

[1] 1 3 2

The %in% operator is especially useful when validating user input or filtering data based on a predefined list of acceptable values.

Challenge

Easy

You will receive three lines of input:

A comma-separated list of items in your inventory (e.g., apple,banana,milk,bread,eggs,cheese)
A comma-separated list of items on your shopping list (e.g., milk,butter,eggs,juice)
A comma-separated list of items that are on sale (e.g., bread,juice,yogurt)

Using the %in% operator:

Create character vectors from each comma-separated input
Check which shopping list items you already have in your inventory and print the logical vector result
Filter and print the shopping list items that you do NOT have in your inventory (items you need to buy)
Filter and print the items you need to buy that are also on sale

For example, if the inputs are:

apple,banana,milk,bread,eggs
milk,butter,eggs,juice,apple
bread,juice,yogurt,butter

The output should be:

[1]  TRUE FALSE  TRUE FALSE  TRUE
[1] "butter" "juice" 
[1] "butter" "juice"

Explanation:

Shopping list checked against inventory: milk (TRUE), butter (FALSE), eggs (TRUE), juice (FALSE), apple (TRUE)
Items NOT in inventory (need to buy): butter, juice
Items to buy that are on sale: butter and juice are both on the sale list

Use the ! operator with %in% to find items NOT in a vector.

Cheat sheet

The %in% operator checks if values exist within a vector, returning TRUE or FALSE:

fruits <- c("apple", "banana", "cherry")
print("banana" %in% fruits)

Output:

[1] TRUE

Check multiple values at once to get a logical vector:

fruits <- c("apple", "banana", "cherry")
check <- c("banana", "grape", "apple")
print(check %in% fruits)

Output:

[1]  TRUE FALSE  TRUE

Use %in% with logical indexing to filter elements:

numbers <- c(1, 5, 3, 8, 2, 9)
allowed <- c(1, 2, 3)
print(numbers[numbers %in% allowed])

Output:

[1] 1 3 2

Use ! with %in% to find items NOT in a vector:

items <- c("milk", "butter", "eggs")
inventory <- c("milk", "eggs")
print(items[!items %in% inventory])

Output:

[1] "butter"

Try it yourself

# Read input
con <- file("stdin", "r")
inventory_input <- suppressWarnings(readLines(con, n = 1))
shopping_list_input <- suppressWarnings(readLines(con, n = 1))
sale_items_input <- suppressWarnings(readLines(con, n = 1))
close(con)

# TODO: Write your code below
# 1. Create character vectors from each comma-separated input using strsplit()
# 2. Check which shopping list items you already have in inventory (use %in%)
# 3. Filter shopping list items you do NOT have in inventory (use ! with %in%)
# 4. Filter items you need to buy that are also on sale