Logical Indexing

Part of the Fundamentals section of Coddy's R journey — lesson 68 of 78.

Instead of specifying exact positions, you can use a logical vector to select elements. This technique, called logical indexing, lets you filter a vector based on conditions rather than positions.

When you apply a comparison to a vector, R returns a logical vector of the same length:

numbers <- c(10, 20, 30, 40, 50)
print(numbers > 25)

Output:

[1] FALSE FALSE  TRUE  TRUE  TRUE

You can use this logical vector directly inside square brackets to extract matching elements:

numbers <- c(10, 20, 30, 40, 50)
print(numbers[numbers > 25])

Output:

[1] 30 40 50

R keeps only the elements where the logical vector is TRUE. This approach is much cleaner than writing loops to filter data. You can combine conditions using logical operators:

numbers <- c(10, 20, 30, 40, 50)
print(numbers[numbers >= 20 & numbers <= 40])

Output:

[1] 20 30 40

Logical indexing is one of R's most powerful features for data manipulation, allowing you to filter datasets with simple, readable expressions.

Challenge

Easy

You will receive two lines of input:

A comma-separated list of numbers (e.g., 15,42,8,67,23,91,34,12,56)
Two threshold values separated by a comma representing a range (e.g., 20,60)

Using logical indexing:

Create a numeric vector from the comma-separated numbers
Extract and print all elements that are less than the first threshold (lower bound)
Extract and print all elements that are greater than the second threshold (upper bound)
Extract and print all elements that are within the range (greater than or equal to the first threshold AND less than or equal to the second threshold)

For example, if the inputs are:

5,25,10,45,60,15,80,35,50
20,50

The output should be:

[1]  5 10 15
[1] 60 80
[1] 25 45 35 50

Explanation:

Elements less than 20: 5, 10, 15
Elements greater than 50: 60, 80
Elements between 20 and 50 (inclusive): 25, 45, 35, 50

Use the & operator to combine conditions for the range filter.

Cheat sheet

Logical indexing allows you to filter a vector based on conditions rather than positions. When you apply a comparison to a vector, R returns a logical vector:

numbers <- c(10, 20, 30, 40, 50)
print(numbers > 25)
# Output: [1] FALSE FALSE  TRUE  TRUE  TRUE

Use the logical vector inside square brackets to extract matching elements:

numbers <- c(10, 20, 30, 40, 50)
print(numbers[numbers > 25])
# Output: [1] 30 40 50

Combine conditions using logical operators like & (AND):

numbers <- c(10, 20, 30, 40, 50)
print(numbers[numbers >= 20 & numbers <= 40])
# Output: [1] 20 30 40

Try it yourself

# Read input
con <- file("stdin", "r")
numbers_input <- suppressWarnings(readLines(con, n = 1))
thresholds_input <- suppressWarnings(readLines(con, n = 1))
close(con)

# Parse the comma-separated numbers into a numeric vector
numbers <- as.numeric(unlist(strsplit(numbers_input, ",")))

# Parse the threshold values
thresholds <- as.numeric(unlist(strsplit(thresholds_input, ",")))
lower_bound <- thresholds[1]
upper_bound <- thresholds[2]

# TODO: Write your code below
# 1. Use logical indexing to extract elements less than lower_bound and print
# 2. Use logical indexing to extract elements greater than upper_bound and print
# 3. Use logical indexing with & operator to extract elements within the range (inclusive) and print