Sorting with order() in R Programming - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this R code using order()?

Consider the following R code that sorts a vector using order(). What will be the output?

R Programming

x <- c(5, 2, 9, 1)
sorted_x <- x[order(x)]
sorted_x

A [1] 9 5 2 1

B [1] 5 2 9 1

C [1] 1 2 5 9

D [1] 2 1 5 9

💡 Hint

The order() function returns the indices that would sort the vector.

The order(x) returns the indices that sort x in ascending order: 4, 2, 1, 3. Using these indices to subset x gives the sorted vector c(1, 2, 5, 9).

❓ Predict Output

intermediate

What is the output when ordering a data frame by two columns?

Given this data frame, what will be the output of ordering by age then score?

R Programming

df <- data.frame(name=c('Ann', 'Bob', 'Cara', 'Dan'), age=c(25, 25, 22, 25), score=c(90, 85, 88, 85))
ordered_df <- df[order(df$age, df$score), ]
ordered_df$name

A [1] "Cara" "Ann" "Bob" "Dan"

B [1] "Cara" "Bob" "Dan" "Ann"

C [1] "Dan" "Bob" "Ann" "Cara"

D [1] "Bob" "Ann" "Dan" "Cara"

💡 Hint

First sort by age ascending, then by score ascending.

The data frame rows sorted by age then score are: Cara (22,88), Bob (25,85), Dan (25,85), Ann (25,90). Since Bob and Dan have the same age and score, their original order is preserved.

🔧 Debug

advanced

Why does this order() call produce an error?

Examine the code below. Why does it produce an error?

R Programming

x <- c(3, 1, 4)
order(x > 2, x)

A No error; returns indices ordering by x > 2 then x

B Error because order() expects vectors of the same length and type

C Error because logical vector cannot be used directly with numeric vector in order()

D Error because > operator is not vectorized

💡 Hint

Check if logical vectors can be used in order().

The order() function can handle logical vectors because they are treated as numeric (FALSE=0, TRUE=1). So no error occurs and it returns indices ordering by x > 2 then x.

📝 Syntax

advanced

Which option has correct syntax to sort a vector in descending order using order()?

Choose the correct R code to sort vector v in descending order using order().

R Programming

v <- c(10, 5, 8, 3)

A v[order(-1 * v)]

B v[order(v, decreasing=TRUE)]

C v[order(v, desc=TRUE)]

D v[order(-v)]

💡 Hint

Remember order() does not have a decreasing argument; use negative values to reverse order.

order() does not support decreasing argument. Using -v reverses the order. Option D and D are similar but D uses multiplication which works but is less idiomatic. Option D is invalid because order() has no decreasing argument. Option D uses a non-existent argument desc.

🚀 Application

expert

How many items are in the sorted vector after removing duplicates?

Given the vector v <- c(4, 2, 4, 3, 2, 1), how many items are in the vector after sorting with order() and removing duplicates?

R Programming

v <- c(4, 2, 4, 3, 2, 1)
unique_sorted_v <- unique(v[order(v)])
length(unique_sorted_v)

A 4

B 5

C 6

D 3

💡 Hint

First sort the vector, then remove duplicates, then count the length.

Sorting v gives c(1, 2, 2, 3, 4, 4). Removing duplicates leaves c(1, 2, 3, 4), which has length 4.