Sorting with order() in R Programming - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the order() function do in R?

The order() function returns the indices that would sort a vector or data frame. You can use these indices to rearrange data in ascending or descending order.

beginner

How do you sort a vector ascending using order()?

Use vec[order(vec)]. This returns the vector elements sorted from smallest to largest.

intermediate

How can you sort a data frame by multiple columns using order()?

Use df[order(df$col1, df$col2), ]. This sorts first by col1, then by col2 within ties.

intermediate

How do you sort in descending order with order()?

Use order(-vec) for numeric vectors. For characters, use order(vec, decreasing = TRUE).

intermediate

Why use order() instead of sort()?

order() gives the positions to rearrange data, useful for sorting data frames or multiple vectors together. sort() just returns a sorted vector.

What does order(c(3,1,2)) return?

A [2, 1, 3]

B [2, 3, 1]

C [3, 1, 2]

D [1, 2, 3]

The order function returns indices that sort the vector. The smallest element is at position 2 (value 1), then position 3 (value 2), then position 1 (value 3).

How do you sort a data frame df by column age ascending?

A df[order(df$age), ]

B order(df$age)

C sort(df$age)

D df[sort(df$age), ]

You use order to get the sorting indices and then reorder the rows of df accordingly.

Which code sorts a vector v in descending order?

A v[order(v)]

B sort(v)

C v[order(-v)]

D order(v, decreasing=TRUE)

Using negative values with order sorts numeric vectors in descending order.

What is the main difference between sort() and order()?

A sort returns sorted values; order returns sorting indices

B order returns sorted values; sort returns indices

C Both return sorted values

D Both return sorting indices

sort() returns the sorted vector, while order() returns the positions to sort the vector.

How do you sort a data frame by two columns, name then score?

A sort(df$name, df$score)

B df[order(df$score, df$name), ]

C order(df$name, df$score)

D df[order(df$name, df$score), ]

You pass multiple columns to order to sort by the first, then the second within ties.

Explain how the order() function works and how it differs from sort().

Think about what you get back from each function and how you use it.

Describe how to sort a data frame by multiple columns using order().

Remember how to handle ties in sorting.

Practice

1. What does the order() function in R do?

easy

A. Sorts the vector and returns the sorted values directly

B. Removes duplicates from a vector

C. Returns the indices that would sort a vector in ascending order

D. Calculates the sum of all elements in a vector

Solution

1. Step 1: Understand the purpose of order()

   The order() function returns the positions (indices) that would sort the vector, not the sorted vector itself.

2. Step 2: Differentiate from sort()

   Unlike sort(), which returns sorted values, order() returns the order of indices to rearrange the vector.

3. Final Answer:

   Returns the indices that would sort a vector in ascending order -> Option C

4. Quick Check:

   order() = indices for sorting [OK]

Hint: Remember: order() gives positions, sort() gives sorted values [OK]

Common Mistakes:

• Confusing order() with sort()
• Expecting order() to return sorted values
• Using order() to remove duplicates

2. Which of the following is the correct syntax to sort a vector v in decreasing order using order()?

easy

A. v[sort(v, decreasing = TRUE)]

B. order(v, decreasing = TRUE)

C. sort(order(v), decreasing = TRUE)

D. v[order(v, decreasing = TRUE)]

Solution

1. Step 1: Use order() with decreasing = TRUE

   To sort vector v in decreasing order, use order(v, decreasing = TRUE) to get indices for sorting from largest to smallest.

2. Step 2: Index vector v with order()

   Use these indices to reorder v as v[order(v, decreasing = TRUE)].

3. Final Answer:

   v[order(v, decreasing = TRUE)] -> Option D

4. Quick Check:

   v[order(v, decreasing=TRUE)] sorts descending [OK]

Hint: Index vector with order() for sorting, add decreasing=TRUE for reverse [OK]

Common Mistakes:

• Using order() without indexing the vector
• Trying to sort order() output directly
• Using sort() on order() output incorrectly

3. What is the output of the following R code?

v <- c(4, 2, 9, 7)
v[order(v)]

medium

A. [1] 2 4 7 9

B. [1] 4 2 9 7

C. [1] 9 7 4 2

D. [1] 2 7 4 9

Solution

1. Step 1: Calculate order(v)

   For v = c(4, 2, 9, 7), order(v) returns indices that sort v: positions of values in ascending order are 2 (2), 1 (4), 4 (7), 3 (9).

2. Step 2: Index v with order(v)

   Using these indices: v[c(2,1,4,3)] gives 2 4 7 9.

3. Final Answer:

   [1] 2 4 7 9 -> Option A

4. Quick Check:

   v[order(v)] = sorted vector ascending [OK]

Hint: order() gives indices to sort; index vector to get sorted values [OK]

Common Mistakes:

• Expecting order() to return sorted values directly
• Mixing up indices and values
• Incorrect indexing order

4. Identify the error in the following R code snippet:

v <- c(5, 3, 8, 1)
sorted_v <- order(v)
print(sorted_v)

medium

A. order() returns indices, not sorted values; should index v with order(v)

B. Missing parentheses in order function call

C. Vector v is not defined before order()

D. print() cannot display vectors

Solution

1. Step 1: Understand what order() returns

   order(v) returns the indices that would sort v, not the sorted vector itself.

2. Step 2: Correct usage to get sorted vector

   To get sorted values, use v[order(v)]. The code assigns indices to sorted_v, so printing it shows positions, not sorted values.

3. Final Answer:

   order() returns indices, not sorted values; should index v with order(v) -> Option A

4. Quick Check:

   order() = indices, index vector to sort [OK]

Hint: Use v[order(v)] to get sorted vector, not order(v) alone [OK]

Common Mistakes:

• Expecting order() to return sorted vector
• Printing order() output directly
• Confusing order() with sort()

5. Given two related vectors names <- c("Anna", "Bob", "Cara") and scores <- c(88, 95, 90), how do you sort names by scores in descending order using order()?

hard

A. order(names, decreasing = TRUE)[scores]

B. names[order(scores, decreasing = TRUE)]

C. scores[order(names, decreasing = TRUE)]

D. sort(names, by = scores, decreasing = TRUE)

Solution

1. Step 1: Use order() on scores with decreasing = TRUE

   To sort names by scores descending, get indices with order(scores, decreasing = TRUE).

2. Step 2: Index names with these order indices

   Use these indices to reorder names: names[order(scores, decreasing = TRUE)].

3. Final Answer:

   names[order(scores, decreasing = TRUE)] -> Option B

4. Quick Check:

   Index names by order(scores, decreasing=TRUE) [OK]

Hint: Sort related vectors by ordering one, index the other [OK]

Common Mistakes:

• Trying to sort names directly by scores without order()
• Using sort() with non-existent 'by' argument
• Mixing order() arguments or indexing incorrectly