Apply functions on matrices in R Programming - Cheat Sheet & Quick Revision

Recall & Review

beginner

What function in R is commonly used to apply a function to the rows or columns of a matrix?

The apply() function is used to apply a function to the rows or columns of a matrix in R.

beginner

How do you specify whether to apply a function to rows or columns in the apply() function?

You use the MARGIN argument: 1 for rows and 2 for columns.

beginner

Example: What does apply(mat, 1, sum) do if mat is a matrix?

It calculates the sum of each row in the matrix mat, returning a vector of row sums.

beginner

Can you use apply() to find the mean of each column in a matrix? How?

Yes. Use apply(mat, 2, mean) to calculate the mean of each column.

intermediate

What is the difference between apply() and lapply() in R?

apply() works on matrices or arrays applying a function over rows or columns, while lapply() works on lists applying a function to each element.

In R, what does apply(mat, 2, sum) do?

A Calculates the sum of each column in mat

B Calculates the sum of each row in mat

C Calculates the sum of all elements in mat

D Calculates the mean of each column in mat

MARGIN = 2 means apply the function to columns, so it sums each column.

Which argument in apply() specifies whether to apply a function to rows or columns?

A FUN

B INDEX

C DATA

D MARGIN

MARGIN is used to specify rows (1) or columns (2).

What will apply(mat, 1, mean) return?

A Mean of each row

B Mean of each column

C Sum of each row

D Sum of each column

MARGIN = 1 applies the function to rows, so it returns the mean of each row.

If you want to apply a function to each element of a list, which function is more appropriate than apply()?

A sapply()

B lapply()

C tapply()

D mapply()

lapply() applies a function to each element of a list.

What type of object does apply() expect as input?

A List

B Data frame

C Matrix or array

D Vector

apply() is designed for matrices or arrays.

Explain how to use the apply() function to calculate the sum of each row in a matrix.

Remember MARGIN = 1 means rows.

Describe the difference between apply() and lapply() in R and when to use each.

Think about the data structure you have.