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Mastering Loop Functions in R: A Practical Guide to lapply and sapply

The Beginner’s Guide to Loop Functions in R:

Loop functions are essential tools for data manipulation and analysis in R programming. Two of the most commonly used loop functions in R are lapply and sapply. In this blog post, we'll explain what these functions are, how they work, and provide some practice material for beginners to intermediate level.


lapply: 

The 'l' stands for 'list'

lapply is a loop function in R that applies a function to each element of a list and returns the result as a list. Here's the basic syntax:

lapply(list, function)

The list argument is the list you want to apply the function to, and the function argument is the function you want to apply. For example, let's say we have a list of numbers and we want to apply the sqrt function to each element:

my_list <- list(1, 4, 9) 
lapply(my_list, sqrt)

This will return a list of the square roots of each element in my_list.

sapply: 

The 's' stands for 'simplify'

sapply is similar to lapply, but it simplifies the output to a vector or matrix if possible. Here's the basic syntax:

sapply(list, function)

The list argument and the function argument are the same as for lapply. For example, let's say we have a list of numbers and we want to apply the sqrt function and simplify the output to a vector:

my_list <- list(1, 4, 9) 
sapply(my_list, sqrt)

This will return a vector of the square roots of each element in my_list.

Practice Material

Here are some exercises to practice using lapply and sapply:

  • Use lapply to apply the sum function to each element of a list of vectors. For example, the list list(c(1, 2, 3), c(4, 5, 6), c(7, 8, 9)) should return the list list(6, 15, 24).
  • Use sapply to apply the mean function and simplify the output to a vector for each column of a matrix. For example, the matrix matrix(c(1, 2, 3, 4, 5, 6), nrow = 2) should return the vector c(2, 4).
  • Use lapply and sapply to calculate the variance of each column of a matrix. For example, the matrix matrix(c(1, 2, 3, 4, 5, 6), nrow = 2) should return the vector c(2.5, 2.5).

  • For more practice you should start swirl's 11th lesson in R Programming. Complete download process of swirl and R Programming is here, click on the link!
  • You can look in to the practice and reading material that is provided in the text book, click here to download the textbook.
  • Lecture slides can be downloaded from here. It would be great if you go through them too.

In conclusion, lapply and sapply are powerful loop functions in R that can help you manipulate and analyze data efficiently. By understanding how these functions work and practicing using them with different data structures, you can improve your R programming skills and become a more effective data analyst.
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