The apply-family

The apply() family of functions in R provides a way to apply a function to elements of matrices, lists, data frames, or grouped data. These functions are vectorized alternatives to loops (for or while), often making code more efficient and readable.

The main functions in the apply() family include:

Function	Works on	Returns	Used for
apply()	Matrices, data frames	Vector, list, matrix	Apply function over rows/columns
sapply()	Vectors, lists	Vector, matrix, list	Simplified output version of lapply()
lapply()	Lists, vectors	Always a list	Apply function to list elements
tapply()	Vectors with grouping factor	Vector or list	Apply function to subsets of data
mapply()	Multiple vectors/lists	Vector or list	Apply function to multiple inputs

The apply() function is used when working with matrices or data frames to apply a function to rows (MARGIN=1) or columns (MARGIN=2).

apply(X, MARGIN, FUN, ...)

The sapply() function is a simplified version of lapply(), which we will discuss in detail here, returning a vector or matrix instead of a list whenever possible.

sapply(X, FUN, ...)

The tapply() function is used when you have grouped data and need to apply a function to each group separately. This grouping variable is given by INDEX, a list of one or more factors, each of same length as X.

tapply(X, INDEX, FUN, ...)

The mapply() function applies a function to multiple vectors or lists simultaneously.