Apply function in R

Question

I am new to R and I have heard a lot about the apply function.

Can someone list down the advantages of apply function?

Thanks.

Answer 1

The apply function allows us to make entry-by-entry changes to data frames and matrices.

The usage in R is as follows:

apply(X, MARGIN, FUN, …)

where:

X is an array or matrix;

MARGIN is a variable that determines whether the function is applied over rows (MARGIN=1), columns (MARGIN=2), or both (MARGIN=c(1,2));

FUN is the function to be applied.

If MARGIN=1, the function accepts each row of X as a vector argument and returns a vector of the results. Similarly, if MARGIN=2 the function acts on the columns of X. Most impressively, when MARGIN=c(1,2) the function is applied to every entry of X.

Advantage:

With the apply function, we can edit every entry of a data frame with a single line command. No auto-filling, no wasted CPU cycles.

Answer 2

apply

Description: “Returns a vector or array or list of values obtained by applying a function to margins of an array or matrix.”

We know about vectors/arrays and functions, but what are these “margins”? 

Simple: either the rows (1), the columns (2) or both (1:2). By “both”, we mean “apply the function to each individual value.” An example:

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

# create a matrix of 10 rows x 2 columns m <- matrix(c(1:10, 11:20), nrow = 10, ncol = 2) # mean of the rows apply(m, 1, mean)  [1]  6  7  8  9 10 11 12 13 14 15 # mean of the columns apply(m, 2, mean) [1]  5.5 15.5 # divide all values by 2 apply(m, 1:2, function(x) x/2)       [,1] [,2]  [1,]  0.5  5.5  [2,]  1.0  6.0  [3,]  1.5  6.5  [4,]  2.0  7.0  [5,]  2.5  7.5  [6,]  3.0  8.0  [7,]  3.5  8.5  [8,]  4.0  9.0  [9,]  4.5  9.5 [10,]  5.0 10.0