在 r 中应用函数（带有矩阵和函数）

Question

I am trying to make 3 rows and 8 columns matrix with apply function to simplify the code, but I have problem... I have to make matrix like this :

    [,1] [,2] [,3]

[1,]    1    1    1

[2,]    2    4    3

[3,]    3    9    6

[4,]    4   16   10

[5,]    5   25   15

[6,]    6   36   21

[7,]    7   49   28

[8,]    8   64   36

x <- matrix(1:8,8,3)
x
m <- apply(x,2,function(z) return(c(z,z^2,(z^2+z)%/%2)))
m

Answer 1

We create a list of functions and loop through the columns with for loop, apply the corresponding functions

f1 <- list(function(x) x, function(x) x^2, function(x) (x^2 + x)%/%2)
for(i in seq_len(ncol(x))) x[,i] <- f1[[i]](x[,i])
x
#      [,1] [,2] [,3]
#[1,]    1    1    1
#[2,]    2    4    3
#[3,]    3    9    6
#[4,]    4   16   10
#[5,]    5   25   15
#[6,]    6   36   21
#[7,]    7   49   28
#[8,]    8   64   36

---

In the OP's solution, the apply is looping through each column, but each of the functions is applied to all the columns and the results are concatenated.

Answer 2

The apply function applies a function to the margins of an array or matrix. You are using the apply function in a correct way, so the anonymous function is being applied to each column of the input matrix and returning a matrix. The only thing is that the columns of your input matrix are similar so all columns in the output matrix will be similar too.

There is a simpler solution to obtain your output matrix:

x <- 1:8
f <- function(z) c(z,z^2,(z^2+z)%/%2)
m <- matrix(f(x), nrow = nrow(x), ncol = 3)

Although it doesn't use apply , I hope it helps.