R: initialize /create data frame in for loop - r

Question

I wonder what would be the best or most appropriate way to create and modify a data frame in a for-loop, using cbind or rbind? For the first iteration, the data frame has no column or rows, so - in the below example - cbind does not work. Only for this first case, I need the if-else-command inside the for-loop. Isn't there a more elegant way wrting the code below, i.e. without if-else?
mydat <- data.frame()
for (j in 1:10) {
if (ncol(mydat) == 0)
mydat <- data.frame(sample(x = j * 5, size = 20, replace = T))
else
mydat <- cbind(mydat, data.frame(sample(x = j * 5, size = 20, replace = T)))
}
colnames(mydat) <- sprintf("x%i", 1:10)

Answer 1

Here is a simple way to combine lapply and the do.call(cbind, list) convention for generating the data.frame you want.
set.seed(1234)
gendata <- function(x) {
sample(x = x*5, size = 20, replace = T)
}
do.call(cbind, lapply(1:10, gendata))
# [,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8] [,9] [,10]
# [1,] 1 4 9 18 24 2 27 21 20 24
# [2,] 4 4 10 1 12 17 33 40 26 18
# [3,] 4 2 5 7 4 9 35 13 20 31
# [4,] 4 1 10 1 14 7 33 20 11 4
# [5,] 5 3 5 5 5 5 18 15 4 48
# [6,] 4 9 8 15 23 10 10 26 29 2
# [7,] 1 6 11 7 10 5 9 30 20 43
# [8,] 2 10 8 11 8 4 18 23 4 32
# [9,] 4 9 4 2 5 14 18 40 37 16
# [10,] 3 1 12 12 23 2 12 24 15 38
# [11,] 4 5 2 3 5 22 34 18 35 32
# [12,] 3 3 5 18 23 4 23 10 27 50
# [13,] 2 4 11 1 4 29 5 4 32 7
# [14,] 5 6 8 16 4 4 15 35 20 45
# [15,] 2 2 3 2 3 7 33 10 16 41
# [16,] 5 8 8 11 13 28 17 40 35 42
# [17,] 2 3 8 8 8 29 32 25 20 42
# [18,] 2 3 12 2 1 9 21 40 26 37
# [19,] 1 10 3 7 8 4 23 16 6 50
# [20,] 2 9 13 14 19 24 31 23 14 32
EDIT:
As was pointed out by Konrad Rudolph, the result I provided was a matrix not a data.frame. Just convert the matrix using as.data.frame:
set.seed(1234)
gendata <- function(x) {
sample(x = x*5, size = 20, replace = T)
}
dat <- as.data.frame(do.call(cbind, lapply(1:10, gendata)))
names(dat) <- sprintf("x%i", 1:10)
head(dat)
# x1 x2 x3 x4 x5 x6 x7 x8 x9 x10
# 1 1 4 9 18 24 2 27 21 20 24
# 2 4 4 10 1 12 17 33 40 26 18
# 3 4 2 5 7 4 9 35 13 20 31
# 4 4 1 10 1 14 7 33 20 11 4
# 5 5 3 5 5 5 5 18 15 4 48
# 6 4 9 8 15 23 10 10 26 29 2