I want to fill an array using the apply function.
For example, my purpose is to simplify the repetition in the following code
tmp <- matrix(1:100, 20, 5)
i <- replicate(2, sample(1:nrow(tmp), 3))
dt <- array(NA, c(3, 5, 2))
# the repetition:
dt[, , 1] <- tmp[i[,1], ]
dt[, , 2] <- tmp[i[,2], ]
The following may be a solution:
for (s in 1:2) {
dt[, , s] <- tmp[i[,s], ]
}
But, I do not want to use the for loop.
The purpose is to make pseudo data sets for a simulation.
That is, "tmp" is a matrix for the population with 20 individuals and 5 variables.
And, each column of "i" is the index of sampled persons for two iteration of the simulation.
Thus, I am trying to stake the two samples in an array.
Here, how can I make the line two lines in the code just one line?
For this, I think, apply function may be a candidate.
Can we reduce the last lines?
(This question is closely related to Fill a NA matrix using apply functions. But, the solution might be pretty different. )
Instead of filling dt, subset tmp by using i in apply and create the array with desired dimensions.
(dt <- array(apply(i, 2, \(x) tmp[x, ]), dim=c(3, 5, 2)))
# , , 1
#
# [,1] [,2] [,3] [,4] [,5]
# [1,] 14 34 54 74 94
# [2,] 4 24 44 64 84
# [3,] 15 35 55 75 95
#
# , , 2
#
# [,1] [,2] [,3] [,4] [,5]
# [1,] 15 35 55 75 95
# [2,] 19 39 59 79 99
# [3,] 20 40 60 80 100
Your for loop, however, is perfectly fine and well understandable.
Data:
tmp <- structure(1:100, dim = c(20L, 5L))
i <- structure(c(14L, 4L, 15L, 15L, 19L, 20L), dim = 3:2)