CODEWITHSUNDEEP
r
Jordan
Jordan

Reputation: 465

How to turn vector into Matrix indices in R

How do I efficiently turn a vector into a matrix so that the values of the vector determine the column and the element ordering determines the row, and the value assigned at those indices are determined afterwards.

X <- c(1,2,3,1,1,3)
Y <- 1:6 
Z <- myfun(X, Y)

Z ## returns matrix
# 1  NA NA
# NA 2  NA
# NA NA 3
# 4  NA NA
# 5  NA NA
# NA NA 6

I am looking for a one-liner better than my for loop

Z0 <- X %o% rep(NA, length(unique(X)))

for(i in 1:length(Y)){  Z0[i, X[i]] <- Y[i] }

Upvotes: 0

Views: 442

Answers (2)

Julius Vainora
Julius Vainora

Reputation: 48191

If you insist on a one-liner, then @djhurio's answer can be converted to

replace(matrix(NA, length(X), max(X)), cbind(1:length(X), X), Y)
#      [,1] [,2] [,3]
# [1,]    1   NA   NA
# [2,]   NA    2   NA
# [3,]   NA   NA    3
# [4,]    4   NA   NA
# [5,]    5   NA   NA
# [6,]   NA   NA    6

Upvotes: 3

djhurio
djhurio

Reputation: 5536

See the example:

X <- c(1,2,3,1,1,3)
Y <- 1:6

myfun <- function(X, Y) {
  Z <- matrix(NA, length(X), max(X))
  Z[cbind(seq_along(X), X)] <- Y
  Z
}

Z <- myfun(X, Y)
Z

Upvotes: 6

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