I am trying to input a vector x onto a matrix where values of x are located on the anti-diagonal of the matrix

x = c(1,2,3,4,5)
ID = function(x){
  n = length(x)
  A = matrix(0, nrow = n, ncol = n)
  for ( i in 1:n){
    A[i, n-i 1] = x[i]
  }
  print(A)  # Why did my code not work when I did not include this
}

Z = ID(x)

I was wondering why does my code not work when I do not include the print(A) within the function. When I don't include Print(A), my z turns out to be NULL.

Thank you in advance.

CodePudding user response：

I would do this without a loop -

x = c(1,2,3,4,5)

ID = function(x){
  n = length(x)
  A = matrix(0, nrow = n, ncol = n)
  diag(A[nrow(A):1, ]) <- x
  A
}
ID(x)

#     [,1] [,2] [,3] [,4] [,5]
#[1,]    0    0    0    0    5
#[2,]    0    0    0    4    0
#[3,]    0    0    3    0    0
#[4,]    0    2    0    0    0
#[5,]    1    0    0    0    0

CodePudding user response：

If a return statement is not encountered before the end of the function body, then the function returns the value of the last "top level" expression. When you exclude print(A), the last top level expression is the for call, which returns NULL invisibly, as documented in ?`for`.

I would modify your function in three ways:

• Replace matrix(0, n, n) with matrix(vector(typeof(x), n * n), n, n) so that the type of the matrix result matches the type of the vector argument.
• Replace the for loop with a vectorized subassignment.
• Replace print(A) with A so that print(ID(x)) doesn't print the matrix twice.

Hence my version of your function would look something like this:

antidiag <- function(x) {
    n <- length(x)
    A <- matrix(vector(typeof(x), n * n), n, n)
    if (n > 0L) {
        A[cbind(n:1, 1:n)] <- x
    }
    A
}

antidiag(1:4)
##      [,1] [,2] [,3] [,4]
## [1,]    0    0    0    4
## [2,]    0    0    3    0
## [3,]    0    2    0    0
## [4,]    1    0    0    0

antidiag(1:4 > 0L)
##       [,1]  [,2]  [,3]  [,4]
## [1,] FALSE FALSE FALSE  TRUE
## [2,] FALSE FALSE  TRUE FALSE
## [3,] FALSE  TRUE FALSE FALSE
## [4,]  TRUE FALSE FALSE FALSE

antidiag(letters[1:4])
##      [,1] [,2] [,3] [,4]
## [1,] ""   ""   ""   "d" 
## [2,] ""   ""   "c"  ""  
## [3,] ""   "b"  ""   ""  
## [4,] "a"  ""   ""   ""