I already tried max(seq_along(x)) but I need it to also return 0 if we, let's say, inputted numeric(0).

So yeah, it works for anything else other than numeric(0). This is what I have so far:

my_length <- function(x){
  max(seq_along(x))
}

CodePudding user response：

Using forloop:

my_length <- function(x){
  l = 0
  for(i in x) l <- l   1  
  return(l)
}

x <- numeric(0)
my_length(x)
# [1] 0

x <- 1:10
my_length(x)
# [1] 10

Another option:

my_length <- function(x) nrow(matrix(x))

CodePudding user response：

You can just include a 0 to the max() call in your attempt:

my_length <- function(x) max(0, seq_along(x))

my_length(10:1)
[1] 10
my_length(NULL)
[1] 0
my_length(numeric())
[1] 0

CodePudding user response：

You can use NROW(). From the documentation:

nrow and ncol return the number of rows or columns present in x. NCOL and NROW do the same treating a vector as 1-column matrix, even a 0-length vector ...

len <- \(x) NROW(x)

Examples:

len(numeric(0))
#> [1] 0

len(letters)
#> [1] 26

len(c(3, 0, 9, 1))
#> [1] 4

CodePudding user response：

Here are a few more functional programming approaches:

1. Using mapping and summation:

   length = function (x) {
       sum(vapply(x, \(.) 1L, integer(1L)))
   }
   

2. Using reduction:

   length = function (x) {
       Reduce(\(x, .) x   1L, x, 0L)
   }
   

3. Using recursion:

   length = function (x, len = 0L) {
       if (is_empty(x)) len else Recall(x[-1L], len   1L)
   }
   

   Alas, the last one needs to define the helper function and that is unfortunately not trivial without using length():

   is_empty = function (x) {
       is.null(x) || identical(x, vector(typeof(x), 0L))
   }