Trying to reproduce cummax function manually (function with for loop)-CodePudding

For the sake of an exercice, I have to recreate the cummax function in R using function() and a for loop. The cummax function calculates the cumulative maximum of its argument.

Thank you so much in advance

Here is what I tried to do until now:


x <- c(1,2,3,2,5,7,6,8,5)

my_cummax <- function(x) {
  for (i in x){
    if (x[i] >  lag(x[i])){
        print(x[i])
    } else {
        lag(x[i])
    }
}
}

my_cummax(x)

CodePudding user response：

One way to do it would be as below:

x <- c(1,2,3,2,5,7,6,8,5)

my_cummax <- function(x) {
  
  # preallocate the vector
  out <- vector(mode = typeof(x), length = length(x))
  # set the first element as current maximum
  cur_max <- x[1]
  
  for (i in seq_along(x)){
    
    # if x[i] is larger than the current maximum use x[i] and update both out[i] and cur_max
    if (x[i] > cur_max){
      out[i] <- cur_max <- x[i]
    # if x[i] is not larger than the current maximum use cur_max
    } else {
      out[i] <- cur_max
    }
  }
  out
}
my_cummax(x)
#> [1] 1 2 3 3 5 7 7 8 8

# compare to:
cummax(x)
#> [1] 1 2 3 3 5 7 7 8 8

^{Created on 2022-10-12 by the reprex package (v2.0.1)}