i have a data frame that look like this :

i want to implement the following slicing:

if the window is 2 that means i have to take the first 2 elements of column a and the last 2 elements of columns b. Sum them and take the minimum of them (which i have done it). For example in the data frame above it must be

and will report 5.

But (and this is the problem) when the window equals the number of rows of the data frame then i want to take the first element of column a and the last element of column b and just sum them. In my data frame that would be 1 5 = 6.

My effort is this but i dont know how to insert this if else in the function

library(tidyverse)
a = seq(1,5,1)
b = seq(1,5,1)
w = tibble(a,b);w
w[1,1] w[nrow(w),2]

im = function(mat,window){
  
  if(window == nrow(mat))
    mat[1,1]   mat[nrow(mat),2]
  else
    SA = mat%>%
      dplyr::select(1)%>%
      dplyr::slice_head(n=window)
  
    SB = mat%>%
      dplyr::dplyr::select(2)%>%
      dplyr::slice_tail(n=window)
      margin = min(SA SB)
  
      return(margin)
  }
im(w,5)

to test it let's say that i have a vector or windows

vec = seq(1,5,1)

i want to run this function im that i have created for all the elements in the vector vec.

How can i do them in R ? Any help

CodePudding user response：

You can do it with a bunch of if/else:

f <- function(n, data){
  if(n == 1){
    data.frame(a = head(data[1], n), 
               b = head(data[2], n)) |>
      transform(result = a   b)
  }
  else{
    if(n == nrow(data)) n <- 1
    data.frame(a = head(data[1], n), 
               b = tail(data[2], n)) |>
      transform(result = a   b)
  }
}

output

lapply(vec, f, data = w)

[[1]]
  a b result
1 1 1      2

[[2]]
  a b result
1 1 4      5
2 2 5      7

[[3]]
  a b result
1 1 3      4
2 2 4      6
3 3 5      8

[[4]]
  a b result
1 1 2      3
2 2 3      5
3 3 4      7
4 4 5      9

[[5]]
  a b result
1 1 5      6

CodePudding user response：

You can use ifelse function , it always simplify things for me more than if else instruction. Try this it will works :

library(tidyverse)
a = seq(1,5,1)
b = seq(1,5,1)
w = tibble(a,b)

im = function(mat,window){
   
   
   SA = unlist(#unlist to transform list type result of ifelse to vector
      ifelse(
              # here is the condition
         window==nrow(mat),
              # if condition is TRUE return first element
               mat%>%
                  dplyr::select(1)%>%
                  dplyr::slice_head(n=1),
             # if condition is FALSE return first window elements
               mat%>%
         dplyr::select(1)%>%
         dplyr::slice_head(n=window))
         
         )

   
   SB = unlist(
      ifelse(window==nrow(mat),
                      mat%>%
                         dplyr::select(2)%>%
                         dplyr::slice_tail(n=1),
                      mat%>%
                         dplyr::select(2)%>%
                         dplyr::slice_tail(n=window))
      )
   
   margin = min(SA SB)
   
   return(margin)
}


im(w,5)