how to create vector with negative subscript in R

Question

I will like to create a vector that will allow negative indexing such as -100 to 100. This means that doing G[-50] will select element at position -50 and not all element except at -50. Could you guys assist me with this. Thanks.

Answer 1

You could use a named vector

x <- 1:201
names(x) <- c(-100:100)
x["-100"]

-100 
   1 

The indexing will be the same, but you can still access positions in the way you want if you put quotes around it

Answer 2

Numeric indices in R must be positive, so there is no way to do what you want. Here are some workarounds, though:

1. If you know what the minimum value is going to be (ie. indices can be from -100 to 100) just add that number (+1) to your indices. So instead of df[-100], you have df[1]

2. You could split positive and negative indices into 2 vectors in a list. So, instead of df[-50] you'd use df[[negative]][50].

See this example:

df <- list('pos' = c(1, 2, 3, 4, 5),
           'neg' = c(-1, -2, -3, -4, -5))

neg_index <- function(df, i) {
    if (i > 0) {
        return(df[['pos']][i])
    } else if (i < 0) {
        return(df[['neg']][abs(i)])
    } else {
        return(NULL)
    }
}

neg_index(df, -2)
[1] -2

neg_index(df, 4)
[1] 4

Answer 3

You can use a dictionary.

> library(hashmap)
> H <- hashmap(-100:100, 1:201)
> H[[-50]]
[1] 51

Answer 4

This is a weird thing to do. You can decide though to create your class to do this. Although I do not recommend you to continue doing so: I will therefore call the class weird

weird = function(x,pos = NULL){
  stopifnot(length(x)%%2==1)
  s = seq_along(x)
  names(x) = s-floor(median(s))
  class(x) = "weird"
  x
}
print.weird = function(x) print(c(unname(x)))
`[.weird` = function(i,j) unname(.Primitive("[")(unclass(i),as.character(j)))



(s = weird(-10:10))
 [1] -10  -9  -8  -7  -6  -5  -4  -3  -2  -1   0   1   2   3   4   5   6   7
[19]   8   9  10

As you can see, this is just a normal vector from -10:10, the thing you cannot see is that the class of this s is weird. you can check by doing class(s)

Now just do:

s[-1]
[1] -1
> s[-10]
[1] -10
> s[0]
[1] 0
> s[3]
[1] 3

Also if you need to use this, then you must specify where index 0 should be. ie for example m = 1:5 what should m[0] be? then you can change the functions accordingly