Subscript out of bounds when using [[]]

Question

I'm attempting to simulate a hash map with the following code and get an index out of bounds in the if statement?

I'm not sure what is causing this since printing the initial null values from a loop works fine.

strings <- c("string1","string2","string3")

test <- NULL

for (i in 1:length(strings)) {
  print(is.null(test[[strings[i]]]))
  }

for (i in 1:length(strings)) {
  if (is.null(test[[strings[i]]])) {
    test[[strings[i]]] <- 1
    }
  }

Answer 1

Instead of test <- NULL you can initialize test as a list:

test <- list()

This should remove the error message and give the desired result:

> test
#$string1
#[1] 1
#
#$string2
#[1] 1
#
#$string3
#[1] 1