Two-Sum with Hash Lookup

Walk the array once, storing seen values in a lookup table. When the complement is already present, the result indices are known.

Algorithm

Basic Implementation

basic.R

arr <- c(2, 7, 11, 4, 5)
target <- 9
seen <- new.env(hash = TRUE, parent = emptyenv())
first <- -1
second <- -1
i <- 1
while (i <= length(arr)) {
  value <- arr[i]
  need <- target - value
  key <- as.character(need)
  if (exists(key, envir = seen, inherits = FALSE)) {
    first <- get(key, envir = seen)
    second <- i - 1
    break
  }
  assign(as.character(value), i - 1, envir = seen)
  i <- i + 1
}
cat("[", first, ", ", second, "]\n", sep = "")

Complexity

Time: O(n) average
Space: O(n)

Implementation notes

arr <- c(2, 7, 11, 4, 5) creates the pinned numeric R vector.
target <- 9 is the scalar target for this run.
seen <- new.env(hash = TRUE, parent = emptyenv()) uses a hashed R environment as the lookup table.
R vector access is 1-based, so the scan starts with i <- 1 and reads value <- arr[i].
The lesson still prints zero-based result indexes: both stored indexes and returned indexes use i - 1.
need <- target - value computes the complement.
key <- as.character(need) converts the complement to a string key because environment bindings are named.
exists(key, envir = seen, inherits = FALSE) checks only the current environment, not parent scopes.
On a hit, get(key, envir = seen) retrieves the earlier zero-based index.
On a miss, assign(as.character(value), i - 1, envir = seen) records the current value under a string key.

Replay steps

start: arr = [2, 7, 11, 4, 5], target = 9, seen = {}
R i=1, value=2: need 7, miss, record key "2" -> 0
R i=2, value=7: need 2, hit key "2", result [0, 1]

break stops the loop after the hit, so 11, 4, and 5 are not scanned.
cat("[", first, ", ", second, "]\n", sep = "") prints [0, 1] with no extra spaces beyond the literal comma-space.

execution replay The checked-in replay follows the language-neutral state table for `array-two-sum-hash`.

cross-language comparison This R DSA version keeps the same data and final output as every other DSA book in this wave.