Add Exponential Search in R (#230)

Author: Siddhram

searches/exponential_search.r

@@ -0,0 +1,56 @@
+# Exponential Search in R – efficient search on sorted arrays
+
+#' Performs exponential search on a sorted numeric vector.
+#'
+#' @param arr Numeric vector sorted in ascending order.
+#' @param target Numeric value to search for in arr.
+#' @return The 1-based index of target in arr if found; otherwise -1.
+#' @details Exponential search first finds a range where the target may reside by
+#' doubling the index (1, 2, 4, 8, ...), then performs a binary search within that range.
+#' Time complexity: O(log n); requires the array to be sorted in ascending order.
+exponential_search <- function(arr, target) {
+  n <- length(arr)
+  if (n == 0) return(-1)
+
+  # If the first element is the target
+  if (arr[1] == target) return(1)
+
+  # Find range for binary search by repeated doubling
+  i <- 2
+  while (i <= n && arr[i] <= target) {
+    i <- i * 2
+  }
+
+  low <- max(1, i %/% 2)
+  high <- min(i, n)
+
+  # Binary search within [low, high]
+  return(.binary_search_range(arr, target, low, high))
+}
+
+# Internal helper: standard binary search on subrange [low, high]
+.binary_search_range <- function(arr, target, low, high) {
+  while (low <= high) {
+    mid <- low + (high - low) %/% 2
+    if (arr[mid] == target) {
+      return(mid)
+    } else if (arr[mid] < target) {
+      low <- mid + 1
+    } else {
+      high <- mid - 1
+    }
+  }
+  return(-1)
+}
+
+# --- Demonstration (mirrors other search files) ---
+arr <- c(2, 3, 4, 10, 15, 18, 20, 25, 30) # sorted input array
+target <- 18 # value to search
+
+idx <- exponential_search(arr, target)
+
+if (idx == -1) {
+  cat("Element", target, "not found in the array.\n")
+} else {
+  cat("Element", target, "found at position", idx, ".\n")
+}