add recursive_binary_search from seminar 11

11_recursion/seminar/recursive_binary_search.cpp

@@ -0,0 +1,95 @@
+/****************************************************************************
+ * This file is part of the "Introduction to programming" course. FMI 2019/20
+ *****************************************************************************/
+
+/**
+ * @file   recursive_binary_search.cpp
+ * @author Ivan Filipov
+ * @date   01.2020
+ * @brief  Example implementation of binary search - recursive variant.
+ */
+
+#include <iostream>
+
+int binary_search_rec(int* arr, int left, int right, int elem) {
+	//there is no such element
+	if (left > right)
+		return -1;
+	//
+	// calculate the middle index
+	// int mid = (L + R) / 2; // <- a bug source, because L + R can easily overflow
+	// solution is
+	int mid = left + (right - left) / 2;
+	// or even better int mid = L + (((unsigned int)R - (unsigned int)L) >> 1);
+	//
+	// is the search element on that index?
+	if (arr[mid] == elem)
+		return mid;
+	//
+	// grater than the middle element -> search in right
+	if (arr[mid] > elem)
+		return binary_search_rec(arr, left, mid - 1, elem);
+	//
+	// less than the middle element -> search in the left
+	if (arr[mid] < elem)
+		return binary_search_rec(arr, mid + 1, right, elem);
+	//
+	// won't reach here
+	return -1;
+}
+
+// wrapper function
+int binary_search(int* arr, size_t size, int elem) {
+
+	return binary_search_rec(arr, 0, (int)size - 1, elem);
+}
+
+// a variant which uses pointers arithmetic,
+// in order to avoid the passing of left and right, but just size
+// returns a pointer to the element, if found
+int* binary_search_rec_ptr(int* arr, size_t size, int elem) {
+
+	if (size == 0 && arr[size] != elem)
+		return nullptr;
+
+	size_t mid = size / 2;
+
+	if (arr[mid] == elem)
+		return arr + mid;
+
+	if (arr[mid] < elem)
+		return binary_search_rec_ptr(arr + mid + 1, size - (mid + 1), elem);
+
+	if (arr[mid] > elem)
+		return binary_search_rec_ptr(arr, mid - 1, elem);
+}
+
+int main() {
+
+	int sorted_arr[] = { -121, -9, 5, 11, 23, 48, 70 };
+	size_t size = sizeof(sorted_arr) / sizeof(int);
+
+	int searched_el = -9;
+
+	int index = binary_search(sorted_arr, size, searched_el);
+
+    if (index != -1) {
+        std::cout << "Found " << searched_el
+                  << " at index: " << index
+                  << std::endl;
+    } else {
+        std::cout << searched_el << " not found." << std::endl;
+    }
+
+    int* elem_ptr = binary_search_rec_ptr(sorted_arr, size, searched_el);
+
+    if (elem_ptr) {
+        std::cout << "Found " << *elem_ptr
+                  << " at index: " << (int)(elem_ptr - sorted_arr)
+                  << std::endl;
+    } else {
+        std::cout << searched_el << " not found." << std::endl;
+    }
+
+	return 0;
+}