Added R version of the binary_search function (#253)

01_introduction_to_algorithms/R/binary_search.R

@@ -0,0 +1,37 @@
+binary_search <- function(list, item) {
+        # low and high keep track of which part of the list you'll search in.
+        # Every data structure in R indexed by starting at 1.
+        low <- 1
+        high <- length(list)
+        
+        # While you haven't narrowed it down to one element ...
+        while (low <= high) {
+                # ... check the middle element
+                mid <- (low + high) %/% 2
+                guess <- list[mid]
+                # Found the item.
+                if (guess == item) {
+                        return(mid)
+                }
+                # The guess was too high.
+                else if (guess > item) {
+                        high <- mid - 1
+                } 
+                else{ # The guess was too low.
+                        low <- mid + 1
+                }
+        }
+        # Item doesn't exist
+        return(NULL)
+}
+
+
+# Set a list
+my_list <- list(1, 3, 5, 7, 9)
+
+# Call the function
+binary_search(my_list, 3) # => 1
+binary_search(my_list, -1) # => NULL
+
+# All above code can be simplified by using "which" function
+which(my_list == 3) # => 1