diff --git a/chapter1/binary.py b/chapter1/binary.py
index 5164f87..35cf75a 100644
--- a/chapter1/binary.py
+++ b/chapter1/binary.py
@@ -6,13 +6,7 @@ logging.basicConfig(level=logging.DEBUG)
 logger = logging.getLogger(__name__)
 
 
-LOWER = 1000
-UPPER = 1000000
-SAMPLE_SIZE = 1000
-
-
 def binary_search(arr, item):
-    count = 1
     low = 0
     high = len(arr) - 1
 
@@ -20,27 +14,29 @@ def binary_search(arr, item):
         mid = (low + high) // 2
         guess = arr[mid]
         if guess == item:
-            logger.debug(f"found item after {count} splits")
             return mid
         elif guess > item:
             high = mid - 1
         else:
             low = mid + 1
 
-        count += 1
-
     return None
 
 
+LOWER = 1000
+UPPER = 1000000
+SAMPLE_SIZE = 1000
+
+
 numbers = random.sample(range(LOWER, UPPER), SAMPLE_SIZE)
 numbers.sort()
-logger.debug(numbers)
 
 start = time.time()
-res = None
-while res is None:
+result = None
+while result is None:
     guess = random.randrange(LOWER, UPPER)
-    res = binary_search(numbers, guess)
+    logger.debug(f"guess: {guess}")
+    result = binary_search(numbers, guess)
 
 
-print(f"Found {guess} at index {res}. Running time {time.time() - start}")
+print(f"Found {guess} at index {result}. Running time {time.time() - start}")
diff --git a/chapter4/binary.py b/chapter4/binary.py
new file mode 100644
index 0000000..1c82ab6
--- /dev/null
+++ b/chapter4/binary.py
@@ -0,0 +1,43 @@
+import logging
+import random
+import time
+
+logging.basicConfig(level=logging.DEBUG)
+logger = logging.getLogger(__name__)
+
+
+def binary_search(arr, low, high, x):
+    # x not found, low has overlapped high so we return None
+    if high < low:
+        return
+
+    mid = (high + low) // 2
+
+    # x found at middle of split, return its index
+    if arr[mid] == x:
+        return mid
+
+    # x in left side of split
+    elif arr[mid] > x:
+        return binary_search(arr, low, mid - 1, x)
+
+    # x in right side of split
+    else:
+        return binary_search(arr, mid + 1, high, x)
+
+
+LOWER = 1000
+UPPER = 1000000
+SAMPLE_SIZE = 1000
+
+numbers = random.sample(range(LOWER, UPPER), SAMPLE_SIZE)
+numbers.sort()
+
+start = time.time()
+result = None
+while result is None:
+    guess = random.randrange(LOWER, UPPER)
+    logger.debug(f"guess: {guess}")
+    result = binary_search(numbers, 0, len(numbers) - 1, guess)
+
+print(f"Found {guess} at index {result}. Running time {time.time() - start}")