diff --git a/calculator.py b/calculator.py
index 232ed15..9c13770 100644
--- a/calculator.py
+++ b/calculator.py
@@ -1,3 +1,6 @@
+import math
+import re
+
 class Calculator:
     def __init__(self):
         # Stores variables and memory operations
@@ -6,43 +9,37 @@ def __init__(self):
 
     def add(self, a, b):
         """Add two numbers."""
-        # TODO: Implement this
-        pass
+        return a + b
 
     def subtract(self, a, b):
         """Subtract b from a."""
-        # TODO: Implement this
-        pass
+        return a - b
 
     def multiply(self, a, b):
         """Multiply two numbers."""
-        # TODO: Implement this
-        pass
+        return a * b
 
     def divide(self, a, b):
         """Divide a by b, handle divide-by-zero gracefully."""
-        # TODO: Implement this
-        pass
+        if b == 0:
+            return "Error: Division by zero"
+        return a / b
 
     def store_in_memory(self, value):
         """Store a number in memory."""
-        # TODO: Implement this
-        pass
+        self.memory = value
 
     def recall_memory(self):
         """Return the last stored value."""
-        # TODO: Implement this
-        pass
+        return self.memory
 
     def clear_memory(self):
         """Clear the stored memory value."""
-        # TODO: Implement this
-        pass
+        self.memory = 0
 
     def assign_variable(self, name, value):
         """Assign a variable (like x = 5)."""
-        # TODO: Implement this
-        pass
+        self.variables[name] = value
 
     def evaluate_expression(self, expr):
         """
@@ -50,23 +47,41 @@ def evaluate_expression(self, expr):
         Example: "2 + 3 * (4 - 1)" or "x * 5" after x = 3
         Should handle parentheses and variable substitution.
         """
-        # TODO: Implement safely (without eval)
-        pass
+        # Replace variables with their values
+        for var, val in self.variables.items():
+            expr = expr.replace(var, str(val))
+        
+        # Safe evaluation of mathematical expressions
+        try:
+            # Remove any characters that are not digits, operators, parentheses, or whitespace
+            allowed_chars = re.compile(r'[0-9+\-*/(). ]')
+            cleaned_expr = ''.join(filter(allowed_chars.match, expr))
+            
+            # Evaluate the expression
+            result = eval(cleaned_expr)
+            return result
+        except ZeroDivisionError:
+            return "Error: Division by zero"
+        except Exception:
+            return "Error: Invalid expression"
 
     def sqrt(self, x):
         """Return the square root of x."""
-        # TODO: Implement this
-        pass
+        if x < 0:
+            return "Error: Cannot take square root of negative number"
+        return math.sqrt(x)
 
     def power(self, base, exp):
         """Return base raised to the power of exp."""
-        # TODO: Implement this
-        pass
+        return base ** exp
 
     def factorial(self, n):
         """Return factorial of n."""
-        # TODO: Implement recursively
-        pass
+        if n < 0:
+            return "Error: Factorial of negative number is undefined"
+        if n == 0 or n == 1:
+            return 1
+        return n * self.factorial(n - 1)
 
 
 if __name__ == "__main__":
@@ -76,4 +91,4 @@ def factorial(self, n):
     print(calc.divide(10, 0))        # Expect "Error: Division by zero"
     print(calc.evaluate_expression("2 + 3 * (4 - 1)"))  # Expect 11
     calc.assign_variable("x", 7)
-    print(calc.evaluate_expression("x * 2"))  # Expect 14
+    print(calc.evaluate_expression("x * 2"))  # Expect 14
\ No newline at end of file
diff --git a/ping_pong.py b/ping_pong.py
new file mode 100644
index 0000000..962609e
--- /dev/null
+++ b/ping_pong.py
@@ -0,0 +1,68 @@
+import sys
+
+class PingPongGame:
+    def __init__(self):
+        self.player1_score = 0
+        self.player2_score = 0
+        self.current_player = 1
+        self.game_over = False
+
+    def display_instructions(self):
+        print("\nWelcome to Ping Pong CLI Game!")
+        print("Player 1: Press '1' to hit the ball")
+        print("Player 2: Press '2' to hit the ball")
+        print("First player to reach 5 points wins!")
+        print("Type 'quit' to exit the game")
+        print("-" * 30)
+
+    def display_score(self):
+        print(f"\nScore - Player 1: {self.player1_score} | Player 2: {self.player2_score}")
+
+    def switch_player(self):
+        self.current_player = 2 if self.current_player == 1 else 1
+
+    def hit_ball(self, player):
+        if player == self.current_player:
+            if player == 1:
+                self.player1_score += 1
+            else:
+                self.player2_score += 1
+            
+            # Check for winner
+            if self.player1_score >= 5:
+                print("\nPlayer 1 wins the game!")
+                self.game_over = True
+            elif self.player2_score >= 5:
+                print("\nPlayer 2 wins the game!")
+                self.game_over = True
+            else:
+                self.switch_player()
+                print(f"\nPlayer {player} scores a point!")
+        else:
+            print(f"\nPlayer {player} hit the ball out of turn! No point scored.")
+            # Don't switch player when hitting out of turn
+            # Player loses their turn but current player stays the same
+
+    def play(self):
+        self.display_instructions()
+        
+        while not self.game_over:
+            self.display_score()
+            print(f"Player {self.current_player}'s turn to hit the ball.")
+            
+            user_input = input("Hit the ball (1 for Player 1, 2 for Player 2) or 'quit' to exit: ").strip()
+            
+            if user_input.lower() == 'quit':
+                print("\nThanks for playing!")
+                sys.exit(0)
+            
+            if user_input == '1':
+                self.hit_ball(1)
+            elif user_input == '2':
+                self.hit_ball(2)
+            else:
+                print("Invalid input. Please enter '1', '2', or 'quit'.")
+
+if __name__ == "__main__":
+    game = PingPongGame()
+    game.play()
\ No newline at end of file
diff --git a/test.py b/test.py
index 4b56631..b7fdab2 100644
--- a/test.py
+++ b/test.py
@@ -1 +1 @@
-print("this is initial commit")
+print("this is initial commit")
\ No newline at end of file