Added power method and fixed some bugs

.gitignore

@@ -1,2 +1,5 @@
 __pycache__/calculator.cpython-312.pyc
 __pycache__/test_calculator.cpython-312.pyc
+*.exe
+*.bin
+*.o

README.md

@@ -1,3 +1,3 @@
 # python-calculator
 
-This is a very simple calculator program for learning how to write unit tests in Python. It contains a Calculator class (*calculator.py*) which perform 4 mathematic operations: add, subtract, multiply, and divide. You have to add unit test cases to test them in the file *test_calculator.py*. 
+This is a very simple calculator program for learning how to write unit tests in Python. It contains a Calculator class (*calculator.py*) which perform 5 mathematic operations: add, subtract, multiply, power, and divide. You have to add unit test cases to test them in the file *test_calculator.py*. 

calculator.py

@@ -3,32 +3,50 @@ def add(self, a, b):
         return a + b
 
     def subtract(self, a, b):
-        return b - a
+        return a - b
 
     def multiply(self, a, b):
         result = 0
-        for i in range(b+1):
+        for _ in range(abs(b)):
             result = self.add(result, a)
-        return result
+        return result if b >= 0 else -result
 
     def divide(self, a, b):
+        if b == 0:
+            raise ZeroDivisionError("Division by zero")
+        sign = -1 if (a < 0) ^ (b < 0) else 1
+        a, b = abs(a), abs(b)
         result = 0
-        while a > b:
+        while a >= b:
             a = self.subtract(a, b)
             result += 1
-        return result
-    
+        return result * sign
+
     def modulo(self, a, b):
-        while a <= b:
-            a = a-b
-        return a
+        if b == 0:
+            raise ZeroDivisionError("Modulo by zero")
+        sign = -1 if a < 0 else 1
+        a, b = abs(a), abs(b)
+        while a >= b:
+            a = self.subtract(a, b)
+        return a * sign
+
+    def power(self, a, b):
+        if b == 0:
+            return 1
+        if b < 0:
+            return 1 / self.power(a, -b)
+        result = 1
+        for _ in range(b):
+            result = self.multiply(result, a)
+        return result
 
-# Example usage:
 if __name__ == "__main__":
     calc = Calculator()
     print("This is a simple calculator class!")
     print("Example: addition: ", calc.add(1, 2))
     print("Example: subtraction: ", calc.subtract(4, 2))
     print("Example: multiplication: ", calc.multiply(2, 3))
     print("Example: division: ", calc.divide(10, 2))
-    print("Example: modulo: ", calc.modulo(10, 3))
+    print("Example: modulo: ", calc.modulo(10, 3))
+    print("Example: power: ", calc.power(2, 3))