Updated postfix_evaluation.py to support Unary operators

data_structures/stacks/postfix_evaluation.py

@@ -1,4 +1,11 @@
 """
+The Reverse Polish Nation also known as Polish postfix notation
+or simply postfix notation.
+https://en.wikipedia.org/wiki/Reverse_Polish_notation
+Classic examples of simple stack implementations
+Valid operators are +, -, *, /.
+Each operand may be an integer or another expression.
+
 Output:
 
 Enter a Postfix Equation (space separated) = 5 6 9 * +
@@ -17,52 +24,205 @@
         Result =  59
 """
 
-import operator as op
+# Defining valid unary operator symbols
+UNARY_OP_SYMBOLS = ("-", "+")
+
+# Defining valid binary operator symbols
+BINARY_OP_SYMBOLS = ("-", "+", "*", "^", "/")
+
+
+def parse_token(token: str | float) -> float | str:
+    """
+    Converts the given data to appropriate number if it is indeed a number, else returns
+    the data as it is with a False flag. This function also serves as a check of whether
+    the input is a number or not.
+
+    Parameters
+    ----------
+    token : str or float
+        The data which needs to be converted to the appropriate number
+
+    Returns
+    -------
+    float or str
+        Returns a float if `token` is a number and returns `token` if it's an operator
+    """
+    if is_operator(token):
+        return token
+    try:
+        return float(token)
+    except ValueError:
+        msg = f"{token} is neither a number nor a valid operator"
+        raise ValueError(msg)
+
+
+def is_operator(token: str | float) -> bool:
+    """
+    Checks whether a given input is one of the valid operators or not.
+    Valid operators being '-', '+', '*', '^' and '/'.
+
+    Parameters
+    ----------
+    token : str
+        The value that needs to be checked for operator
+
+    Returns
+    -------
+    bool
+        True if data is an operator else False.
+    """
+    return token in BINARY_OP_SYMBOLS
+
+
+def evaluate(post_fix: list[str], verbose: bool = False) -> float:
+    """
+    Function that evaluates postfix expression using a stack.
+    >>> evaluate(["2", "1", "+", "3", "*"])
+    9.0
+    >>> evaluate(["4", "13", "5", "/", "+"])
+    6.6
+    >>> evaluate(["2", "-", "3", "+"])
+    1.0
+    >>> evaluate(["-4", "5", "*", "6", "-"])
+    -26.0
+    >>> evaluate([])
+    0
+    >>> evaluate(["4", "-", "6", "7", "/", "9", "8"])
+    Traceback (most recent call last):
+    ...
+    ArithmeticError: Input is not a valid postfix expression
+
+    Parameters
+    ----------
+    post_fix : list
+        The postfix expression tokenized into operators and operands and stored as a
+        python list
 
+    verbose : bool
+        Display stack contents while evaluating the expression if verbose is True
 
-def solve(post_fix):
+    Returns
+    -------
+    float
+        The evaluated value
+    """
     stack = []
-    div = lambda x, y: int(x / y)  # noqa: E731 integer division operation
+    valid_expression = []
     opr = {
-        "^": op.pow,
-        "*": op.mul,
-        "/": div,
-        "+": op.add,
-        "-": op.sub,
+        "^": lambda p, q: p**q,
+        "*": lambda p, q: p * q,
+        "/": lambda p, q: p / q,
+        "+": lambda p, q: p + q,
+        "-": lambda p, q: p - q,
     }  # operators & their respective operation
-
-    # print table header
-    print("Symbol".center(8), "Action".center(12), "Stack", sep=" | ")
-    print("-" * (30 + len(post_fix)))
-
-    for x in post_fix:
-        if x.isdigit():  # if x in digit
+    if len(post_fix) == 0:
+        return 0
+    # Checking the list to find out whether the postfix expression is valid
+    valid_expression = [parse_token(token) for token in post_fix]
+    if verbose:
+        # print table header
+        print("Symbol".center(8), "Action".center(12), "Stack", sep=" | ")
+        print("-" * (30 + len(post_fix)))
+    for x in valid_expression:
+        if not is_operator(x):
             stack.append(x)  # append x to stack
-            # output in tabular format
-            print(x.rjust(8), ("push(" + x + ")").ljust(12), ",".join(stack), sep=" | ")
-        else:
+            if verbose:
+                # output in tabular format
+                print(
+                    str(x).rjust(8),
+                    ("push(" + str(x) + ")").ljust(12),
+                    stack,
+                    sep=" | ",
+                )
+            continue
+        # If x is operator
+        # If only 1 value is inside stack and + or - is encountered
+        # then this is unary + or - case
+        if x in UNARY_OP_SYMBOLS and len(stack) < 2:
             b = stack.pop()  # pop stack
+            if x == "-":
+                b *= -1  # negate b
+            stack.append(b)
+            if verbose:
+                # output in tabular format
+                print(
+                    "".rjust(8),
+                    ("pop(" + str(b) + ")").ljust(12),
+                    stack,
+                    sep=" | ",
+                )
+                print(
+                    str(x).rjust(8),
+                    ("push(" + str(x) + str(b) + ")").ljust(12),
+                    stack,
+                    sep=" | ",
+                )
+            continue
+        b = stack.pop()  # pop stack
+        if verbose:
             # output in tabular format
-            print("".rjust(8), ("pop(" + b + ")").ljust(12), ",".join(stack), sep=" | ")
+            print(
+                "".rjust(8),
+                ("pop(" + str(b) + ")").ljust(12),
+                stack,
+                sep=" | ",
+            )
 
-            a = stack.pop()  # pop stack
+        a = stack.pop()  # pop stack
+        if verbose:
             # output in tabular format
-            print("".rjust(8), ("pop(" + a + ")").ljust(12), ",".join(stack), sep=" | ")
-
-            stack.append(
-                str(opr[x](int(a), int(b)))
-            )  # evaluate the 2 values popped from stack & push result to stack
+            print(
+                "".rjust(8),
+                ("pop(" + str(a) + ")").ljust(12),
+                stack,
+                sep=" | ",
+            )
+        # evaluate the 2 values popped from stack & push result to stack
+        stack.append(opr[str(x)](a, b))
+        if verbose:
             # output in tabular format
             print(
-                x.rjust(8),
-                ("push(" + a + x + b + ")").ljust(12),
-                ",".join(stack),
+                str(x).rjust(8),
+                ("push(" + str(a) + str(x) + str(b) + ")").ljust(12),
+                stack,
                 sep=" | ",
             )
+    # If everything executed correctly, the stack will contain
+    # only one element which is the result
+    if len(stack) != 1:
+        raise ArithmeticError("Input is not a valid postfix expression")
+    return float(stack[0])
+
+
+def is_yes(val: str) -> bool:
+    """
+    Function that checks whether a user has entered any representation of a Yes (y, Y).
+    Any other input is considered as a No.
+
+    Parameters
+    -----------
+    val : str
+        The value entered by user
 
-    return int(stack[0])
+    Returns
+    -------
+    bool
+        True if Yes, otherwise False
+    """
+    return val in ("Y", "y")
 
 
 if __name__ == "__main__":
-    Postfix = input("\n\nEnter a Postfix Equation (space separated) = ").split(" ")
-    print("\n\tResult = ", solve(Postfix))
+    loop = True
+    # Creating a loop so that user can evaluate postfix expression multiple times
+    while True:
+        expression = input("Enter a Postfix Expression (space separated): ").split(" ")
+        choice = input(
+            "Do you want to see stack contents while evaluating? [y/N]: "
+        ).strip()
+        display = is_yes(choice)
+        output = evaluate(expression, display)
+        print("Result = ", output)
+        choice = input("Do you want to enter another expression? [y/N]: ")
+        if not is_yes(choice):
+            break