prefix & postfix evaluation

Author: mohanakrishnavh

DataStructures/Stack/postfix_evaluation.py

@@ -0,0 +1,77 @@
+from Stack import Stack
+
+
+def is_operator(expr):
+    '''
+    Checks if a given expression is operator
+    :param expr: Input expression
+    :return: True if the expression is an operator, else False
+    '''
+    if expr == '+' or expr == '-' or expr == '*' or expr == '/':
+        return True
+    return False
+
+
+def is_operand(expr):
+    '''
+    Checks if a given expression is digit or not
+    :param expr: Input expression
+    :return: True if the expression is a digit, else False
+    '''
+    if expr.isdigit():
+        return True
+    else:
+        return False
+
+
+def perform(operator, op1, op2):
+    '''
+    Performs arithmetic operation on the given operands
+    :param operator: Arithmetic operator
+    :param op1: Operand 1
+    :param op2: Operand 2
+    :return: Result of arithmetic operation on operands
+    '''
+    if operator == '+':
+        return op1 + op2
+    elif operator == '-':
+        return op1 - op2
+    elif operator == '*':
+        return op1 * op2
+    elif operator == '/':
+        return op1 / op2
+    else:
+        print("Invalid Operation")
+        return -1
+
+
+def evaluate_postfix_expr(postfix_expr):
+    '''
+    Evaluates a postfix expression using stack
+    :param postfix_expr: Postfix expression
+    :return: evaluated result
+    '''
+    stack = Stack()
+    n = len(postfix_expr)
+
+    for i in range(n):
+        if is_operand(postfix_expr[i]):
+            stack.push(int(postfix_expr[i]))
+        elif is_operator(postfix_expr[i]):
+            op2 = stack.peek()
+            stack.pop()
+            op1 = stack.peek()
+            stack.pop()
+            result = perform(postfix_expr[i], op1, op2)
+            stack.push(result)
+    return stack.peek()
+
+
+def main():
+    postfix_expr = "2,3,*,5,4,*,+,9,-"
+    postfix_expr = postfix_expr.split(',')
+    print(evaluate_postfix_expr(postfix_expr))
+
+
+if __name__ == '__main__':
+    main()

DataStructures/Stack/prefix_evaluation.py

@@ -0,0 +1,77 @@
+from Stack import Stack
+
+
+def is_operator(expr):
+    '''
+    Checks if a given expression is operator
+    :param expr: Input expression
+    :return: True if the expression is an operator, else False
+    '''
+    if expr == '+' or expr == '-' or expr == '*' or expr == '/':
+        return True
+    return False
+
+
+def is_operand(expr):
+    '''
+    Checks if a given expression is digit or not
+    :param expr: Input expression
+    :return: True if the expression is a digit, else False
+    '''
+    if expr.isdigit():
+        return True
+    else:
+        return False
+
+
+def perform(operator, op1, op2):
+    '''
+    Performs arithmetic operation on the given operands
+    :param operator: Arithmetic operator
+    :param op1: Operand 1
+    :param op2: Operand 2
+    :return: Result of arithmetic operation on operands
+    '''
+    if operator == '+':
+        return op1 + op2
+    elif operator == '-':
+        return op1 - op2
+    elif operator == '*':
+        return op1 * op2
+    elif operator == '/':
+        return op1 / op2
+    else:
+        print("Invalid Operation")
+        return -1
+
+
+def evaluate_prefix_expr(prefix_expr):
+    '''
+    Evaluates a postfix expression using stack
+    :param postfix_expr: Postfix expression
+    :return: evaluated result
+    '''
+    stack = Stack()
+    n = len(prefix_expr)
+
+    for i in range(n-1, -1, -1):
+        if is_operand(prefix_expr[i]):
+            stack.push(int(prefix_expr[i]))
+        elif is_operator(prefix_expr[i]):
+            op1 = stack.peek()
+            stack.pop()
+            op2 = stack.peek()
+            stack.pop()
+            result = perform(prefix_expr[i], op1, op2)
+            stack.push(result)
+    return stack.peek()
+
+
+def main():
+    postfix_expr = "-,+,*,2,3,*,5,4,9"
+    postfix_expr = postfix_expr.split(',')
+    print(evaluate_prefix_expr(postfix_expr))
+
+
+if __name__ == '__main__':
+    main()