8. Evaluation of Prefix expression

Evaluation of Prefix expression¶

In [1]:

prefix_expression = '- + 2 * 3 4 / 16 ^ 2 3'

prefix_expression_list = prefix_expression.split()

prefix_expression = '- + 2 * 3 4 / 16 ^ 2 3' prefix_expression_list = prefix_expression.split()

In [2]:

def prefix_expression_evaluation(expression):
    operators = ["+", "-", "/", "*", "^"]
    
    rev_prefix_expression = expression[::-1]
    print(rev_prefix_expression)
    
    stack = []
    for i in rev_prefix_expression:
        if i in operators:
            a = stack.pop()
            b = stack.pop()
            
            if i == "^":
                i = "**"
            s = a+i+b
            print(s)
            ans = str(eval(a+i+b))
            print("pushing {} onto stack".format(ans))
            stack.append(ans)
        else:
            print("pushing {} onto stack".format(i))
            stack.append(i)
            
    
    return float(stack[-1])

def prefix_expression_evaluation(expression): operators = ["+", "-", "/", "*", "^"] rev_prefix_expression = expression[::-1] print(rev_prefix_expression) stack = [] for i in rev_prefix_expression: if i in operators: a = stack.pop() b = stack.pop() if i == "^": i = "**" s = a+i+b print(s) ans = str(eval(a+i+b)) print("pushing {} onto stack".format(ans)) stack.append(ans) else: print("pushing {} onto stack".format(i)) stack.append(i) return float(stack[-1])

In [3]:

prefix_expression_evaluation(prefix_expression_list)

prefix_expression_evaluation(prefix_expression_list)

['3', '2', '^', '16', '/', '4', '3', '*', '2', '+', '-']
pushing 3 onto stack
pushing 2 onto stack
2**3
pushing 8 onto stack
pushing 16 onto stack
16/8
pushing 2.0 onto stack
pushing 4 onto stack
pushing 3 onto stack
3*4
pushing 12 onto stack
pushing 2 onto stack
2+12
pushing 14 onto stack
14-2.0
pushing 12.0 onto stack

Out[3]:

12.0