CODEWITHSUNDEEP
pythonrecursion
user1647372
user1647372

Reputation:

solving fully parenthesized expressions with recursion

I am having trouble coming up with a recursive method that can solve fully parenthesized equations..such as ((3+2)/(1+4)). I was able to come up with a recursive solution for solving infix expressions like +*+3421 using recursion, but for something like ((3+2)/(1+4)) I am a little stuck.

def evalPrefix(exp):
    it = iter(exp)
    return evalPrefixInner(it)

def evalPrefixInner(it):
    item = it.next()
    if isInt(item):
        return int(item)
    else: 
        operand1 = evalPrefixInner(it)
        operand2 = evalPrefixInner(it)
        return execute(item, operand1, operand2)

Upvotes: 4

Views: 1218

Answers (3)

Ashwini Chaudhary
Ashwini Chaudhary

Reputation: 250961

Try the shunting-yard algorithm :

dic={"+": lambda x, y:x+y,
     "-": lambda x, y:x-y,
     "/": lambda x, y:x/y,
     "%": lambda x, y:x%y,
     "*": lambda x, y:x*y}

def algo(x, op=None, nums=None):
    if x != "":
        op = op if op else []
        nums = nums if nums else []
        item = x[0]
        if item in ("+","-","%","/","*"):
            op.append(item)
        if item.isdigit():
            nums.append(item)
        if item==")":
            operator = op.pop()
            opd1, opd2 = int(nums.pop()), int(nums.pop())
            ans = dic[operator](opd1, opd2)
            nums.append(ans)
        return algo(x[1:], op, nums)
    else:
        if op and nums:
            operator = op.pop()
            opd1, opd2 = int(nums.pop()), int(nums.pop())
            return dic[operator](opd1, opd2)
        else:
            return nums[-1]

print algo("((3+2)/(1+4))")  #output :1
print algo("((5+2)*3*(2+5))") #output :147

Upvotes: 2

mackworth
mackworth

Reputation: 5953

Your grammar is:

expr ::= int | ( expr op expr )

correct?

So, ignoring error checking, something like...

def evalExpr(it):
    item = it.next()
    if isInt(item):
        return int(item)
    else: 
        //item should = lparen
        operand1 = evalExpr(it)
        op = it.next()        
        operand2 = evalExpr(it)
        rparen = it.next() 
        return execute(op, operand1, operand2)

Upvotes: 2

alvonellos
alvonellos

Reputation: 1062

Parse the input for valid data and then use the compiler module to parse the expression like so:

import compiler
expr = compiler.compile(r'((3+2)/(4+1))', 'err', 'eval')

and then you can use:

eval(expr)

Upvotes: 0

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