Question

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)

Answer 1

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)