Infix evaluation in Python

Question

I am trying to convert the code here http://www.geeksforgeeks.org/expression-evaluation/ to python. However, I am running into some trouble and can't figure out.

class evaluateString:

def evalString(self,expression):
    valueStack = []
    opStack = []
    i=0
    while(i<len(expression)):
        if(expression[i] == ' '):
            continue
        if(expression[i]>='0' and expression[i] <= '9'):
            charNumber = [] #for storing number
            while(i<len(expression) and expression[i]>='0' and expression[i] <= '9'):
                charNumber.append(expression[i])
                i+=1
            valueStack.append(int(''.join(charNumber)))

        elif (expression[i]=='('):
            opStack.append(expression[i])

        elif (expression[i]==')'):
            while(opStack[-1]!='('):
                valueStack.append(self.applyOperation(opStack.pop(),valueStack.pop(),valueStack.pop()))
                opStack.pop()
        elif(expression[i]=='+'or expression[i]=='-'or expression[i]=='*'or expression[i]=='/'):
            while( (len(opStack)!=0) and ( self.opPrecedence(expression[i],opStack[-1]) ) ):
                valueStack.append(self.applyOperation(opStack.pop(),valueStack.pop(),valueStack.pop()))
                opStack.append(expression[i])
        i = i + 1

    while(len(opStack)!=0):
        valueStack.append(self.applyOperation(opStack.pop(),valueStack.pop(),valueStack.pop()))

    return valueStack.pop()


def applyOperation(self,op,a,b):
    if op=='+':
        return a+b
    elif op=='-':
        return a-b
    elif op=='*':
        return a*b
    elif op=='/':
        return a/b
    else:
        return 0

def opPrecedence(self,op1,op2):
    if (op2 == '(' or op2 == ')'):
        return False
    if ((op1 == '*' or op1 == '/') and (op2 == '+' or op2 == '-')):
        return False
    else:
        return True

a = evaluateString()
print(a.evalString("(5+7)"))

I am able to get the right numbers in the valueStack. However, there seems to be problem in the last two elseif. Can someone point me in the right direction?

Answer 1

I have done some fixes and it works for some operations. But I haven't tested it for all cases. Also, operations are only integers, no floats (eg check last output below).

class evaluateString:

  def evalString(self,expression):
    valueStack = []
    opStack = []
    i=0

    while(i<len(expression)):
        if(expression[i] == ' '):
            continue
        if(expression[i]>='0' and expression[i] <= '9'):
            charNumber = [] #for storing number
            j = i
            while(j<len(expression) and expression[j]>='0' and expression[j] <= '9'):
                charNumber.append(expression[j])
                j += 1

            i = (j-1)
            valueStack.append(int(''.join(charNumber)))

        elif (expression[i]=='('):
            opStack.append(expression[i])

        elif (expression[i]==')'):
            while(opStack[-1]!='('):
                valueStack.append(self.applyOperation(opStack.pop(),valueStack.pop(),valueStack.pop()))
            opStack.pop()
        elif(expression[i]=='+'or expression[i]=='-'or expression[i]=='*'or expression[i]=='/'):
            while( (len(opStack)!=0) and ( self.opPrecedence(expression[i],opStack[-1]) ) ):
                valueStack.append(self.applyOperation(opStack.pop(),valueStack.pop(),valueStack.pop()))
            opStack.append(expression[i])
        i = i + 1

    while(len(opStack)!=0):
        valueStack.append(self.applyOperation(opStack.pop(),valueStack.pop(),valueStack.pop()))

    return valueStack.pop()


  def applyOperation(self,op,a,b):
    if op=='+':
        return a+b
    elif op=='-':
        return b-a
    elif op=='*':
        return a*b
    elif op=='/':
        return b/a
    else:
        return 0

  def opPrecedence(self,op1,op2):
    if (op2 == '(' or op2 == ')'):
        return False
    if ((op1 == '*' or op1 == '/') and (op2 == '+' or op2 == '-')):
        return False
    else:
        return True

a = evaluateString()
print(a.evalString("8*12"))        #prints 96
print(a.evalString("(122-434)"))   #prints -312
print(a.evalString("(232+12)/2"))  #print 122
print(a.evalString("232/12+2"))    #prints 21