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?
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 (e.g. 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