Building an Infix Calculator in Python
Motivation
When writing mathematical expressions, we generally utilise what is known as infix notation. In infix form, an operator is written in between the two operands on which the operation is to be evaluated.
Evaluation of these operators is then in the order of PEMDAS:
- Parentheses
- Exponents
- Multiplication
- Division
- Addition
- Subtraction
Given that the computer is naive, we must resort to alogithmically evaluating a mathematical infix expression by converting it first into either postfix or prefix notation which is easier for a computer to process systematically.
If we evaluate an infix expression A * B - (C + D) + E, it is convertible into a postfix expression A B * C D + - E +. This represents the stacking of two operands followed by the operator that is to be used on the prior operands.
My code for a CLI infix calculator calculator.py takes an infix expression as a string at runtime in the terminal from where it is called, performs validation checks to ensure that it is a valid infix expression, algorithmically converts it into a postfix expression, then evaluates the postfix expression to yield the correct answer.
The source code for this project is available on my Github[1] for both personal use and as an educational sample under the MIT license.
Source Code
#!/usr/bin/python3
import sys
def compareOperations(operation, target):
if operation == "^":
return True
if operation == "*":
if target == "^":
return False
else:
return True
elif operation == "/":
if target == "*" or target == "^":
return False
else:
return True
elif operation == "+":
if target == "+" or target == "-":
return True
else:
return False
else:
return False
class InfixExpression:
infix = []
operands = []
operators = []
def __init__(self, infixexpression):
operations = ["+", "-", "*", "/", "^"]
for character in infixexpression:
if character not in operations and (character == "." or character.isnumeric()):
if len(self.infix) - 1 < 0:
self.infix.append(character)
else:
self.infix[len(self.infix)-1] += character
elif character in operations:
if len(self.infix) - 1 < 0:
self.infix.append(character)
elif self.infix[len(self.infix)-1] in operations or self.infix[len(self.infix)-1] == "":
if self.infix[len(self.infix)-1] == "-":
self.infix.append(character)
else:
self.infix[len(self.infix)-1] += character
if character != "-":
self.infix.append("")
else:
self.infix.append(character)
self.infix.append("")
if self.infix[len(self.infix)-1] == "":
self.infix.pop(-1)
if self.check_infix() == False:
print("Error: Invalid infix expression!")
#print("Infix Expression:", self.infix)
return None
def check_infix(self):
operations = ["+", "-", "*", "/", "^"]
for element in self.infix:
if element not in operations:
self.operands.append(element)
elif element in operations:
self.operators.append(element)
if len(self.operands) == 0 and len(self.operators) == 0:
return True
if len(self.operands) > 0 and len(self.operators) == 0:
return True
if len(self.operands) == 0 and len(self.operators) > 0:
return False
if len(self.operands) <= len(self.operators) and self.infix[0] != "-":
return False
elif self.infix[0] == self.operators[0] and self.infix[0] != "-":
return False
elif self.infix[len(self.infix)-1] == self.operators[len(self.operators)-1]:
return False
else:
operations = ["+", "-", "*", "/", "^"]
i = 0
while i < len(self.infix)-1:
if self.infix[i] in operations and self.infix[i+1] in operations:
return False
i += 1
return True
def infixtopostfix(self):
operations = ["+", "-", "*", "/", "^"]
buffer = []
self.operands = []
self.operators = []
for element in self.infix:
if element not in operations:
buffer.append(element)
elif element in operations:
thisOps = element
while self.operators != [] and compareOperations(self.operators[-1], thisOps) == True:
buffer.append(self.operators.pop(-1))
self.operators.append(thisOps)
while self.operators != []:
buffer.append(self.operators.pop(-1))
print("Postfix Expression:", buffer)
return buffer
def evaluate(self):
operations = ["+", "-", "*", "/", "^"]
self.operands = []
self.operators = []
for element in self.infix:
if element not in operations:
self.operands.append(element)
elif element in operations:
thisOps = element
while self.operators != [] and compareOperations(self.operators[-1], thisOps) == True:
o = self.operators.pop(-1)
b = self.operands.pop(-1)
a = self.operands.pop(-1)
a, b = float(a), float(b)
if o == "^":
self.operands.append(a**b)
elif o == "*":
self.operands.append(a*b)
elif o == "/":
self.operands.append(a/b)
elif o == "+":
self.operands.append(a+b)
elif o == "-":
self.operands.append(a-b)
self.operators.append(thisOps)
while self.operators != []:
o = self.operators.pop(-1)
b = self.operands.pop(-1)
a = self.operands.pop(-1)
a, b = float(a), float(b)
if o == "^":
self.operands.append(a**b)
elif o == "*":
self.operands.append(a*b)
elif o == "/":
self.operands.append(a/b)
elif o == "+":
self.operands.append(a+b)
elif o == "-":
self.operands.append(a-b)
print("Evaluated:", self.operands)
return self.operands
def main():
if len(sys.argv) == 2:
expression = str(sys.argv[1:])
calculation = InfixExpression(expression)
if calculation.check_infix() == False:
sys.exit()
else:
calculation.infixtopostfix()
calculation.evaluate()
else:
print('Error in expression. Please type "./calculator.py <expression>" or "python calculator.py <expression>" with the expression surrounded by quotation marks.')
if __name__ == '__main__':
main()
Open Source
[1] Github Repository: Infix Calculator