# How to Program a Calculator in Python We’re going to write a calculator in Python (a reverse Polish calculator) that can run from the command line. This is based on a possible Python homework that we can help with.

import sys

def calc(expr):

return expr

def main(args):

print(calc(” “.join(args[1:])))

if __name__ == “__main__”:

main(sys.argv)

At the moment it just displays the arguments as passed from the command line (skipping the program which is args). So we want to tokenize it, this is probably best done using regular expressions.

import sys, re

tokenize = re.compile(“\w+|-?[0-9.]+|[+-/*]”, re.IGNORECASE)

def calc(expr):

expr = tokenize.findall(expr)

return expr

Now we want to start evaluating the expression, well add a couple of functions initially to ensure the code works.

functions = {“*” : (lambda stack : stack.pop() * stack.pop()),
“+” : (lambda stack : stack.pop() + stack.pop())}

def is_value(token):

try:

if ‘.’ in token:

return float(token)

return int(token)

except:

return

def calc(expr):

stack = []

for token in tokenize.findall(expr):

value = is_value(token)

if value is None:

func = functions.get(token)

if func:

stack.append(func(stack))

else:

stack.append(value)

return stack

Lets extend it so you can add functions, here is a simple example.

> def square: dup *;8 square
64

And the code to implement this.

import sys, re

tokenize = re.compile(“\w+|-?[0-9.]+|[+-/*;:]”, re.IGNORECASE)

def sub(stack):

r, l = stack.pop(), stack.pop()

return l – r

def div(stack):

r, l = stack.pop(), stack.pop()

return l / r

functions = {“*” : (lambda stack : stack.pop() * stack.pop()),

“+” : (lambda stack : stack.pop() + stack.pop()),

“/” : div,

“-” : sub,

“dup” : (lambda stack : stack[-1])

}

definitions = {}

def is_value(token):

try:

if ‘.’ in token:

return float(token)

return int(token)

except:

return

def tokens(expr):

new = False

defining = None

for token in tokenize.findall(expr):

if token in definitions:

for token in definitions[token]:

yield token

else:

if token in (“def”, “:”):

new = True

continue

if new and not defining:

defining = token

definitions[defining] = []

continue

if defining:

if token == “;”:

defining = None

new = False

else:

definitions[defining].append(token)

continue

yield token

def calc(expr):

try:

stack = []

for token in tokens(expr):

value = is_value(token)

if value is None:

func = functions.get(token)

if func:

stack.append(func(stack))

else:

raise Exception(“Invalid token ” + token)

else:

stack.append(value)

if len(stack) != 1:

print(stack)

raise Exception(“Expression unbalanced”)

return stack

except Exception as e:

print(e)

def main(args):

if len(args) > 1:

print(calc(” “.join(args[1:])))

#print(calc(“def f2c: 32- 9/5*;212 f2c”))

while True:

expr = input(“> “)

if expr in (“q”, “quit”, “exit”):

break

print(calc(expr))

if __name__ == “__main__”:

main(sys.argv)

It can easily be extended with variables and saving the state between sessions, you can also add extra functions such as sin, cos, sqrt, etc. The code uses a generator to simplify the main logic in the calc function, as that is totally unaware of the user defined functions. So if you are looking for python assignment help and would be interested in using our service then we can deliver quality results.

At Programmingassignments.com we can write this type of code in a matter of hours and can write far more sophisticated code if required.