1 Task

Write a file named calculator.py that has a function binop which can evaluate strings representing simple mathematical expressions.

binop accepts a single string parameter consisting of two positive integers and a singe operator between them, where the operator is one of +, -, *, or /. There may also be arbitrarily many spaces before or after either number and/or the operator. The function returns the int or float value created by evaluating the expression.

Python has several built-in ways to do this (eval, exec, compile, etc.); do not use them. calculator.py also should not import anything.

2 Example Invocations

When you run calculator.py, nothing should happen. It defines a function, it does not run it.

If in another file (which you do not submit) you write the following:

import calculator

print(calculator.binop("111 * 10"))
print(calculator.binop("    11+11     "))
print(calculator.binop("   965   -      123"))

you should get the following output:


3 Thought Questions

How hard would it be to

  • Return the number if all there is is one number?
  • Deal with negative numbers too, like "-123 - -456"
  • Deal with a string with two operators, like "1 + 23 * 456"?

4 Troubleshooting

There are four operators that could be in the string, so you probably want four cases in a big if-elif-type statement.

Those string methods you learned about in lab will come in handy.

Since you can’t rely on the number of spaces, it probably makes sense to find the operator and slice off the part before and after that to turn into ints.

int can deal with spaces just fine (int(" 12 ") works).