How can I draw from a list of math operators and have it interpreted as a math operation and not a string?

Question

I'm trying to use this loop to condense existing code, but storing the operators in a list and then drawing from it doesn't work, it throws this error: TypeError: unsupported operand type(s) for +: 'float' and 'str'

I understand that it's not interpreting the operator variable as an actual math operation, it's reading it as a str, but I'm not sure how to fix that. Here's the complete loop I've built.

operators = ["+", "-", "*", "/", "^"]

for operator in operators:
    math_expression = input("Enter your math expression, or (q)uit: ")
    print("Operator found " + operator)
    operator_position = math_expression.find(operator) # find operator
    print("Found " + operator + "operator at index " + str(operator_position))
    first_number = math_expression[:operator_position] # find first number
    second_number = math_expression[operator_position + 1:] # find second number
    answer = float(first_number) + operator + float(second_number)

What you're trying is equivalent to something like `1.5 + "*" + 0.5`, which would just result in that error, since you're trying to concatenate floating point numbers with strings. You could be using something like [`eval`](https://docs.python.org/3/library/functions.html#eval) or `exec`, have you tried those? They should only be used if you're not trying to create something secure though. — Random Davis, Nov 23 '20 at 16:50
Don't use `eval` or `exec` though! They can be used to execute any arbitrary code. — AKX, Nov 23 '20 at 16:51
@RandomDavis I have not tried that, I'll look into it, thanks! — aidenmitchell, Nov 23 '20 at 16:51
Create a *mapping* (i.e. a dict) from the string to a function which executes whatever operation you want. e.g. `{"+": lambda x,y: return x+y, "-": lambda x,y: x - y, ...}`, or using `import operator`, you can use `{"+": operator.add, "-": operator.sub, ...}` — juanpa.arrivillaga, Nov 23 '20 at 16:51
@Iguananaut That seems a little complicated, but, yes, I'm trying to create a calculator. — aidenmitchell, Nov 23 '20 at 16:52
@juanpa.arrivillaga Yeah I'd love to start with a different format, but I can't store them otherwise, at least I don't think. — aidenmitchell, Nov 23 '20 at 16:53
@aidenmitchell **of course you can**. See my edit to my previous comment — juanpa.arrivillaga, Nov 23 '20 at 16:53
@aidenmitchell well, because creating a calculator, "correctly" *is complicated*. It would involve writing a parser, at least, for the input strings you want to process. You can, of course, use `eval` or `exec` which uses the Python parser, but as noted, generally you want to avoid `eval`ing strings from user input, that is very unsafe — juanpa.arrivillaga, Nov 23 '20 at 16:54
I picked one existing question about this at random (there are many). But implementing a simple calculator program is a fairly typical exercise and something you can search around about. You can get sophisticated with it but for starters if you just want to handle expressions with a single infix operator, create a mapping from the operator (eg the string '+' to a function that computes it (eg. operator.add) — Iguananaut, Nov 23 '20 at 16:55

score 1 · Accepted Answer · answered Nov 23 '20 at 16:55

The operator module has functions that represent basic operations, such as the ones you're looking for.

Here, we

stuff those operators into a dict
loop over the symbol and function for each operator
if the symbol is found in the user input, the str.partition() function splits the string into segments before and after the symbol...
and then we just call the function.

import operator

operators = {
    "+": operator.add,
    "-": operator.sub,
    "*": operator.mul,
    "/": operator.truediv,
    "^": operator.pow,
}

expression = "5 + 3"  # elided input to make debugging faster

for symbol, func in operators.items():
    if symbol in expression:
        a, _, b = expression.partition(symbol)
        answer = func(float(a), float(b))
        print(answer)
        break

How can I draw from a list of math operators and have it interpreted as a math operation and not a string?

1 Answers1