Why does the last if statement print when the condition is false

Question

Why does "Invalid Operator" print when the if statement is false? Could I use an elif statement to make it work?

num1 = float(input("Enter the first number: "))
operator = input("Enter your operator here: ")
num2 = float(input("Enter your second number here: "))


if operator == "+":
    print(num1 + num2)

if operator == "-":
    print(num1 - num2)
if operator == "*":
    print(num1 * num2)
if operator == "/":
    print(num1 / num2)
if operator != ("+", "-", "*", "/"):
    print("Invalid Operator.")

What do you think `operator != ("+", "-", "*", "/")` means and why? — lurker, Feb 26 '21 at 20:48
`operator` isn't equal to that tuple, that's why. You probably meant to use `not in` instead of `!=`. (But an `if/elif/else` chain is probably most normal here.) — Robin Zigmond, Feb 26 '21 at 20:48
That last condition should probably be `if operator not in '+-*/':` — Cory Kramer, Feb 26 '21 at 20:49
Does this answer your question? [How to test multiple variables against a value?](https://stackoverflow.com/questions/15112125/how-to-test-multiple-variables-against-a-value) — Makoto, Feb 26 '21 at 20:53

score 3 · Answer 1 · answered Feb 26 '21 at 20:49

3

You are compairing an operator to a tuple. Instead, you can check if operator in tuple, like this:

if operator not in ("+", "-", "*", "/"):
    print("Invalid Operator.")

answered Feb 26 '21 at 20:49

Arseniy

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score 1 · Answer 2 · answered Feb 26 '21 at 20:53

The alternative is elifs:

if operator == "+":
    print(num1 + num2)
elif operator == "-":
    print(num1 - num2)
elif operator == "*":
    print(num1 * num2)
elif operator == "/":
    print(num1 / num2)
else:
    print("Invalid Operator.")

With this, when you add a new operator you don't need to modify the not in (...) condition.

Niels Henkens · Answer 3 · 2021-03-02T08:01:03.277

The last if-statement is not correct. Now it checks if it is equal to the entire tuple. You could check if the operator is not in the list.

num1 = float(input("Enter the first number: "))
operator = input("Enter your operator here: ")
num2 = float(input("Enter your second number here: "))


if operator == "+":
    print(num1 + num2)
if operator == "-":
    print(num1 - num2)
if operator == "*":
    print(num1 * num2)
if operator == "/":
    print(num1 / num2)
if operator not in ("+", "-", "*", "/"):
    print("Invalid Operator.")

Another option is indeed using elif and else. If it’s not one of the first for options, then in will be an Invalid Operator (else).

if operator == "+":
    print(num1 + num2)
elif operator == "-":
    print(num1 - num2)
elif operator == "*":
    print(num1 * num2)
elif operator == "/":
    print(num1 / num2)
else:
    print("Invalid Operator.")

Edit: Changed if not in ... from list to tuple. Tuples are more efficient in this context.

Your right. I tested both out and using a tuble instead of a list is about twice as fast. Thanks. — Niels Henkens, Mar 02 '21 at 07:59

score 0 · Answer 4 · answered Feb 26 '21 at 21:00

operator != ("+", "-", "*", "/") # condition_1

where operator is a string object, where as ("+", "-", "*", "/") is a tuple so both are not equal so above condition_1 result into True and last if block will get executed. Either use if/elif/else statement or change last condition correctly like this

operator not in ("+", "-", "*", "/") # itemwise ckech takeplace 
operator not in "+-*/" # string is also a iterable object

Why does the last if statement print when the condition is false

4 Answers4