Basic calculator will only add values

Question

I'm new to python and am trying to make a basic calculator that takes two numbers and performs the operation specified by the user, but regardless of the operation I enter, the numbers always add.

num1 = int(input("Enter a number. "))
print(" ")
num2 = int(input("Enter another number. "))
print(" ")
operation = input("Type Add, Subtract, Multiply, or Divide" )

if operation == "Add" or "add":
    add = num1 + num2
    print(add)
elif operation == "Subtract" or "subtract":
    sub = num1 - num2
    print(sub)
elif operation == "Multiply" or "multiply":
    mult = num1 * num2
    print(mult)
elif operation == "Divide" or "divide":
    div = num1 / num2
    print(div)
else:
    print("Enter a valid operation: ")

With what I have, if num1 = 10 and num2 = 5 and I enter "multiply" the result I get is 15, not 50.

`operation == "Add" or "add"` should be `operation == "Add" or operation == "add"` — BlackPearl, May 12 '19 at 05:56
Or `operation = input("Type Add, Subtract, Multiply, or Divide" ).lower()` followed by the *single* checks like `if operation == "add"` - this will handle *any* amount of case mixing. — paxdiablo, May 12 '19 at 06:05

AdamGold · Answer 1 · 2019-05-12T06:08:58.913

if operation == "Add" or "add"

These are two conditions:

if operation == "Add" which does what you expect.
if "add" - Always evaluates to True because it's a constant string. So you could just as well have written: if True. You need to replace it with:

if operation == "Add" or operation == "add"

Anyway, I would like to suggest a few things.

Lowercase then check only once

You should lowercase your input string instead of double checking "Add" or "add":

num1 = int(input("Enter a number. "))
print(" ")
num2 = int(input("Enter another number. "))
print(" ")
operation = input("Type Add, Subtract, Multiply, or Divide" ).lower()  # Notice the `lower` here

if operation == "add":
    add = num1 + num2
    print(add)
elif operation == "subtract":
    sub = num1 - num2
    print(sub)
elif operation "multiply":
    mult = num1 * num2
    print(mult)
elif operation "divide":
    div = num1 / num2
    print(div)
else:
    print("Enter a valid operation: ")

Use the built-in `operator` module

The operator module contains exactly the methods that you seek for. You could use it for convenience (haven't tested, should work):


import operator
keywords = {"add": "add", "subtract": "sub", "multiply": "mul", "divide": "truediv"}

num1 = int(input("Enter a number. "))
print(" ")
num2 = int(input("Enter another number. "))
print(" ")
operation = input("Type Add, Subtract, Multiply, or Divide" )
op_func = keywords.get(operation.lower(), "add")  # defaults to add
print(getattr(operator, op_func)(num1, num2))

Thank you! I'll definitely keep this in mind. – Matt May 12 '19 at 06:08 — Matt, May 12 '19 at 06:08

score 0 · Answer 2 · answered May 12 '19 at 06:01

def fun(operation, num1,num2):
    operation = operation.lower()

    if operation == "add":
        add = num1 + num2
        print(add)
    elif operation == "subtract":
        sub = num1 - num2
        print(sub)
    elif operation == "multiply":
        mult = num1 * num2
        print(mult)
    elif operation == "divide":
        div = num1 / num2
        print(div)
    else:
        print("Enter a valid operation: ")

fun('multiply', 10,5)

Why:
if operation == "add" or "Add": this is always True
We can use if operation == "add" or operation == "Add":

score 0 · Answer 3 · answered May 12 '19 at 06:02

You have to do like this:

num1 = int(input("Enter a number. ")) 
print(" ") 
num2 = int(input("Enter another number. ")) 
print(" ") 
operation = str(input("Type Add, Subtract, Multiply, or Divide" ))
if operation == "Add" or operation =="add": 
    add = num1 + num2 
    print(add) 
elif operation == "Subtract" or operation =="subtract": 
    sub = num1 - num2 
    print(sub) 
elif operation == "Multiply" or operation =="multiply":
    mult = num1 * num2 
    print(mult) 
elif operation == "Divide" or operation =="divide": 
    div = num1 / num2 
    print(div) 
else: 
    print("Enter a valid operation: ")

Because of or operation with string, all your conditions will be true. That's why it will only go to first condition no matter what you give in the input.

Basic calculator will only add values

3 Answers3

Lowercase then check only once

Use the built-in operator module

Use the built-in `operator` module