Mathematical operation in one single input

Question

I'm a python beginner and i'm trying to make a calculator that instead of asking the user enter first number than an operator and than antoher number to return the result, the user can be able to input like for example 3 * 3 in one single input so the computer returns 9.

num1 = float(input("Enter a number: "))
op = input("Enter an operator: ")
num2 = float(input("Enter another number: "))

if op == "+":
 print(num1 + num2)
elif op == "-":
 print(num1 - num2)
elif op == "*":
 print(num1 * num2)
elif op == "/":
 print(num1 / num2)

else:
 print(" Enter a valid operator...")

There's plenty to say about this, but what exactly is your question? — Grismar, Nov 26 '21 at 06:01
Eval is highly unsafe and not recommended at all. Especially for what you are trying to do with your calculator. — Akshay Sehgal, Nov 26 '21 at 06:11
Does this answer your question? [Evaluating a mathematical expression in a string](https://stackoverflow.com/questions/2371436/evaluating-a-mathematical-expression-in-a-string) — Tomerikoo, Nov 26 '21 at 06:41

score 0 · Answer 1 · edited Nov 26 '21 at 18:17

0

The eval function may help:

inputstr = '3 * 3'
print(eval(inputstr))

edited Nov 26 '21 at 18:17

answered Nov 26 '21 at 06:02

prof_FL

156
9

score 0 · Answer 2 · answered Nov 26 '21 at 06:05

0

If you are sure the input will be completely formatted, use eval()

s = input("Enter an expression: ")
try:
    print(eval(s))
except:
    print("Enter a valid expression...")

answered Nov 26 '21 at 06:05

Chloe

1

score 0 · Answer 3 · answered Nov 26 '21 at 06:09

0

As others mentioned, the unsafe way is to use eval(), but a safe way could be

vals = input("Enter an operation: ").split()
num1 = float(vals[0])
op = vals[1]
num2 = float(vals[2])

# The rest of your code as you've written it

answered Nov 26 '21 at 06:09

rchome

2,623
8
21

Akshay Sehgal · Accepted Answer · 2021-11-26T06:15:52.873

IIUC, you are looking to get the complete input from the user in one go, so that your calculator works without asking for inputs 3 times.

NOTE: Using eval is highly unsafe and is very much discouraged. Your approach is absolutely fine and doesn't need eval(). The question you are trying to solve is not related to eval() to begin with. Read more here.

Try this -

num1, op, num2 = input('Enter equation with spaces in-between: ').split(' ')
num1, num2 = float(num1), float(num2)

if op == "+":
    print(num1 + num2)
elif op == "-":
    print(num1 - num2)
elif op == "*":
    print(num1 * num2)
elif op == "/":
    print(num1 / num2)

else:
    print(" Enter a valid operator...")

enter equation: 3 * 3
9.0

The str.split(' ') will break the string input into 3 parts based on the spaces. You can then store these in variables, convert the num1 and num2 into float and use your function.

score 0 · Answer 5 · answered Nov 26 '21 at 06:14

Use .split().

This works assuming there's a space separating each value and operator, and there are only two values involved.

def equation():
    string = input("Enter an equation: ")
    #this splits the input string into its parts and puts them into a list
    string_as_list = string.split(' ')
    print(string_as_list)
    
    num1 = float(string_as_list[0])
    op = string_as_list[1]
    num2 = float(string_as_list[2])
    if op == "+":
        result = num1 + num2
    elif op == "-":
        result = num1 - num2
    elif op == "*":
        result = num1 * num2
    elif op == "/":
        result = num1 / num2
    
    return result

Mathematical operation in one single input

5 Answers5