不能做负数的减法

Question

print('=====CALCULADORA==========')
conta = str(input('')).strip()
operadores = ('+', '-', 'x', '/')
for o in range(0,4):
    if operadores[o] in conta[1:len(conta)]:
        if operadores[o] == '+':
            print(f'{conta[0:conta.find("+")].strip()} + {conta[conta.find("+") + 1:].strip()} = {int(conta[0:conta.find("+")]) + int(conta[conta.find("+") + 1:])}')
        if operadores[o] == '-':
            print(f'{conta[0:conta.find("-")].strip()} - {conta[conta.find("-") + 1:].strip()} = {int(conta[0:conta.find("-")]) - int(conta[conta.find("-") + 1:])}')

Im creating a simple calculator code.

Doing additions works, but when I try to do operations like -4-6, the program dont work. An error appears, saying: " ValueError: invalid literal for int() with base 10: '' "

The code was supposed to do the expressions correctly, it parcially works, it seems the problem is when I repeat the operators. -4 + 3, or +4 - 3 actually works, but +4 + 3, or -4-3, dont.

Answer 1

Take a close look at what your code does when you enter '-4-6' .

First the data entry:

conta = str(input('')).strip()

That works as expected, although this would do the exact same:

conta = input().strip()

Then a defintion of operators, and your looping over it:

operadores = ('+', '-', 'x', '/')
for o in range(0,4):
    ...  # code using operadores[o]

This also works, although again it's more complicated than it needs to be:

operadores = '+-x/'
for o in operadores:
    ...  # code using o as the current operator

Then the problematic part:

    if operadores[o] in conta[1:len(conta)]:

You specifically skip the first character, probably to avoid detecting the - at the start as the actual operator. However, the - is still there in '-4-6' .

Then, later:

        if operadores[o] == '-':
            print(f'{conta[0:conta.find("-")].strip()} - {conta[conta.find("-") + 1:].strip()} = {int(conta[0:conta.find("-")]) - int(conta[conta.find("-") + 1:])}')

Here, you use conta.find("-") to find the position of the operator, but of course here it actually finds the first - , not the second.

You could fix this code like this:

print('=====CALCULADORA==========')
conta = str(input('')).strip()
operadores = '+-x/'
for o in operadores:
    if o in conta[1:len(conta)]:
        if o == '+':
            print(f'{conta[0:conta[1:].find("+")+1].strip()} + {conta[conta[1:].find("+")+2:].strip()} = {int(conta[0:conta[1:].find("+")+1].strip()) + int(conta[conta[1:].find("+")+2:].strip())}')
        if o == '-':
            print(f'{conta[0:conta[1:].find("-")+1].strip()} - {conta[conta[1:].find("-")+2:].strip()} = {int(conta[0:conta[1:].find("-")+1].strip()) - int(conta[conta[1:].find("-")+2:].strip())}')

However, that's still wildly complicated with a lot of reptition. It also doesn't consider something like '4--2' , which is subtracting -2 from 4 , and 6 should be the result. Never mind longer expressions, like '4+3+2' .

It makes more sense to break up an input string into tokens and then perform the operations as they should be. So, '-4+3+2' would be broken up in ['-4', '+', '3', '+', '2'] and you can compute the result going over that.

Of course this problem has been solved many times before, so there's good examples out there of even better strategies.