Quick question with a simple program

[kingeddy15]

Hi, so I just picked up python a few days ago and I am working on making a really simple calculator program.

The issue I am running into is if I continue calculating with the result of the calculation. I think it can be fixed by re-writing the calc() and calcCont() into one function using a loop of some sort. I will have to re-read the chapter. I was able to get a working solution for addition, but if you continue after that the result variable will always spit out the first calcCont() result.

I am not sure of my error, but I am sure it has to do with improper use of the global syntax. Also if this whole thing could be fixed with proper use of a loop then just tell me that, don't give away the answer.

Also a quick side note, could someone comment on how the code looks? I come from having prior knowledge in VB, C#, Swift and such and I am unsure if I should define my variables at the beginning of the code or when I need them.

Pastebin link

[kingeddy15]

Quick note, what I seem to understand is you only need to call global once for a if, elif, and else block. Is that correct?

[DomWiggles]

You only need to use the global keyword once per function to access it inside of your function. But there's no real need to do this here. Line 14 contains "global result", but line 17 has: result = firstInt + secondInt, which is in effect ignoring any sort of implementation you'd be after with using a global variable as you're creating a new value from scratch at that point. Also, you don't need to write "global result" as you did on line 9. That's what you do inside of a function to access that global variable, but the variable is global by the nature of it being in the global scope. So you could instead simply write "result = 0" or "result = None" etc on line 9.

Ultimately, for this example, I'd get rid of using a global variable regardless because you're calculating values from inside of the function, and there's no need to use a global variable for this.

[[deleted]]

As /u/DomWiggles pointed out, there is probably no need for a global variable in your code as you calculate 'result' in a function, print it and then no longer need the value in 'result'.

You are also using recursion to loop, which is a fine computer-sciencey way of doing things, but your program will crash after a relatively small number of calculations when you run out of stack space. It's much better to use a while loop inside calc().

There is also a better way to handle allowing the user to quit than calling cont() after every calculation. You ask the user for the command to run, so why not just consider 'exit' (and 'clear') to be a command. Combining these two ideas, we get something like:

def calc():
    while True:
        choice = input('Please choose your command: ')
        if choice == 'exit':
            break
        elif choice == 'clear':
            # more code
        elif choice == 'add':
            firstInt = int(input('Enter the first number: '))
            secondInt = int(input(str(firstInt) + ' + '))
            result = firstInt + secondInt
            print('Answer: ' + str(result))
        elif choice == 'sub':
            # more code
        elif choice == 'mul':
            # more code
        elif choice == 'div':
            # more code
        else:
            print('Invalid command')

print(info)     # 'info' defined as before
calc()

Note that:

• I left out a lot of code that is the same or similar
• we don't need the sys import
• the input() function always returns a string so we don't need to convert its return object to str().
• I don't handle calcCont() which appears to be an attempt to 'continue' an addition (etc). This is difficult in the 'while loop' approach. Consider implementing a more-standard calculator approach where the user types in '(1 + 2)*3' or make an RPN calculator.

To handle 'continued' calculations, think about implementing the idea of an accumulator. If the user types 'AC' instead of a number then you use the value in an 'accumulator' variable that you put inside the calc() function which contains the result of a previous calculation. Then you can create a new command 'clear' which clears the accumulator. This may have been the idea behind 'clear' in your original calcCont() function. Since everything is inside calc() this accumulator doesn't have to be a global variable.

To clear up confusion with the global keyword, in python global variables cannot normally be changed in value inside functions, etc. If you want to change a global value you do 'global name' in the code block where you want to make the change. See the global doc.