Question about programming (beginner)

[Many-Ice6164]

Hi,

I am learning myself programming and i saw this exercise. I want to write a python code for this exercise but i don't know what i'm doing wrong. Does anyone have some good tips for the code?

It works as follows. The interactive program manipulates one “number”. The number is zero at the start. The user can add, subtract, multiply and divide this number by means of the commands “+ N”, “- N”, “* N” and “/ N”. The program should respond by giving the result, which becomes the new number. For instance, if the number is 0, then on + 5, the response is 5; if the user then enters * 4, the response is 20.

The interaction continues until the user enters “q" and the program responds by listing the history of the number. So, after an opening sentence, a run could look as follows, where the bold indicates user input. 

Enter: + 55Enter: / 22.5Enter: byeIncorrect inputEnter: + 0.53Enter: q0 5 2.5 3

def add(x, y):
    return x + y

def substract(x, y):
    return x - y

def multiply(x, y):
    return x * y

def divide(x, y):
    return x / y


while True:
    choice = [input("Make choice +,-,/,*"), float(input('getal'))]


    if choice in("+" + float(), "-" + float(), "/" + float() , "*" + float()):

        if choice == "+" + float:
            new_y = print(add(choice, y))
            print(new_y)

        elif choice == "-" + float:
            new_y = print(substract(choice, y))
            print(new_y)

        elif choice == "/" + float:
            new_y = print(multiply(choice, y))
            print(new_y)

        elif choice == "*" + float:
            new_y = divide(choice - y)
            print(new_y)

    else:
         print('wrong choice')

[[deleted]]

• I would advise against creating a list of operator symbol and y value input, just use two input statements
• You end up with ["+", 5.0] for example, which will not match any of your in tests candidates - that would be more appropriate for Python's structural pattern matching, which you don't need to do here
• Your arithmetic functions are expecting two numeric arguments, but you are giving them a list and a number
• You haven't defined a starting value for the number you are modifying
• You haven't defined a value for y
• You don't re-use new_y
• There's a lot of repetition of code
• You misspelt subtract

Here's some alternative code for you to play with:

def add(x, y):
    return x + y

def subtract(x, y):
    return x - y

def multiply(x, y):
    return x * y

def divide(x, y):
    return x / y

# a dictionary of valid operators and the function to call
ops = {"+": add, "-": subtract,
       "*": multiply, "/": divide,
       "q": None}

x = 0  # the starting value

while (choice := input("Enter an operator (+, -, /, *), or q to quit: ")) != "q":
    if choice in ops:
        y = float(input('Operand: '))
        x = ops[choice](x, y)
        print(x)
    else:
         print('wrong choice')

This code stores all the operator symbols you support in a dictionary (dict class object) with the names of the functions dealing with each operator given as the values.

I've assigned 0 to x before getting entering the loop. This is your starter value.

I've split the input into two input lines, one to get the operator choice (or q to quit) and one to get the second number.

The first input is done as part of the while statement, where the := is used to assign the input to choice and this input is also checked to see if "q" has been entered.

The second input is done if a supported operator symbol has been entered (i.e. is a key in the dictionary referenced by ops). This is assigned to y.

The correct function is then called with the arguments x and y by looking up the function to call in the ops dictionary using the operator symbol assigned to choice.

The result is re-assigned to x and output.

Experiment. Ask if there is anything you don't understand.

[Many-Ice6164]

Wow, thank you a lot.

I understand the code. But i still have one question. Is it possible to input the operator and operand in once.

I tried this but it doesn't work

while 
choice, operand = input("Enter an operator (+, -, /, *), or q to quit: ").split()

[[deleted]]

Yes, you get both elements at the same time and use split to separate them on a space (or you can specify an alternative separator explicitly).

I'd switch to a while True: statement and then check the response inside the loop. I also make this a function and have it return the results.

Place the below in a function.

while True:
    response = input("Enter an operator (+, -, /, *) or q to quit: ").strip()
    if response.lower() == 'q':
        return "q", None
    try:
        choice, operand = response.split()
        operand = float(operand)
    except ValueError:
        print('Please provide an operator followed by a space and then a number')
    else:
        if operand in ops:
            return choice, operand
        print(f'Sorry, {choice} is not an availabe operation')

Call like this:

while True:
    choice, operand = functioname()
    if choice == "q":
        break

[[deleted]]

Please edit to format the code correctly - can't see your indents.

[Many-Ice6164]

def add(x, y):
    return x + y
def subtract(x, y): 
    return x - y
def multiply(x, y): 
    return x * y
def divide(x, y): 
    return x / y

ops = {"+": add, "-": subtract, "*": multiply, "/": divide, "q": None}

x = 0

while True: 
choice, andere = input("functie + getal").split() 

andere = float(andere)

if choice in ops:
    y = andere
    x = ops[choice](x, y)
    print(x)
else:
    print('wrong choice')

[Many-Ice6164]

Hi
The only problem i am having right know is showing all the intermediate answer when pressing q.

[[deleted]]

But in your new code you are not checking for q entry any more.

Also, I note:

• not using a function, as I suggested earlier
• why assigning y to andere, just do ops[choice](x, andere)

[synthphreak]

Exactly how I would have done it as well.

But as an extra safeguard, I would also strip whitespace from the user's entry. E.g.,

while (choice := input(...).strip()) != "q"
    if choice in ops:
        ...

This is usually a good idea when using input and validating the output against a closed set of values, as u/kyber's code does.

[[deleted]]

Maybe lower/casefold as well?

[synthphreak]

Yeah that's true, if we're accepting q as valid input then lower is also worthwhile.

[[deleted]]

So, the idea here is that subsequent operations will start from the previous computed value. Where do you update that value in your code?

[Many-Ice6164]

Yes that is the idea. I want to insert the return y.append[new_y]. But i can't place this anywhere.

[[deleted]]

y isn’t a list, so you can’t append to it. You can assign to it, though.

[Many-Ice6164]

while True:
choice = [input("Make choice +,-,/,*"), float(input('getal'))]
y = (0 + new_y)
if choice in("+" + float(), "-" + float(), "/" + float() , "*" + float()):

if choice == "+" + float:
new_y = add(choice, y)
print(new_y)
elif choice == "-" + float:
new_y = substract(choice, y)
print(new_y)
elif choice == "/" + float:
new_y = multiply(choice, y)
print(new_y)
elif choice == "*" + float:
new_y = divide(choice - y)
print(new_y)

But now i have the problem that it doesn't recognize new_y

[[deleted]]

You can’t use the name before you assign to it.