How to return to a line of code when not in a WHILE or FOR loop in Python?-CodePudding

This code aims to check whether inputs are in a pre-determined list and, if not, force the user to start over again.

I understand I can use a FOR or WHILE loop and have the IF and ELSE lines nested in the loops. However, I've realised this gets rather messy and tedious on a large scale.

Is there a way to send read previous lines of code after a loop has been exited? The line that concerns us is the one with the "#problem area"; here is my code:

# Variables:

healthy = ["potato", "tomato"]

counter_ = 0

limit_ = 3

while counter_ < limit_:
    
    shopping_list = []
    wanted_item = input("What to get? \n").lower()
    if wanted_item not in healthy:
        counter_  = 1
        print("Try again")
        continue
    if wanted_item in healthy:
        shopping_list.append(wanted_item)
        break
    else:
        break

if counter_ >= limit_:

    print("Start over")
    #problem area

else:

    print("That's a good list")
    print(shopping_list)

CodePudding user response：

You can try this. This can cause an infinite loop if the input is never a "potato" or "tomato".

healthy = ["potato", "tomato"]

counter_ = 0

limit_ = 3

while counter_ < limit_:
    
    shopping_list = []
    wanted_item = input("What to get? \n").lower()

    if wanted_item in healthy:
        shopping_list.append(wanted_item)
        break
    else: # if wanted_item not in healthy
        counter_  = 1
        print("Try again")
        continue

    if counter_ >= limit_:

       print("Start over")
       counter_ = 0

CodePudding user response：

You are talking about a goto mechanism; and my answer is no, such a thing does not exist in Python. Thank God.

I've realised this gets rather messy and tedious on a large scale.

I think this is a strong misconception. Especially on large scale, goto becomes absolutely unmaintainable. (Although some people probably disagree)

But ending my anti-goto-rage, let's see how your code can be refactored to do what you want while still being readable and maintainable, to demonstrate that it's in most cases just a matter of thinking about code structure.

Let's first refactor your while loop:

the second if breaks in both cases, so just move the break out:

while counter_ < limit_:
    shopping_list = []
    wanted_item = input("What to get? \n").lower()
    if wanted_item not in healthy:
        counter_  = 1
        print("Try again")
        continue
    if wanted_item in healthy:
        shopping_list.append(wanted_item)
    break

both ifs are opposed to each other, so let's make an else statement instead and let's move the break back in (because the break is only reachable in the else case, as otherwise continue would be hit)

while counter_ < limit_:
    shopping_list = []
    wanted_item = input("What to get? \n").lower()
    if wanted_item not in healthy:
        counter_  = 1
        print("Try again")
        continue
    else:
        shopping_list.append(wanted_item)
        break

let's switch the if and else case:

while counter_ < limit_:
    shopping_list = []
    wanted_item = input("What to get? \n").lower()
    if wanted_item in healthy:
        shopping_list.append(wanted_item)
        break
    else:
        counter_  = 1
        print("Try again")
        continue

ditch the continue because it is now redundant, and remove the else as the if is already breaking, so everything after the if is automatically the else:

while counter_ < limit_:
    shopping_list = []
    wanted_item = input("What to get? \n").lower()
    if wanted_item in healthy:
        shopping_list.append(wanted_item)
        break
    counter_  = 1
    print("Try again")

So this is as far as I can help:

healthy = ["potato", "tomato"]
counter_ = 0
limit_ = 3

while counter_ < limit_:
    shopping_list = []
    wanted_item = input("What to get? \n").lower()
    if wanted_item in healthy:
        shopping_list.append(wanted_item)
        break
    counter_  = 1
    print("Try again")

if counter_ >= limit_:

    print("Start over")
    #problem area

else:

    print("That's a good list")
    print(shopping_list)

I'd like to help you further, but it's unclear from your code what you are trying to achieve.

Why do you break after the first correct item is entered?
Why do you want to start over? Like, what's the criterium? That a person entered more than limit items incorrectly? What do you mean with start over? Back to the start?
Is it intentional that you empty the shopping list in every loop iteration?

I'll continue my refactoring as soon as I get answers.

CodePudding user response：

so if your only issue is that you want to call code again and again until someone has inputted a desired value you can make use of a recursive function

we define a function (here just called function). Everything except the declaration of the healthy array into it. So when the function is now called (with the expression "function()") it will just run the code once like it did in your example. But defining that chunk of code as a function gives us the opportunity to call it again and again and again, so every time the function is called the code inside the function runs from top to bottom.

Now coming to the "problem area" - calling the function() itself right here will call the function each time we reach the "Try again" part of our if statement. -> so we start from the top again.

We will only leave the function if we reach the point where the function does not call itself, which is at "This is a good list"

all we need to do now (after doing the function definition part, is to call the function (this is done by writing function())

I use functions (to be more precise a recursive function) - so if you need additional resources you can use those keywords ("function" - "recursive function") to find them I would highly advise you to get familiar with functions ... functions are an essential part of most programming languages

healthy = ["potato", "tomato"]

def function():

    limit_ = 3

    counter_ = 0

    while counter_ < limit_:
    
        shopping_list = []
        wanted_item = input("What to get? \n").lower()
        if wanted_item not in healthy:
            counter_  = 1
            print("Try again")
            continue
        if wanted_item in healthy:
            shopping_list.append(wanted_item)
            break
        else:
            break

    if counter_ >= limit_:

        print("Start over")
        function()

    else:

        print("That's a good list")
        print(shopping_list)


function()