如果 Python 中的输入为负数，如何结束循环？

Question

I am writing a program for an assignment that has the following conditions:

1. 12 bars per case, represented by bars_per_case

2. A price of 1 per bar: price_per_bar

3. A case cost of 8: cost_per_case

4. A 10% cut: SGA

The program should ask the user how many bars were sold and calculate the SGA amount and total net gain cheer_proceeds . The program should have a different message depending on cheer_proceeds : "Congratulations ..." when cheer_proceeds >= 500 or "Sorry ..." when cheer_proceeds < 500 .

The program should NOT accept a negative number of bars. If a negative number is entered, the program should NOT move forward to calculate totals until valid data is entered.

My problems are:

1. When I use the while loop, the last lines the if cheer_proceeds... keeps looping
2. When entering a negative number the code still continues

Where would I insert a break in order to stop the loop?

My code:

    # program computes money cheerleaders raised
    
    cost_per_case = 8  # $8.00
    bars_per_case = 12
    price_per_bar = 1  # $1.00
    SGA = 0.10  # 10 percent earnings
    cheergoal = 500  # $500 is their goal
    
    # ask user how many bars were sold
    barsSold = int(input('Bars sold: '))
    
    # calculate and display proceeds
    while barsSold >= 0:
        total_cost_candy = (barsSold / 12) / 8
        total_profit = (barsSold * price_per_bar) - total_cost_candy
        sga_proceeds = total_profit * 0.1
        cheer_proceeds = total_profit - sga_proceeds
    
    # display proceeds and if goal was met or not
    print(sga_proceeds, cheer_proceeds)
    if cheer_proceeds >= 500:
        print('Congrats! You raised $500 or mroe!')
    else:
        print('Sorry! You did not meet your goal of $500!')

Answer 1

When the user enters an amount, just loop until the amount is non-negative:

# program computes money cheerleaders raised

cost_per_case = 8  # $8.00
bars_per_case = 12
price_per_bar = 1  # $1.00
SGA = 0.10  # 10 percent earnings
cheergoal = 500  # $500 is their goal

# ask user how many bars were sold
barsSold = int(input('Bars sold: '))

# calculate and display proceeds
while barsSold < 0:   # loop until valid number
    print('invalid amount')
    barsSold = int(input('Bars sold: '))
    
total_cost_candy = (barsSold / 12) / 8
total_profit = (barsSold * price_per_bar) - total_cost_candy
sga_proceeds = total_profit * 0.1
cheer_proceeds = total_profit - sga_proceeds

# display proceeds and if goal was met or not
print(sga_proceeds, cheer_proceeds)
if cheer_proceeds >= 500:
    print('Congrats! You raised $500 or mroe!')
else:
    print('Sorry! You did not meet your goal of $500!')

Answer 2

What makes the same code run over and over within the same program? Loops do. So anytime your problem description includes repeating something (like asking a question until a positive number is given), you'll want a loop.

Any time you aren't doing something over and over (like calculating the bars sold), you don't want a loop.

To directly answer your questions...

1. I'm not sure why if cheer_proceeds would keep looping as that's outside a loop. Did you provide all of the code?
2. The code moves forward because it's not in a loop.
3. You don't want a break. There shouldn't be a loop there.

Answer 3

while loop is not useful here, use nested if condition.

cost_per_case = 8  # $8.00
bars_per_case = 12
price_per_bar = 1  # $1.00
SGA = 0.10  # 10 percent earnings
cheergoal = 500  # $500 is their goal
    
    # ask user how many bars were sold
barsSold = int(input('Bars sold: '))
    
    # calculate and display proceeds
if barsSold > 0:
    total_cost_candy = (barsSold / 12) / 8
    total_profit = (barsSold * price_per_bar) - total_cost_candy
    sga_proceeds = total_profit * 0.1
    cheer_proceeds = total_profit - sga_proceeds

    # display proceeds and if goal was met or not
    
    if cheer_proceeds >= 500:
        print(sga_proceeds, cheer_proceeds)
        print('Congrats! You raised $500 or mroe!')
    else:
        print(sga_proceeds, cheer_proceeds)
        print('Sorry! You did not meet your goal of $500!')    
else:
    print('Invalid Input')
    

Answer 4

I believe you have some issues with indentation as your code currently stands. I had to fudge with it a bit to get it to run at all. As the other user mentioned this does not need to be done with a while loop. BUT, it can be done this way.

1. when i use the while loop the last lines (the if cheer_proceeds.....) keeps looping.

• Yes, if you have a while loop, as long as the condition is met the loop will continue forever. You need the break in there or the condition needs to be met for the loop to exit

2. when entering a negative number the code still tries to move foward.. (how to resolve this ?)

• The issue here is the ordering of your code. If you ask for barsSold before the loop, and the user inputs a not accepted option, the program will not ask them again to input an appropriate answer. Not only that, but you have not put any conditional statement to tell the program what to do if the user inputs a negative answer.

3. where would i insert a break in order to stop said loop?

• the break should be inserted in a conditional statement (the if loop), so that you can get the loop to break at a specific point (ie when someone enters a non-negative answer)

Here is a working version of the code

# program computes money cheerleaders raised
cost_per_case = 8  # $8.00
bars_per_case = 12
price_per_bar = 1  # $1.00
SGA = 0.10  # 10 percent earnings
cheergoal = 500  # $500 is their goal


# calculate and display proceeds
while True:
    # ask user how many bars were sold
    barsSold = int(input('Bars sold: '))
    if barsSold >=0:
        total_cost_candy = (barsSold / 12) / 8
        total_profit = (barsSold * price_per_bar) - total_cost_candy
        sga_proceeds = total_profit * 0.1
        cheer_proceeds = total_profit - sga_proceeds
        print(sga_proceeds, cheer_proceeds)
        
        if cheer_proceeds >= 500:
            print('Congrats! You raised $500 or mroe!')
        else:
            print('Sorry! You did not meet your goal of $500!')

        break 
    else:
        print("That is not a suitable input. Please try again.")