带2个循环的while循环内部循环到错误的位置Python3

Question

我有一个用python 3编写的真正的基本计算程序。第一个是这样，因此在给出答案后可以提示用户执行另一个问题。 后续循环用于验证用户输入。

由于某种原因，如果我选择关闭程序，则内部循环会正常工作，而主循环会正常工作，但是，如果我选择执行其他问题，则循环会要求输入数字而不是开始询问什么问题。

while True:
# User inputs type of problem & input is validated
    while True:
        problem = input()

        if problem not in ('1', '2', '3', '4'):
            print('You did not make a valid selection, please try again.')
        else:
            break

# User inputs numbers for problems & input is validated
    while True:
        num1 = input('Please input the first number in the problem')
        num2 = input('Please input the second number in the problem')
        isnum1 = is_number(num1)
        isnum2 = is_number(num2)

        if isnum1 is False or isnum2 is False:
            print('You did not enter a number')
        else:
            num1 = float(num1)
            num2 = float(num2)
            break

# Perform appropriate math
    ans = 0
    if problem == '1':
        ans = num1 + num2
        print(num1, '+', num2, '=', ans)
    elif problem == '2':
        ans = num1 - num2
        print(num1, '-', num2, '=', ans)
    elif problem == '3':
        ans = num1 * num2
        print(num1, '*', num2, '=', ans)
    elif problem == '4':
        ans = num1 / num2
        print(num1, '/', num2, '=', ans)

# Ask if user would like to perform another problem or exit
    nxt = input('Would you like to do another problem?\n 1 - Yes\n 2 - No')

    if nxt not in ('1', '2'):
        print('You did not make a valid selection.')
    elif nxt == '2':
        break

exit(0)

Answer 1

您的问题是problem不会超出范围，因此仍然被定义，您跳出了第一个循环。

这是您的第一个循环：

while True:
    problem = input()

    if problem not in ('1', '2', '3', '4'):
        print('You did not make a valid selection, please try again.')
    else:
        break

在此之前未定义problem 。 但这是在此之后定义的。 因此，当外部while循环循环时，问题仍然存在。 因此，您的第二个if子句（ else ）将执行并中断。

为了证明这一点，请执行以下操作：

while True:
    problem = input()

    if problem not in ('1', '2', '3', '4'):
        print('You did not make a valid selection, please try again.')
    else:
        print('You have made a valid selection: {}".format(problem))  # This will display, showing that the loop is executed.
        break

要解决此问题，请执行以下操作：

problem = None
while True:
    problem = input()

    if problem not in ('1', '2', '3', '4'):
        print('You did not make a valid selection, please try again.')
    else:
        print('You have made a valid selection: {}".format(problem))
        break

Answer 2

正如纳撒尼尔（Nathaniel）所说，我对problem定义没有确定。 但是，如果我在循环之前仅将problem = None设置problem = None ，则没有说明提示用户选择问题。

所以我改了这段代码：

    if nxt not in ('1', '2'):
        print('You did not make a valid selection.')
    elif nxt == '2':
        break

对此：

if nxt not in ('1', '2'):
    print('You did not make a valid selection.')
elif nxt =='1':
    print('Please select the type of problem you would like to complete.\n 1 - Addition\n 2 - Subtraction\n 3 - Multiplication\n 4 - Division\n')
    problem = 0
elif nxt == '2':
    break

现在一切都很好。 感谢Nathaniel朝正确方向的观点。