Python while 循环减法

Question

这是一个非常愚蠢的问题，但我已经有一段时间没有使用它了，不记得该怎么做。我正在做一个递归问题，需要显示倒计时的数字，以基本上显示递归是如何工作的。但是，我似乎无法弄清楚。这是我的代码。

def main():

    #have user input first number
    x = int(input('Enter the first number: '))

    #have user input second number
    y = int(input('Enter the second number: '))

    #calculate result by calling recursive function
    result = mult(x, y)

    #print the result
    print(x, 'times', y, 'is', result)

#define a function that uses recursion
def mult(x, y):
    # create a loop to display the numbers
    count = y
    while count > 0:
        print('First number =', x, 'Second number =', count)
        count -= 1
    #use recursion to multiply the numbers
    if x == 0:
        return 0
    elif x == 1:
        return y
    else:
        return y + mult(x - 1, y)

main()

我需要输出来说明这一点:

Enter the first number: 5
Enter the second number: 7
First number = 5 Second Number = 7
First number = 5 Second Number = 6
First number = 5 Second Number = 5
First number = 5 Second Number = 4
First number = 5 Second Number = 3
First number = 5 Second Number = 2
First number = 5 Second Number = 1
5 times 7 = 35

所以它在大部分情况下都可以工作，但是现在它显示的是:

Enter the first number: 5
Enter the second number: 7
First number = 5 Second number = 7
First number = 5 Second number = 6
First number = 5 Second number = 5
First number = 5 Second number = 4
First number = 5 Second number = 3
First number = 5 Second number = 2
First number = 5 Second number = 1
First number = 4 Second number = 7
First number = 4 Second number = 6
First number = 4 Second number = 5
First number = 4 Second number = 4
First number = 4 Second number = 3
First number = 4 Second number = 2
First number = 4 Second number = 1
First number = 3 Second number = 7
First number = 3 Second number = 6
First number = 3 Second number = 5
First number = 3 Second number = 4
First number = 3 Second number = 3
First number = 3 Second number = 2
First number = 3 Second number = 1
First number = 2 Second number = 7
First number = 2 Second number = 6
First number = 2 Second number = 5
First number = 2 Second number = 4
First number = 2 Second number = 3
First number = 2 Second number = 2
First number = 2 Second number = 1
First number = 1 Second number = 7
First number = 1 Second number = 6
First number = 1 Second number = 5
First number = 1 Second number = 4
First number = 1 Second number = 3
First number = 1 Second number = 2
First number = 1 Second number = 1
5 times 7 is 35

Answer 1

打印这两个数字的目的是为了跟踪递归。因此，很自然地，数字必须从递归函数内部打印。换句话说，您不想创建额外的循环(使用while)来显示数字，但递归就是循环.

您也只想在每个递归步骤中打印一次数字。

另外可能让您感到困惑的是，您在所需的输出和参数顺序之间交换了“first”/x 和“second”/y 的角色的递归。 (您希望输出中的“第二个”数字减少，但您减少了 mult 的first 参数 (x)。)

它应该看起来像这样:

def mult(x, y):
    """Calculate x * y by using recursion."""
    print('First number =', x, 'Second number =', y)
    if y == 0:
        return 0
    elif y == 1:
        return x
    else:
        return x + mult(x, y - 1)

if __name__ == '__main__':
    x = int(input('Enter the first number: '))
    y = int(input('Enter the second number: '))

    # calculate result by calling recursive function
    result = mult(x, y)

    # print the result
    print(x, 'times', y, 'is', result)