Question

我试图绕过递归，当我编写一小段代码时，我不知道例如它如何到达特定的输出。

x = 0
def recursion():
    global x
    x += 1
    print(x)
    while x < 5:
        recursion()
        recursion()
        print("example")
    print("test")
recursion()

输出：

1
2
3
4
5
test
6
test
example
test
7
test
example
test
8
test
example
test
9
test
example
test

如果有人可以逐步解释它，我将不胜感激。

Answer 1

这是逐步发生的情况。首次通话时：

def recursion():
      global x
      x += 1
      print(x)  # x ==1
      while x < 5:
        recursion() # on hold, executing recursion

def recursion():
      global x
      x += 1
      print(x)  # x ==2
      while x < 5:
        recursion() # on hold, executing recursion
.
.
.
.

def recursion():
      global x
      x += 1  
      print(x) # x == 5, skipping while loop
      while x < 5:
        recursion()
      print('test') # printing test

现在恢复所有保留的呼叫：

def recursion():
      global x
      x += 1
      print(x)
      while x < 5:
        recursion() # Called when x == 1
        recursion() # Resuming, this will put x to 6 and print it, won't enter the loop, so prints test directly
        print('example') # print example


def recursion():
      global x
      x += 1
      print(x)
      while x < 5:
        recursion() # Called when x == 2
        recursion() # Resuming, this will put x to 7 and print it, won't enter the loop, so prints test directly
        print('example') # print example

直到x == 4的呼叫

Answer 2

我同意@abarnert关于使用交互式可视化工具的建议。但是，此代码段足够短，我们可以对其进行全面检测：

indent = 0

print('\t' * indent, "x = 0"); x = 0

def recursion(indent):
    global x; indent += 1

    print('\t' * indent, "x += 1"); x += 1
    print('\t' * indent, 'print(x)'); print(x)

    while x < 5:
        print('\t' * indent, "while x < 5:")
        print('\t', '\t' * indent, "recursion()"); recursion(indent + 1)
        print('\t', '\t' * indent, "recursion()"); recursion(indent + 1)

        print('\t', '\t' * indent, 'print("example")'); print("example")

    print('\t' * indent, 'print("test")'); print("test")

print('\t' * indent, "recursion()"); recursion(indent)

运行它，我们将在左侧显示您的结果，并缩进该程序在每个步骤中所做的工作：

> python3 test.py
 x = 0
 recursion()
     x += 1
     print(x)
1
     while x < 5:
         recursion()
             x += 1
             print(x)
2
             while x < 5:
                 recursion()
                     x += 1
                     print(x)
3
                     while x < 5:
                         recursion()
                             x += 1
                             print(x)
4
                             while x < 5:
                                 recursion()
                                     x += 1
                                     print(x)
5
                                     print("test")
test
                                 recursion()
                                     x += 1
                                     print(x)
6
                                     print("test")
test
                                 print("example")
example
                             print("test")
test
                         recursion()
                             x += 1
                             print(x)
7
                             print("test")
test
                         print("example")
example
                     print("test")
test
                 recursion()
                     x += 1
                     print(x)
8
                     print("test")
test
                 print("example")
example
             print("test")
test
         recursion()
             x += 1
             print(x)
9
             print("test")
test
         print("example")
example
     print("test")
test
>

很显然，仅对小例子有用。（但是值得在学习递归过程中至少做一次！）本质上，这是交互式可视化工具会为您执行的操作，从而节省了很多时间和麻烦。

了解Python堆栈中的递归

2 个答案: