Ruby:递归方法 [英] Ruby: recursive method

问题描述

def reverse_append(arr, n)  
    return arr if n < 0 
    reverse_append(arr, n-1)
    arr << n
    arr
end 

reverse_append([],4) #=> [0, 1, 2, 3, 4]

我似乎无法理解这种递归方法.它产生一个从 0 到 n 的数组.

I can't seem to understand this recursive method. It produces an array from 0 up to n.

谁能给我解释一下?

推荐答案

1. 方法reverse_append([],4)被调用
2. 由于 4 >= 0，return 语句不会被调用.
3. 方法 reverse_append([],3) 被调用.
4. 由于 3 >= 0，return 语句不会被调用.
5. 方法 reverse_append([],2) 被调用.
6. 由于 2 >= 0，return 语句不会被调用.
7. 方法 reverse_append([],1) 被调用.
8. 由于 1 >= 0，return 语句不会被调用.
9. 方法 reverse_append([],0) 被调用.
10. 由于 0 >= 0，return 语句不会被调用.
11. 方法 reverse_append([],-1) 被调用.
12. 由于 -1 <0，返回数组([]).
13. 我们在调用堆栈中弹出一层，到 n = 0 和 arr = [].
14. arr << 和 arr 被返回，所以现在 arr = [0].
15. 我们在调用堆栈中弹出一层，到 n = 1 和 arr = [0].
16. arr << 和 arr 被返回，所以现在 arr = [0, 1].
17. 我们在调用堆栈中弹出一层，到 n = 2 和 arr = [0, 1].
18. arr << 和 arr 被返回，所以现在 arr = [0, 1, 2].
19. 我们在调用堆栈中弹出一层，到 n = 3 和 arr = [0, 1, 2].
20. arr << 和 arr 被返回，所以现在 arr = [0, 1, 2, 3].
21. 我们在调用堆栈中弹出一层，到 n = 4 和 arr = [0, 1, 2, 3].
22. arr << 和 arr 被返回，所以现在 arr = [0, 1, 2, 3, 4].
23. 最后，顶级"方法返回，我们得到了最终结果.

1. The method reverse_append([],4) is called
2. Since 4 >= 0, the return statement does not get called.
3. The method reverse_append([],3) is called.
4. Since 3 >= 0, the return statement does not get called.
5. The method reverse_append([],2) is called.
6. Since 2 >= 0, the return statement does not get called.
7. The method reverse_append([],1) is called.
8. Since 1 >= 0, the return statement does not get called.
9. The method reverse_append([],0) is called.
10. Since 0 >= 0, the return statement does not get called.
11. The method reverse_append([],-1) is called.
12. Since -1 < 0, the array ([]) is returned.
13. We pop up one level in our call stack, to where n = 0 and arr = [].
14. arr << n and arr is returned, so now arr = [0].
15. We pop up one level in our call stack, to where n = 1 and arr = [0].
16. arr << n and arr is returned, so now arr = [0, 1].
17. We pop up one level in our call stack, to where n = 2 and arr = [0, 1].
18. arr << n and arr is returned, so now arr = [0, 1, 2].
19. We pop up one level in our call stack, to where n = 3 and arr = [0, 1, 2].
20. arr << n and arr is returned, so now arr = [0, 1, 2, 3].
21. We pop up one level in our call stack, to where n = 4 and arr = [0, 1, 2, 3].
22. arr << n and arr is returned, so now arr = [0, 1, 2, 3, 4].
23. Finally, the "top-level" method returns, and we have our final result.

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