[英]Result of recursive method
I am trying to understand this recursive method but even with a debugger I couldn't come up to something that makes sense to me so i hope someone here is motivated to explain me what is going on here. 我试图理解这种递归方法,但是即使使用调试器,我也无法解决对我来说有意义的事情,因此我希望这里的人有动力向我解释这里的情况。 I know how recursion works basically but the way this method is written down troubles me. 我知道递归的工作原理,但是写下这种方法的方式困扰着我。 I know the conditional operator in java and I worked out a new code but still I don't understand it, so what I expect here is. 我知道Java中的条件运算符,我编写了一个新代码,但是仍然不明白,所以我期望这里是。
What is the result for m(5) AND m(15). m(5)和m(15)的结果是什么。 How did you calculate it? 您是如何计算的? Thanks 谢谢
Edit after the answers. 答案后编辑。 I made a table with my results for future readers 我整理了一张表格,供以后的读者参考
m(n)::|0|1|2|3|4|5|6|7|8|9|10|11|12|13|14|15|...
result|0|0|1|1|1|2|3|4|6|9|13|19|28|41|60|88|...
I checked only the result of 15 with my program. 我只用程序检查了15的结果。
public class practice {
/**
* Try to understand the way this works
* @param n
* @return
*/
static long m(long n) {
return n <= 1 ? 0 : n == 2 ? 1 : m(n - 1) + m(n - 3);
}
/**
* This is what i tried so far.
* @param n
* @return
*/
static long ma(long n) {
System.out.println("Method called");
System.out.println("N is: " + n);
if (n <= 1) {
System.out.println("N<=1: " + n);
System.out.println("0 is returned");
return 0;
} else if (n == 2) {
System.out.println("N==2: " + n);
System.out.println("1 is returned");
return 1;
} else {
System.out.println("Inside ELSE, N is: " + n);
return ma(n - 1) + ma(n - 3);
}
}
public static void main(String[] args) {
ma(15);
}
}
Wrote it this way to make it more understandable: 用这种方式编写它,使它更易于理解:
m(0) = 0
m(1) = 0
m(2) = 1
m(n) = m(n - 1) + m(n - 3) // n >= 3
When we know the values for m(0)
, m(1)
and m(2)
, we can calculate any m(n)
, where n >= 3
, using m(n - 1) + m(n - 3)
. 当我们知道m(0)
, m(1)
和m(2)
,我们可以使用m(n - 1) + m(n - 3)
计算任何m(n)
,其中n >= 3
。 For any negative input, the result is 0. 对于任何负输入,结果均为0。
For example: 例如:
m(3) = m(3 - 1) + m(3 - 3) = m(2) + m(0) = 1 + 0 = 1
m(4) = m(4 - 1) + m(4 - 3) = m(3) + m(1) = 1 + 0 = 1
m(5) = m(5 - 1) + m(5 - 3) = m(4) + m(2) = 1 + 1 = 2
And so on... 等等...
Pencil and paper are you friends. 铅笔和纸是您的朋友。
There are 3 cases (two of them are base conditions) 有3种情况(其中两种是基本情况)
if n <= 1 then return 0
if n == 2 then return 1
else recursive call to m(n-1) + m(n-3)
So you know that on every recursive call we are approaching one of the base conditions. 因此,您知道在每个递归调用中,我们都接近基本条件之一。
Here is a stack trace of what happens on m(5)
这是m(5)
发生的堆栈跟踪
m(5)
m(4) + m(2)
m(3) + m(1) return 1
m(2) + m(0) return 0
return 1 return 0
Adding all the returns gives 1 + 0 + 0 + 1
which is 2
将所有收益相加得出1 + 0 + 0 + 1
,即2
So 所以
m(5) == 2
Method m
in pseudo code. 伪代码中的方法m
。 (this is some kind of scala/python mash up) (这是某种scala / python混搭)
def m (number n)
if (n <= 1) 0
else if (n == 2) 1
else m(n - 1) + m(n - 3)
Looking at this you can see that anything <= 2
is a terminal operation, returning 0
or 1
based on the input. 查看此内容,您可以看到<= 2
任何内容都是终端操作,根据输入返回0
或1
。 If n
is > 2, the fun begins. 如果n
> 2,则乐趣开始。 Take for example n=3
. 以n=3
为例。 Since 3
is greater than 2
, we need to run the third if
. 由于3
大于2
,因此我们需要运行第三个if
。 When we look at that line we see that we nee to return m(n - 1) + m(n - 3)
. 当我们看那条线时,我们看到nee返回m(n - 1) + m(n - 3)
。 Plugging n = 3
in, it looks like this: m(3 - 1) + m(3 - 3)
, or, more simplified, like this: m(2) + m(0)
. 插入n = 3
后,它看起来像是: m(3 - 1) + m(3 - 3)
,或更简单地说,是这样的: m(2) + m(0)
。 This will now terminate with 1
because neither 2
or 0
for n
will result in more calling of m
. 现在这将以1
终止,因为n
2
或0
都不会导致m
更多调用。
So now we have something we understand we can now workout what m(15)
and m(5)
will return. 现在我们有了一些了解,现在我们可以锻炼m(15)
和m(5)
会返回什么。
I will only work it out for 5
since the call stack for 15
would be way to long 我只会计算5
因为15
的调用栈会很长
m(5)
/ \
m(5-1) + m(5-3)
/ \ |
m(4-1) + m(4-3) |
/ \ | |
m(3-1) + m(3-3) | |
| | | |
1 + 0 + 0 + 1
2
Hope it helps! 希望能帮助到你!
m(5) = m(4)+m(2) // 5>2, so the third condition
m(4) + m(2) = m(3)+m(1) + 1 // 4>2, so the third condition
m(3) + m(1) + 1 = m(2)+m(0) + 0 + 1 // 3>2, so the third condition
m(2) + m(0) + 0 + 1 = 1 + 0 + 0 + 1 = 2
Now, between transformations, m(2)
s instantly replaced with 1
, and m(n<=1)
is instantly replaced with 0
. 现在,在转换之间, m(2)
立即替换为1
,而m(n<=1)
立即替换为0
。 This is how you can analyze this on paper. 这就是您可以在纸上进行分析的方式。 Computer, however, would wirst calculate m(4)
before calculating m(2)
and adding the results in the first line - this happens because of order of poerations within the recursive functions. 但是,计算机在计算m(2)
并将结果加到第一行之前会先计算m(4)
-发生这种情况是由于递归函数中的插补顺序。
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