C中的递归二进制搜索 [英] Recursive binary search in C
问题描述
我的函数不起作用，我需要执行递归二进制搜索，返回指向我正在搜索的数字位置的指针，如果不存在则返回null。
代码：
My function does not work, i need to preform a recursive binary search that returns a pointer to the location of the number i was searching, or null if doesn't exist.
code:
int* binsearch(int arr[], int low, int size, int *p)
{
int mid;
mid = (low + size)/2;
if (arr[mid] == *p) // found result
{
*p = arr[mid];
return p;
}
if (size <= 1) // meaning the number is not in the array
{
printf("Number not found\n");
return NULL;
}
if (arr[mid] > *p) // if its on the left half of the array
{
binsearch(arr, low, size/2, p);
}
if (arr[mid] < size) // if its on the right half of the array
{
binsearch(arr+mid, mid+1, size/2, p);
}
}
我尝试过：
发布了我试过的代码，还有几个变种但似乎没什么用处
What I have tried:
posted the code i tried, and a couple more variations but nothing seems to work
推荐答案
我的功能不起作用
这不是提供信息。函数调用woukd的一个例子也很有帮助。
你的代码中有很多问题：
This is not informative. An example of function calling woukd be also helpful.
There is numerous problems in your code:
binsearch(arr, low, size/2, p);
binsearch(arr+mid, mid+1, size/2, p);
取决于 size of arr 奇数或偶数，你的代码在 size / 2 上失败。它只有1点失败。
当你不明白你的代码在做什么或为什么它做它的作用时，答案是调试器。
它也是一个很好的学习工具，因为它向你展示了现实，你可以看到哪种期望与现实相符。
使用调试器查看代码正在执行的操作。只需设置断点并查看代码执行情况，调试器允许您逐行执行第1行并在执行时检查变量，这是一个令人难以置信的学习工具。
调试器 - 维基百科，免费的百科全书 [ ^ ]
掌握Visual Studio 2010中的调试 - 初学者指南 [ ^ ]
使用Visual Studio 2010进行基本调试 - YouTube [ ^ ]
调试器在这里显示你的代码在做什么你的任务是与它应该做的事情进行比较。
调试器中没有魔法，它没有发现错误，它只是帮助你。当代码没有达到预期的效果时，你就接近了一个bug。
Depending on the size of arr being odd or even, your code fails on size/2. And it just 1 point of failure.
When you don't understand what your code is doing or why it does what it does, the answer is debugger.
It is also a great learning tool because it show you reality and you can see which expectation match reality.
Use the debugger to see what your code is doing. Just set a breakpoint and see your code performing, the debugger allow you to execute lines 1 by 1 and to inspect variables as it execute, it is an incredible learning tool.
Debugger - Wikipedia, the free encyclopedia[^]
Mastering Debugging in Visual Studio 2010 - A Beginner's Guide[^]
Basic Debugging with Visual Studio 2010 - YouTube[^]
The debugger is here to show you what your code is doing and your task is to compare with what it should do.
There is no magic in the debugger, it don't find bugs, it just help you to. When the code don't do what is expected, you are close to a bug.
你没有从你的递归binsearch（）调用中返回一个指针......
You don't RETURN a pointer from your recursive binsearch() calls ...
编译并不意味着您的代码是正确的！ ：笑：
将开发过程想象成编写电子邮件：成功编译意味着您使用正确的语言编写电子邮件 - 例如英语而不是德语 - 而不是电子邮件包含您的邮件想发送。
所以现在你进入第二阶段的发展（实际上它是第四或第五阶段，但你将在之后的阶段进入）：测试和调试。
首先查看它的作用，以及它与你想要的有何不同。这很重要，因为它可以为您提供有关其原因的信息。例如，如果程序旨在让用户输入一个数字并将其翻倍并打印答案，那么如果输入/输出是这样的：
Compiling does not mean your code is right! :laugh:
Think of the development process as writing an email: compiling successfully means that you wrote the email in the right language - English, rather than German for example - not that the email contained the message you wanted to send.
So now you enter the second stage of development (in reality it's the fourth or fifth, but you'll come to the earlier stages later): Testing and Debugging.
Start by looking at what it does do, and how that differs from what you wanted. This is important, because it give you information as to why it's doing it. For example, if a program is intended to let the user enter a number and it doubles it and prints the answer, then if the input / output was like this:
Input Expected output Actual output
1 2 1
2 4 4
3 6 9
4 8 16然后很明显问题出在将它加倍的位 - 它不会将自身加到自身上，或者将它乘以2，它会将它自身相乘并返回输入的平方。
所以，你可以查看代码和很明显，它在某处：
Then it's fairly obvious that the problem is with the bit which doubles it - it's not adding itself to itself, or multiplying it by 2, it's multiplying it by itself and returning the square of the input.
So with that, you can look at the code and it's obvious that it's somewhere here:
private int Double(int value)
{
return value * value;
}
一旦你知道可能出现的问题，就开始使用teh调试器找出原因。在你的线上设一个断点：
Once you have an idea what might be going wrong, start using teh debugger to find out why. Put a breakpoint on your line:
mid = (low + size)/2;
并运行你的应用程序。在执行代码之前，请考虑代码中的每一行应该做什么，并将其与使用Step over按钮依次执行每一行时实际执行的操作进行比较。它符合您的期望吗？如果是这样，请转到下一行。
如果没有，为什么不呢？它有何不同？
这是一项非常值得开发的技能，因为它可以帮助你在现实世界和发展中。和所有技能一样，它只能通过使用来改善！
是的，我可能会告诉你问题是什么 - 但这并不难做到，你会在同一时间学到一些非常值得的东西！
and run your app. Think about what each line in the code should do before you execute it, and compare that to what it actually did when you use the "Step over" button to execute each line in turn. Did it do what you expect? If so, move on to the next line.
If not, why not? How does it differ?
This is a skill, and it's one which is well worth developing as it helps you in the real world as well as in development. And like all skills, it only improves by use!
Yes, I could probably tell you what "the problem" is - but it's not difficult to do this yourself, and you will learn something really worthwhile at the same time!
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