使用cython将双精度型转换为char * [英] Convert a double to char* with cython

问题描述

在Cython中，如何在不使用Python对象（例如 bytes <的情况下）转换以生成C的C字符串（ char * ）表示形式/ code>或 str ）作为中间物？

In Cython, how can I convert produce a C string (char *) representation of a C double without using Python objects (e.g. bytes or str) as an intermediate?

事实上，我已经在C扩展文件（.pyx），如下所示：

In fact, I have defined my function in a C-extension file (.pyx) as follows:

cdef void function(self, char* var1) nogil:

    cdef char* chaine =""
    cdef double inter = 0.0
    #Here there is some treatment which modifies the value of the local variable 'inter' so that it contains a double value different from 0
    strcat(chaine ,  "(")
    strcat(chaine , <char*>inter)
    strcat(chaine ,  ")**beta")

    strcpy(&var1,  chaine)

编译文件后，我有了错误 C2440：不可能将'double'转换为'char *'和 C2168：strcat nombre deparamètres的功能固有不足

After compilation of the file, I have the errors C2440 : impossible de convertir de 'double' en 'char*' and C2168 : strcat nombre de paramètres de fonction intrinséque insuffisant

如何我可以解决问题了吗？

How can I fix the problem please?

推荐答案

抛开是否值得在python或C上这样做的问题在您的示例代码中，似乎在C级别上存在一些严重的误解。有很多掩饰，所以我将只提供一些指导以帮助您朝正确的方向发展。一旦对C和cython感到更满意，请随时发布正确的代码版本作为答案。

Setting aside the issue of whether or not it even is worth doing this at the python or C level, it looks like there are several critical misunderstandings at the C level presented in your sample code. There is a lot of cover, so I will just give few pointers to help steer you in the right direction; feel free to post a corrected version of your code as an answer once you feel more comfortable with C and cython.

首先，一个有关指针的词。指针只是保存内存地址的变量。该内存地址指向内存中的某些内容。以下是一个可能清除此问题的简单示例：

First, a word about pointers. A pointer is just a variable that holds a memory address. This memory address "points" to some contents in memory. Here is a simple sample that might clear this up:

cdef int a_number = 42#just a regular int, nothing special here :)
cdef int* a_pointer = &a_number#"referencing" to get the address of a_number
cdef int b_number = a_pointer[0]#"dereferencing" to get the value at a_pointer
#note the dereferencing syntax is different in cython than in C!!!

第二个是函数的工作方式。在C语言中，一切都是传递值。这意味着每当您将参数传递给函数时，都会对参数进行副本，并对该副本进行操作。这包括指针；如果您像尝试使用函数那样尝试设置 var1 指针，则实际的 var1 指针不变，并且仅修改了 function 范围内的本地副本。显然，这不是我们想要的！

Second is how functions work. In C, everything is pass-by-value. This means that whenever you pass parameters to a function, a copy of the parameter is made and operations take place on this copy. This includes pointers; if you try to set your var1 pointer like you are attempting in function, the actual var1 pointer is unchanged and only a local copy within the scope of function is being modified. Clearly not what we want!

第三，我们需要查看字符串如何用C表示。字符串基本上是您关心的字符列表，后跟一个null终止符 \0 。我敢肯定，您可以在线上阅读许多有关 char * 和 char [] 之间区别的资料，我强烈建议您看看它们。我在这里只说 char * 只是一个指针，因此它仅指向第一个字符。 char * 也没有字符串长度的概念。

Third, we need to see how strings are represented in C. Strings are basically a list of characters that you care about, followed by a null terminator \0. I am sure there are lots of sources you can read online about the difference between say char* and char[], and I strongly suggest you take a look at them. I will just say here that char* is just a pointer, and so it only points to the first character. char* also has no concept of the length of the string.

一旦您能很好地处理所有字符串这些概念之后，您就可以开始查看Linux手册页上的功能，例如 strcpy 和 strcat 。我还会查找 sprintf ，这有点相似python的 format 格式，可能比将一堆碎片串联在一起更聪明。希望这对您的学习之旅有所帮助，祝您好运！

Once you have a good handle of all of these concepts, you can then start looking at functions on the linux man pages like strcpy and strcat. I would also look up sprintf, which is sort of similar to python's format and might be smarter than concatenating a bunch of pieces together. Hope this helps you on your learning journey, and good luck!

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