C++ M_PI 标记为未声明的标识符
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M_PI flagged as undeclared identifier
提问by Eunsu Kim
When I compile the code below, I got these error messages:
当我编译下面的代码时，我收到以下错误消息：
(Error 1 error C2065: 'M_PI' : undeclared identifier
2 IntelliSense: identifier "M_PI" is undefined)
What is this?
这是什么？
#include <iostream>
#include <math.h>
using namespace std;
double my_sqrt1( double n );`enter code here`
int main() {
double k[5] = {-100, -10, -1, 10, 100};
int i;
for ( i = 0; i < 5; i++ ) {
double val = M_PI * pow( 10.0, k[i] );
cout << "n: "
<< val
<< "\tmysqrt: "
<< my_sqrt1(val)
<< "\tsqrt: "
<< sqrt(val)
<< endl;
}
return 0;
}
double my_sqrt1( double n ) {
int i;
double x = 1;
for ( i = 0; i < 10; i++ ) {
x = ( x + n / x ) / 2;
}
return x;
}
回答by Shep
It sounds like you're using MS stuff, according to their docs
根据他们的文档，听起来您正在使用 MS 的东西
Math Constants are not defined in Standard C/C++. To use them, you must first define _USE_MATH_DEFINES and then include cmath or math.h.
标准 C/C++ 中未定义数学常量。要使用它们，您必须首先定义 _USE_MATH_DEFINES，然后包含 cmath 或 math.h。
So you need something like
所以你需要类似的东西
#define _USE_MATH_DEFINES
#include <cmath>
as a header.
作为标题。
回答by Hemant Gangwar
math.hdoes not define M_PIby default.
math.hM_PI默认不定义。
So go with this:
所以用这个：
#ifndef M_PI
#define M_PI 3.14159265358979323846
#endif
This will handle both cases either your header have M_PIdefined or not.
这将处理您的标头已M_PI定义或未定义的两种情况。
回答by legends2k
M_PIis supported by GCC too, but you've to do some work to get it
M_PIGCC 也支持，但你必须做一些工作才能得到它
#undef __STRICT_ANSI__
#include <cmath>
or if you don't like to pollute your source file, then do
或者，如果您不想污染源文件，请执行以下操作
g++ -U__STRICT_ANSI__ <other options>
回答by Metric Crapton
As noted by shep above you need something like
正如上面shep所指出的，你需要类似的东西
#define _USE_MATH_DEFINES
#include <cmath>
However you also include iostream.
但是，您还包括iostream.
iostreamincludes a lot of stuff and one of those things eventually includes cmath. This means that by the time you include it in your file all the symbols have already been defined so it is effectively ignored when you include it and the #define _USE_MATH_DEFINESdoesn't work
iostream包括很多东西，其中一项最终包括cmath. 这意味着当您将它包含在文件中时，所有符号都已定义，因此当您包含它时它会被有效地忽略并且#define _USE_MATH_DEFINES不起作用
If you include cmathbefore iostreamit should give you the higher precision constants like M_PI
如果你cmath在iostream它之前包含它应该给你更高精度的常量，比如M_PI
#define _USE_MATH_DEFINES
#include <cmath>
#include <iostream>
回答by Eruthon
I used C99 in NetBeans with remote linux host with its build tools.
Try adding #define _GNU_SOURCEand add the -lmduring linking.
我在 NetBeans 中使用 C99 和远程 linux 主机及其构建工具。
尝试在链接期间添加#define _GNU_SOURCE和添加-lm。
