问题
When I compile the code below, I got these error messages:
(Error 1 error C2065: 'M_PI' : undeclared identifier
2 IntelliSense: identifier "M_PI" is undefined)
What is this?
#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1:
It sounds like you're using MS stuff, according to their docs
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.
So you need something like
#define _USE_MATH_DEFINES
#include <cmath>
as a header.
回答2:
math.h
not defines M_PI
by default.
So go with this:
#ifndef M_PI
#define M_PI 3.14159265358979323846
#endif
This will handle both cases either your header have M_PI
defined or not.
回答3:
M_PI
is supported by GCC too, but you've to do some work to get it
#undef __STRICT_ANSI__
#include <cmath>
or if you don't like to pollute your source file, then do
g++ -U__STRICT_ANSI__ <other options>
回答4:
As noted by shep above you need something like
#define _USE_MATH_DEFINES
#include <cmath>
However you also include iostream
.
iostream
includes 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_DEFINES
doesn't work
If you include cmath
before iostream
it should give you the higher precision constants like M_PI
#define _USE_MATH_DEFINES
#include <cmath>
#include <iostream>
回答5:
I used C99 in NetBeans with remote linux host with its build tools.
Try adding #define _GNU_SOURCE
and add the -lm
during linking.
来源:https://stackoverflow.com/questions/26065359/m-pi-flagged-as-undeclared-identifier