c++ pow function- invalid result?

问题

Why is the output of the dResult invalid ?
Env: Visual Studio 2008

int _tmain(int argc, _TCHAR* argv[])  
{  
   double dN = - 0.091023604111478473 ;  
   double dD = 0.127777777777777;  
   double dResult =  pow( dN,dD );   
   //dResult = -1.#IND000000000000  
   return 0;  
}   

回答1:

See http://www.cplusplus.com/reference/clibrary/cmath/pow/

double pow (double base, double exponent );

"If base is negative and exponent is not an integral value, or if base is zero and exponent is negative, a domain error occurs, setting the global variable errno to the value EDOM."

回答2:

If your dD value was .25 instead of the fraction you've presented then you could see that it is really taking the fourth root instead of an exponential of a negative number. Your fraction is close to the eighth root. You need complex numbers to express the answer that function should give.

回答3:

That's the expected result because dN is negative. The result of pow( dN,dD ); is only defined if either dN is positive or if dD is an integer. Otherwise, the result is a complex number. For example, pow(-1., 0.5) won't work either.

回答4:

Well, 0.127777777 is what number? its smt like 1277777777/(10....0)

1277777777777 is not an even number, therefore -smt to the power of 12777777777 is a negative number, and 10...0th root from that is not a real number.

I'm reffering to a fact that a^(b/c) = (c-th root from)(a^b)

来源：https://stackoverflow.com/questions/4108411/c-pow-function-invalid-result