I try to pow(x, 1.0/3.0) and get -1.#IND00, why does it happens?

I tried to do pow(x, 1./3), pow(x, (1.0/3.0)), but nothing positive.

It doesn't works only when x less than 0. And x is float.

Example:

   x = -3.12;
   y = pow(x, 1.0 / 3.0);
   printf("%f", y);

The code above prints y as -1.#IND00

CodePudding user response：

The pow function doesn't compute roots of negative numbers.

Section 7.12.7.4p1-2 of the C standard regarding the pow functions states:

1. double pow(double x, double y);

2. The pow functions compute x raised to the power y. A domain error occurs if x is finite and negative and y is finite and not an integer value

So the values you're giving constitute a domain error, which is described in section 7.12.1p2:

For all functions, a domain error occurs if an input argument is outside the domain over which the mathematical function is defined. ... On a domain error, the function returns an implementation-defined value

If you want to compute the cube root of a number, you should instead use cbrt which accepts negative values.

CodePudding user response：

The power function with negative base is defined in the real numbers only when the exponent is integer. The operation -3.12^0.3 is not defined in mathematics (in real numbers).^*

This fact is described in the documentation of pow():

If base is finite and negative and exponent is finite and non-integer, a domain error occurs and a range error may occur.

^* This is technically not quite correct but for most exponents the value of x^y when x is negative is a complex number, not a real number.