### 20.9 Complex Numbers

ISO C99 introduces support for complex numbers in C. This is done with a new type qualifier, `complex`. It is a keyword if and only if `complex.h` has been included. There are three complex types, corresponding to the three real types: `float complex`, `double complex`, and `long double complex`.

To construct complex numbers you need a way to indicate the imaginary part of a number. There is no standard notation for an imaginary floating point constant. Instead, `complex.h` defines two macros that can be used to create complex numbers.

— Macro: const float complex _Complex_I

> This macro is a representation of the complex number “0+1i”. Multiplying a real floating-point value by `_Complex_I` gives a complex number whose value is purely imaginary. You can use this to construct complex constants:

>               3.0 + 4.0i = 3.0 + 4.0 * _Complex_I

> Note that `_Complex_I * _Complex_I` has the value `-1`, but the type of that value is `complex`.

`_Complex_I` is a bit of a mouthful. `complex.h` also defines a shorter name for the same constant.

— Macro: const float complex I

> This macro has exactly the same value as `_Complex_I`. Most of the time it is preferable. However, it causes problems if you want to use the identifier `I` for something else. You can safely write

>               #include <complex.h>
>               #undef I

> if you need `I` for your own purposes. (In that case we recommend you also define some other short name for `_Complex_I`, such as `J`.)