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.
Likewise, on machines that have support for _FloatN or
_FloatNx enabled, the complex types _FloatN
complex and _FloatNx complex are also available if
complex.h has been included; see Mathematics.
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.
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.
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.)