_Imaginary_I
Defined in header <complex.h>


#define _Imaginary_I /* unspecified */ 
(since C99)  
The _Imaginary_I
macro expands to a value of type const float _Imaginary with the value of the imaginary unit.
As with any pure imaginary number support in C, this macro is only defined if the imaginary numbers are supported.
A compiler that defines __STDC_IEC_559_COMPLEX__ is not required to support imaginary numbers. POSIX recommends checking if the macro 
(since C99) (until C11) 
Imaginary numbers are supported if __STDC_IEC_559_COMPLEX__ is defined. 
(since C11) 
Notes
This macro allows for the precise way to assemble a complex number from its real and imaginary components, e.g. with (double complex)((double)x + _Imaginary_I * (double)y). This pattern was standardized in C11 as the macro CMPLX. Note that if _Complex_I is used instead, this expression is allowed to convert negative zero to positive zero in the imaginary position.
Example
Output:
z1 = 0.0+Infi z2 = NaN+Infi
References
 C11 standard (ISO/IEC 9899:2011):
 7.3.1/5 _Imaginary_I (p: 188)
 G.6/1 _Imaginary_I (p: 537)
 C99 standard (ISO/IEC 9899:1999):
 7.3.1/3 _Imaginary_I (p: 170)
 G.6/1 _Imaginary_I (p: 472)
See also
(C99) 
the complex unit constant i (macro constant) 
(C99) 
the complex or imaginary unit constant i (macro constant) 