_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 `_Imaginary_I` is defined to identify imaginary number support.	(since C99) (until C11)
Imaginary numbers are supported if __STDC_IEC_559_COMPLEX__ is defined.	(since C11)

[edit] 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.

[edit] Example

Run this code

#include <stdio.h>
#include <complex.h>
 
int main(void)
{
    double complex z = 0.0 + -0.0 * _Imaginary_I;
    printf("z = %.1f%+.1fi\n", creal(z), cimag(z));
}

Output:

z = 0.0-0.0i

[edit] See also

_Complex_I (C99)	the complex unit constant i (macro constant)
I (C99)	the complex or imaginary unit constant i (macro constant)

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