_Imaginary_I

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.

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

#include <stdio.h>
#include <complex.h>
#include <math.h>
 
int main(void)
{
    double complex z1 = 0.0 + INFINITY * _Imaginary_I;
    printf("z1 = %.1f%+.1fi\n", creal(z1), cimag(z1));
 
    double complex z2 = 0.0 + INFINITY * _Complex_I;
    printf("z2 = %.1f%+.1fi\n", creal(z2), cimag(z2));
}

z1 = 0.0+Infi 
z2 = NaN+Infi

References