# _Imaginary_I

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< c‎ | numeric‎ | complex

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)

### 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

Run this code

    #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));
    }

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

_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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