# _Imaginary_I

< c‎ | numeric‎ | complex

C
 Language Headers Type support Program utilities Variadic function support Error handling Dynamic memory management Date and time utilities Strings library Algorithms Numerics Input/output support Localization support Atomic operations (C11) Thread support (C11) Technical Specifications

Complex number arithmetic
Types and the imaginary constant
 imaginary _Imaginary_I I
Manipulation
Power and exponential functions
Trigonometric functions
Hyperbolic functions

 Defined in header  #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)

## Contents

### 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));
}

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)