_Imaginary_I

 

C

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Technical Specifications

 

Numerics

Common mathematical functions

Floating-point environment

Complex number arithmetics

Random number generation

Type-generic math

 

Complex number arithmetic

Types and the imaginary constant
complex _Complex_I CMPLX	imaginary _Imaginary_I I
Manipulation
cimag creal carg	cabs conj cproj
Power and exponential functions
cexp clog	cpow csqrt
Trigonometric functions
ccos csin ctan	cacos casin catan
Hyperbolic functions
ccosh csinh ctanh	cacosh casinh catanh

 

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)