I

The I macro expands to either _Complex_I or _Imaginary_I . If the implementation does not support imaginary types, then the macro always expands to _Complex_I .

A program may undefine and perhaps then redefine the macro I .

Notes

The macro is not named i , which is the name of the imaginary unit in mathematics, because the name i was already used in many C programs, e.g. as a loop counter variable.

The macro I is often used to form complex numbers, with expressions such as x + y * I . If I is defined as _Complex_I , then such expression may create a value with imaginary component +0.0 even when y is -0.0 , which is significant for complex number functions with branch cuts. The macro CMPLX provides a way to construct a complex number precisely.

GCC provides a non-portable extension that allows imaginary constants to be specified with the suffix i on integer literals: 1.0fi , 1.0i , and 1.0li are imaginary units in GNU C. A similar approach is part of standard C++ as of C++14 ( 1.0if , 1.0i , and 1.0il are the imaginary units in C++)

Example

#include <stdio.h>
#include <complex.h>
 
int main(void)
{
    printf("I = %.1f%+.1fi\n", creal(I), cimag(I));
 
    double complex z1 = I * I;     // imaginary unit squared
    printf("I * I = %.1f%+.1fi\n", creal(z1), cimag(z1));
 
    double complex z = 1.0 + 2.0*I; // usual way to form a complex number pre-C11
    printf("z = %.1f%+.1fi\n", creal(z), cimag(z));
}

I = 0.0+1.0i
I * I = -1.0+0.0i
z = 1.0+2.0i

References