#define I /* unspecified */
(since C99)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.
[edit] NotesThe 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++)
#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));
}
Output:
I = 0.0+1.0i I * I = -1.0+0.0i z = 1.0+2.0i[edit] References
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