I
Defined in header <complex.h>


#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.
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 nonportable 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 preC11 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
References
 C11 standard (ISO/IEC 9899:2011):
 7.3.1/6 I (p: 188)
 G.6/1 I (p: 537)
 C99 standard (ISO/IEC 9899:1999):
 7.3.1/4 I (p: 170)
 G.6/1 I (p: 472)
See also
(C99) 
the imaginary unit constant i (macro constant) 
(C99) 
the complex unit constant i (macro constant) 
(C11)(C11)(C11) 
constructs a complex number from real and imaginary parts (function macro) 