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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.

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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));
-}

Output:

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

References

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)
C++ documentation for operator""i
-

- © cppreference.com
Licensed under the Creative Commons Attribution-ShareAlike Unported License v3.0.
- https://en.cppreference.com/w/c/numeric/complex/I -

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