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`

.

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++).

#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

- 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)

(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) |

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