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### 20.9 Complex Numbers

ISO C99 introduces support for complex numbers in C. This is done with a new type qualifier, `complex`. It is a keyword if and only if complex.h has been included. There are three complex types, corresponding to the three real types: `float complex`, `double complex`, and `long double complex`.

Likewise, on machines that have support for `_FloatN` or `_FloatNx` enabled, the complex types ```_FloatN complex``` and `_FloatNx complex` are also available if complex.h has been included; see Mathematics.

To construct complex numbers you need a way to indicate the imaginary part of a number. There is no standard notation for an imaginary floating point constant. Instead, complex.h defines two macros that can be used to create complex numbers.

Macro: const float complex _Complex_I

This macro is a representation of the complex number “0+1i”. Multiplying a real floating-point value by `_Complex_I` gives a complex number whose value is purely imaginary. You can use this to construct complex constants:

```3.0 + 4.0i = `3.0 + 4.0 * _Complex_I`
```

Note that `_Complex_I * _Complex_I` has the value `-1`, but the type of that value is `complex`.

`_Complex_I` is a bit of a mouthful. complex.h also defines a shorter name for the same constant.

Macro: const float complex I

This macro has exactly the same value as `_Complex_I`. Most of the time it is preferable. However, it causes problems if you want to use the identifier `I` for something else. You can safely write

```#include <complex.h>
#undef I
```

if you need `I` for your own purposes. (In that case we recommend you also define some other short name for `_Complex_I`, such as `J`.)

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