12.4 Imaginary Constants

A complex number consists of a real part plus an imaginary part. (Either or both parts may be zero.) This section explains how to write numeric constants with imaginary values. By adding these to ordinary real-valued numeric constants, we can make constants with complex values.

The simple way to write an imaginary-number constant is to attach the suffix ‘i’ or ‘I’, or ‘j’ or ‘J’, to an integer or floating-point constant. For example, 2.5fi has type _Complex float and 3i has type _Complex int. The four alternative suffix letters are all equivalent.

The other way to write an imaginary constant is to multiply a real constant by _Complex_I, which represents the imaginary number i. Standard C doesn’t support suffixing with ‘i’ or ‘j’, so this clunky way is needed.

To write a complex constant with a nonzero real part and a nonzero imaginary part, write the two separately and add them, like this:

4.0 + 3.0i

That gives the value 4 + 3i, with type _Complex double.

Such a sum can include multiple real constants, or none. Likewise, it can include multiple imaginary constants, or none. For example:

_Complex double foo, bar, quux;

foo = 2.0i + 4.0 + 3.0i; /* Imaginary part is 5.0. */
bar = 4.0 + 12.0; /* Imaginary part is 0.0. */
quux = 3.0i + 15.0i; /* Real part is 0.0. */

See Complex Data Types.