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| <h3 class="section">6.9 Complex Numbers</h3> |
| |
| <p><a name="index-complex-numbers-2221"></a><a name="index-g_t_0040code_007b_005fComplex_007d-keyword-2222"></a><a name="index-g_t_0040code_007b_005f_005fcomplex_005f_005f_007d-keyword-2223"></a> |
| ISO C99 supports complex floating data types, and as an extension GCC |
| supports them in C90 mode and in C++, and supports complex integer data |
| types which are not part of ISO C99. You can declare complex types |
| using the keyword <code>_Complex</code>. As an extension, the older GNU |
| keyword <code>__complex__</code> is also supported. |
| |
| <p>For example, ‘<samp><span class="samp">_Complex double x;</span></samp>’ declares <code>x</code> as a |
| variable whose real part and imaginary part are both of type |
| <code>double</code>. ‘<samp><span class="samp">_Complex short int y;</span></samp>’ declares <code>y</code> to |
| have real and imaginary parts of type <code>short int</code>; this is not |
| likely to be useful, but it shows that the set of complex types is |
| complete. |
| |
| <p>To write a constant with a complex data type, use the suffix ‘<samp><span class="samp">i</span></samp>’ or |
| ‘<samp><span class="samp">j</span></samp>’ (either one; they are equivalent). For example, <code>2.5fi</code> |
| has type <code>_Complex float</code> and <code>3i</code> has type |
| <code>_Complex int</code>. Such a constant always has a pure imaginary |
| value, but you can form any complex value you like by adding one to a |
| real constant. This is a GNU extension; if you have an ISO C99 |
| conforming C library (such as GNU libc), and want to construct complex |
| constants of floating type, you should include <code><complex.h></code> and |
| use the macros <code>I</code> or <code>_Complex_I</code> instead. |
| |
| <p><a name="index-g_t_0040code_007b_005f_005freal_005f_005f_007d-keyword-2224"></a><a name="index-g_t_0040code_007b_005f_005fimag_005f_005f_007d-keyword-2225"></a>To extract the real part of a complex-valued expression <var>exp</var>, write |
| <code>__real__ </code><var>exp</var>. Likewise, use <code>__imag__</code> to |
| extract the imaginary part. This is a GNU extension; for values of |
| floating type, you should use the ISO C99 functions <code>crealf</code>, |
| <code>creal</code>, <code>creall</code>, <code>cimagf</code>, <code>cimag</code> and |
| <code>cimagl</code>, declared in <code><complex.h></code> and also provided as |
| built-in functions by GCC. |
| |
| <p><a name="index-complex-conjugation-2226"></a>The operator ‘<samp><span class="samp">~</span></samp>’ performs complex conjugation when used on a value |
| with a complex type. This is a GNU extension; for values of |
| floating type, you should use the ISO C99 functions <code>conjf</code>, |
| <code>conj</code> and <code>conjl</code>, declared in <code><complex.h></code> and also |
| provided as built-in functions by GCC. |
| |
| <p>GCC can allocate complex automatic variables in a noncontiguous |
| fashion; it's even possible for the real part to be in a register while |
| the imaginary part is on the stack (or vice-versa). Only the DWARF2 |
| debug info format can represent this, so use of DWARF2 is recommended. |
| If you are using the stabs debug info format, GCC describes a noncontiguous |
| complex variable as if it were two separate variables of noncomplex type. |
| If the variable's actual name is <code>foo</code>, the two fictitious |
| variables are named <code>foo$real</code> and <code>foo$imag</code>. You can |
| examine and set these two fictitious variables with your debugger. |
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