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<title>The GNU Fortran Compiler: ATAN2</title>
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<a name="ATAN2"></a>
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<div class="header">
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<p>
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Next: <a href="ATANH.html#ATANH" accesskey="n" rel="next">ATANH</a>, Previous: <a href="ATAN.html#ATAN" accesskey="p" rel="prev">ATAN</a>, Up: <a href="Intrinsic-Procedures.html#Intrinsic-Procedures" accesskey="u" rel="up">Intrinsic Procedures</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<a name="ATAN2-_002d_002d_002d-Arctangent-function"></a>
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<h3 class="section">9.22 <code>ATAN2</code> — Arctangent function</h3>
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<a name="index-ATAN2"></a>
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<a name="index-DATAN2"></a>
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<a name="index-trigonometric-function_002c-tangent_002c-inverse-1"></a>
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<a name="index-tangent_002c-inverse-1"></a>
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<dl compact="compact">
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<dt><em>Description</em>:</dt>
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<dd><p><code>ATAN2(Y, X)</code> computes the principal value of the argument
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function of the complex number <em>X + i Y</em>. This function can
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be used to transform from Cartesian into polar coordinates and
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allows to determine the angle in the correct quadrant.
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</p>
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</dd>
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<dt><em>Standard</em>:</dt>
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<dd><p>Fortran 77 and later
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</p>
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</dd>
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<dt><em>Class</em>:</dt>
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<dd><p>Elemental function
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</p>
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</dd>
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<dt><em>Syntax</em>:</dt>
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<dd><p><code>RESULT = ATAN2(Y, X)</code>
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</p>
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</dd>
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<dt><em>Arguments</em>:</dt>
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<dd><table>
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<tr><td width="15%"><var>Y</var></td><td width="70%">The type shall be <code>REAL</code>.</td></tr>
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<tr><td width="15%"><var>X</var></td><td width="70%">The type and kind type parameter shall be the same as <var>Y</var>.
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If <var>Y</var> is zero, then <var>X</var> must be nonzero.</td></tr>
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</table>
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</dd>
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<dt><em>Return value</em>:</dt>
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<dd><p>The return value has the same type and kind type parameter as <var>Y</var>. It
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is the principal value of the complex number <em>X + i Y</em>. If <var>X</var>
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is nonzero, then it lies in the range <em>-\pi \le \atan (x) \leq \pi</em>.
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The sign is positive if <var>Y</var> is positive. If <var>Y</var> is zero, then
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the return value is zero if <var>X</var> is strictly positive, <em>\pi</em> if
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<var>X</var> is negative and <var>Y</var> is positive zero (or the processor does
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not handle signed zeros), and <em>-\pi</em> if <var>X</var> is negative and
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<var>Y</var> is negative zero. Finally, if <var>X</var> is zero, then the
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magnitude of the result is <em>\pi/2</em>.
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</p>
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</dd>
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<dt><em>Example</em>:</dt>
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<dd><div class="smallexample">
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<pre class="smallexample">program test_atan2
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real(4) :: x = 1.e0_4, y = 0.5e0_4
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x = atan2(y,x)
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end program test_atan2
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</pre></div>
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</dd>
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<dt><em>Specific names</em>:</dt>
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<dd><table>
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<tr><td width="20%">Name</td><td width="20%">Argument</td><td width="20%">Return type</td><td width="25%">Standard</td></tr>
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<tr><td width="20%"><code>ATAN2(X, Y)</code></td><td width="20%"><code>REAL(4) X, Y</code></td><td width="20%"><code>REAL(4)</code></td><td width="25%">Fortran 77 and later</td></tr>
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<tr><td width="20%"><code>DATAN2(X, Y)</code></td><td width="20%"><code>REAL(8) X, Y</code></td><td width="20%"><code>REAL(8)</code></td><td width="25%">Fortran 77 and later</td></tr>
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</table>
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</dd>
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</dl>
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<p>
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Next: <a href="ATANH.html#ATANH" accesskey="n" rel="next">ATANH</a>, Previous: <a href="ATAN.html#ATAN" accesskey="p" rel="prev">ATAN</a>, Up: <a href="Intrinsic-Procedures.html#Intrinsic-Procedures" accesskey="u" rel="up">Intrinsic Procedures</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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