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<h3 class="section">8.22 <code>ATAN2</code> — Arctangent function</h3>
<p><a name="index-ATAN2-421"></a><a name="index-DATAN2-422"></a><a name="index-trigonometric-function_002c-tangent_002c-inverse-423"></a><a name="index-tangent_002c-inverse-424"></a>
<dl>
<dt><em>Description</em>:<dd><code>ATAN2(Y, X)</code> computes the principal value of the argument
function of the complex number X + i Y. This function can
be used to transform from Cartesian into polar coordinates and
allows to determine the angle in the correct quadrant.
<br><dt><em>Standard</em>:<dd>Fortran 77 and later
<br><dt><em>Class</em>:<dd>Elemental function
<br><dt><em>Syntax</em>:<dd><code>RESULT = ATAN2(Y, X)</code>
<br><dt><em>Arguments</em>:<dd>
<p><table summary=""><tr align="left"><td valign="top" width="15%"><var>Y</var> </td><td valign="top" width="70%">The type shall be <code>REAL</code>.
<br></td></tr><tr align="left"><td valign="top" width="15%"><var>X</var> </td><td valign="top" width="70%">The type and kind type parameter shall be the same as <var>Y</var>.
If <var>Y</var> is zero, then <var>X</var> must be nonzero.
<br></td></tr></table>
<br><dt><em>Return value</em>:<dd>The return value has the same type and kind type parameter as <var>Y</var>.
It is the principal value of the complex number X + i Y. If
<var>X</var> is nonzero, then it lies in the range -\pi \le \atan (x) \leq \pi.
The sign is positive if <var>Y</var> is positive. If <var>Y</var> is zero, then
the return value is zero if <var>X</var> is positive and \pi if <var>X</var>
is negative. Finally, if <var>X</var> is zero, then the magnitude of the result
is \pi/2.
<br><dt><em>Example</em>:<dd>
<pre class="smallexample"> program test_atan2
real(4) :: x = 1.e0_4, y = 0.5e0_4
x = atan2(y,x)
end program test_atan2
</pre>
<br><dt><em>Specific names</em>:<dd>
<p><table summary=""><tr align="left"><td valign="top" width="20%">Name </td><td valign="top" width="20%">Argument </td><td valign="top" width="20%">Return type </td><td valign="top" width="25%">Standard
<br></td></tr><tr align="left"><td valign="top" width="20%"><code>ATAN2(X, Y)</code> </td><td valign="top" width="20%"><code>REAL(4) X, Y</code> </td><td valign="top" width="20%"><code>REAL(4)</code> </td><td valign="top" width="25%">Fortran 77 and later
<br></td></tr><tr align="left"><td valign="top" width="20%"><code>DATAN2(X, Y)</code> </td><td valign="top" width="20%"><code>REAL(8) X, Y</code> </td><td valign="top" width="20%"><code>REAL(8)</code> </td><td valign="top" width="25%">Fortran 77 and later
<br></td></tr></table>
</dl>
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