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### 9.25 `ATAN2`

— Arctangent function

*Description*:`ATAN2(Y, X)`

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.*Standard*:Fortran 77 and later

*Class*:Elemental function

*Syntax*:`RESULT = ATAN2(Y, X)`

*Arguments*:`Y`The type shall be `REAL`

.`X`The type and kind type parameter shall be the same as `Y`. If`Y`is zero, then`X`must be nonzero.*Return value*:The return value has the same type and kind type parameter as

`Y`. It is the principal value of the complex number*X + i Y*. If`X`is nonzero, then it lies in the range*-\pi \le \atan (x) \leq \pi*. The sign is positive if`Y`is positive. If`Y`is zero, then the return value is zero if`X`is strictly positive,*\pi*if`X`is negative and`Y`is positive zero (or the processor does not handle signed zeros), and*-\pi*if`X`is negative and`Y`is negative zero. Finally, if`X`is zero, then the magnitude of the result is*\pi/2*.*Example*:program test_atan2 real(4) :: x = 1.e0_4, y = 0.5e0_4 x = atan2(y,x) end program test_atan2

*Specific names*:Name Argument Return type Standard `ATAN2(X, Y)`

`REAL(4) X, Y`

`REAL(4)`

Fortran 77 and later `DATAN2(X, Y)`

`REAL(8) X, Y`

`REAL(8)`

Fortran 77 and later *See also*:

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