Fortran Wiki
atan2

Description

atan2(y, x) computes the arctangent 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. To convert from Cartesian Coordinates $(x,y)$ to polar coordinates $(r,\theta)$:

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 $-\pi \leq \atan (x) \leq \pi$. 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

Note: In Return value, NNemec changed the first $\leq$ to $\lt$. Joe Krahn reverted the edit, following the Fortran 2008 specs.