# Fortran Wiki atan2

## Description

atan2(y, x) computes the arctangent of the complex number $X+iY$.

## 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 $X+iY$. If x is nonzero, then it lies in the range $-\pi \le atan\left(x\right)\le \pi$. The sign is positive if y is positive. If y is zero, then the return value is zero if x is positive and $\pi$ if x is negative. 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 $\le$ to $<$. Joe Krahn reverted the edit, following F2008 specs.

category: intrinsics