Arctangent (inverse tangent) function. The result is the principal value of the nonzero complex number (X, Y) formed by the real arguments Y and X.

**Y**- must be of type real.
**X**- must be of the same type and kind type parameter as Y. If Y has the value zero, X must not have the value zero.

**Class**

Elemental function

**Result Type and Attributes**

Same as X.

**Result Value**

- It is expressed in radians and has a value equal to the principal value of the argument of the complex number (X, Y).
- It is in the range -&pi. < ATAN2(Y, X) <= &pi..
- If X <> 0, the result approximates arctan(Y/X).
- If Y > 0, the result is positive.
- If Y < 0, the result is negative.
- If Y = 0 and X > 0, the result is zero.
- If Y = 0 and X < 0, the result is &pi..
- If X = 0, the absolute value of the result is &pi./2.

**Examples**

ATAN2 (1.5574077, 1.0) has the value 1.0.

Given that:

```
Y = | 1 1 | X = | -1 1 |
| -1 -1 | | -1 1 |
```

the value of ATAN2(Y,X) is approximately:

```
ATAN2 (Y, X) = | 3&pi./4 &pi./4 |
| -3&pi./4 -&pi./4 |
```

Specific Name | Argument Type | Result Type | Pass As Arg? |
---|---|---|---|

ATAN2 | default real | default real | yes |

DATAN2 | double precision real | double precision real | yes |

QATAN2 (1)
| REAL(16) | REAL(16) | yes |

**Notes: **

- IBM Extension.