[ Arithmetic | The ECLiPSe Built-In Predicates | Reference Manual | Alphabetic Index ]

# atan(+Y, +X, -Result)

Computes the arc tangent function of two variables and unifies the resulting value with Result.

Y
  A number.
X
  A number.
Result
  A variable, float or breal.

## Description

This predicate is used by the ECLiPSe compiler to expand evaluable arithmetic expressions. So the call to atan(Y, X, Result) is equivalent to

        Result is atan(Y, X)

which should be preferred for portability.

It is similar to calculating the arc tangent of Y/X, except that the signs of both arguments are used to determine the quadrant of the result. The result lies in the interval -pi..pi. The operation is valid even if X is zero, in which case the result is pi/2 or -pi/2. One application is the conversion of cartesian to polar coordinates, where this function computes the angle component (in radians).

In coroutining mode, if X or Y is uninstantiated, the call to atan/3 is delayed until both variables are instantiated.

### Modes and Determinism

- atan(+, +, -) is det

### Exceptions

(4) instantiation fault
  X or Y is not instantiated (non-coroutining mode only).
(5) type error
  Result is a number but not a float or breal
(24) number expected
  Number is not of a numeric type.
(24) number expected
  Result is neither a number nor a variable.

## Examples

    Success:
          atan( 0.0, -1.0, Result).	(gives Result =  3.141592)
          atan( 1.0, -1.0, Result).	(gives Result =  2.356194)
          atan( 1.0,  0.0, Result).	(gives Result =  1.570796)
          atan( 1.0,  1.0, Result).	(gives Result =  0.785398)
          atan( 0.0,  0.0, Result).	(gives Result =  0.0)
          atan(-1.0,  1.0, Result).	(gives Result = -0.785398)
          atan(-1.0,  0.0, Result).	(gives Result = -1.570796)
          atan(-1.0, -1.0, Result).	(gives Result = -2.356194)
          atan(-0.0, -1.0, Result).	(gives Result = -3.141592)

          atan( 7.0,  7.0, Result).	(gives Result =  0.785398)

    Error:
          atan(A, 0.0, 6.0).              (Error 4).
          atan(1.55741, 0.0, 1).          (Error 5).
          atan(1 + 0.55741, 1.0, R).      (Error 24).

## See Also

is / 2