'''Purpose:''' Explain the math function '''atan'''.
----
The call,

    [expr { atan( $x ) }]

returns the inverse tangent of the number, $x.  The resulting angle
is measured in radians.
----
Most uses of ''atan'' are perilous.  The problem is that there is
always uncertainty about the quadrant of the angle:

 % set PI 3.1415926535897932
 3.1415926535897932
 % puts [expr {tan ( 0.25 * $PI ) }]
 0.99999999999999989
 % puts [expr { tan ( 1.25 * $PI ) }]
 0.99999999999999967

In other words, when you say,

    [expr { atan( 1.0 ) }]

it is hard to know whether you mean 
 0.25 * $PI
or
 1.25 * $PI

For this reason, ''atan'' should generally be avoided in favor of
''[atan2]'', which does not suffer from this problem.

----
[Math function help] - [Arts and crafts of Tcl-Tk programming]
- [Category Mathematics]