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 ) }]
0.99999999999999967In 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