'''Purpose:''' Show how to convert between rectangular and polar co-ordinates in Tcl.
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If you have rectangular co-ordinates $x and $y, and you need polar
co-ordinates $r and $theta, you can use the following:

 proc rect_to_polar { x y } {
     set r [expr { hypot( $x, $y ) }]
     set theta [expr { atan2( $y, $x ) }]
     return [list $r $theta]
 }

 foreach { r theta } [rect_to_polar $x $y] break

Note the use of [hypot] and [atan2] in the code above.  These functions
are much more robust than any corresponding code using [atan] and [sqrt].
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If you have polar co-ordinates $r and $theta, and you need rectangular co-ordinates $x and $y, you can use the following:

 proc polar_to_rect { r theta } {
     set x [expr { $r * cos($theta) }]
     set y [expr { $r * sin($theta) }]
     return [list $x $y]
 }

 foreach { x y } [polar_to_rect $r $theta] break

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[Category Mathematics]