Data commit

Task/Pentagram/Tcl/pentagram.tcl

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+package require Tk 8.6   	;# lmap is new in Tcl/Tk 8.6
+
+set pi [expr 4*atan(1)]
+
+pack [canvas .c] -expand yes -fill both     ;# create the canvas
+
+update          ;# draw everything so the dimensions are accurate
+
+set w [winfo width .c]          ;# calculate appropriate dimensions
+set h [winfo height .c]
+set r [expr {min($w,$h) * 0.45}]
+
+set points [lmap n {0 1 2 3 4 5} {
+    set n [expr {$n * 2}]
+    set y [expr {sin($pi * 2 * $n / 5) * $r + $h / 2}]
+    set x [expr {cos($pi * 2 * $n / 5) * $r + $w / 2}]
+    list $x $y
+}]
+set points [concat {*}$points]  ;# flatten the list
+
+puts [.c create line $points]
+
+;# a fun reader exercise is to make the shape respond to mouse events,
+;# or animate it!