Rolf Ade posted a question to the chat asking how to form the Cartesian product of a set of lists. That is, given a list like,
{ { a b c } { d e f } }
he wanted
{{a d} {a e} {a f} {b d} {b e} {b f} {c d} {c e} {c f}}
He also wanted it to generalize to higher dimension: given
{{a b} {c d} {e f}}
he wanted
{{a c e} {a c f} {a d e} {a d f} {b c e} {b c f} {b d e} {b d f}}
and so on.
Kevin Kenny proposed the following:
proc crossProduct { listOfLists } { if { [llength $listOfLists] == 0 } { return [list [list]] } else { set result [list] foreach elt [lindex $listOfLists 0] { foreach combination [crossProduct [lrange $listOfLists 1 end]] { lappend result [linsert $combination 0 $elt] } } return $result } } puts [crossProduct {{a b c} {d e f} {g h i}}]
This solution is by no means the fastest available, but it appears to work for the purpose.