tree_group

tree_group   xs   { node_level "id" }

Divides a list into groups. The first element of the list provides the key value that starts a new group. Example: tree_group {1 2 3 2 1 2 1 1 2 2} = {1 2 3 2} {1 2} 1 {1 2 2} The optional argument node_level is a function that extracts the value of an element. By default, this is the identity function id. Example: tree_group {{a 1} {b 2} {c 1}} snd = {{a 1} {b 2}} {c 1}

Source code:

    if { $xs=={} } return {}
    set key [$node_level [head $xs]]
    set list {}
    set sublist {}
    foreach x $xs {
	if { $sublist=={} || [$node_level $x]!=$key } {
	    lappend sublist $x
	} else {
	    lappend list $sublist
	    set sublist [list $x]
	}
    }
    lappend list $sublist
    return $list