unzip

unzip   xs

unzip takes a list of tuples {x1 y1} {x2 y2} {x3 y3} ... and returns a tuple of lists {x1 x2 x3 ...} {y1 y2 y3 ...}.

Source code:

    set left {}
    set right {}
    foreach x $xs {
	# assertion: x is a tuple
	lappend left [lindex $x 0]
	lappend right [lindex $x 1]
    }
    return [list $left $right]