I'm trying to use the xat(n) and yat(n) in the Field Calculator but can't get them to work. Does anybody have an example how to use them? I need start and end coordinates for line segments, does anybody know how to get thoose?

In the below list the xat() and yat() functions are listed. http://hub.qgis.org/wiki/17/List_of_Field_Calculator_Functions