BUILT-IN MATHEMATICA SYMBOL
GatherBy
GatherBy[list, f]
gathers into sublists each set of elements in list that gives the same value when f is applied.
GatherBy[list, {f1, f2, ...}]
gathers list into nested sublists using 
at level i.
- GatherBy[{e1, e2, e3, ...}, f] evaluates
for each i to determine how to gather the 
.
- GatherBy[list, ...] gives results that follow the ordering of elements in list.
- Within each sublist generated by GatherBy[list, ...], elements appear in the same order as in list; the complete sublists are ordered so that their first elements appear in the same order as in list.
- GatherBy[list] is equivalent to GatherBy[list, Identity], which is also equivalent to Gather[list]. »
- GatherBy effectively produces an equivalence partition.
- GatherBy[list, {f1, f2}] is equivalent to Map[GatherBy[#, f2]&, GatherBy[list, f1]].
New in 7
