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List rearrangement and summing


I have a list of the form:
{{a, x, y}, {b, x, x}, {c, y, y}, {d, x, y}, {e, y, y}}
How can I  rearrange it to:
{{a + d, x, y}, {b, x, x}, {c + e, y, y}}
i.e. to sum 1st elements for all sublists with equal 2nd and 3d elements

POSTED BY: Ivan Morozov
3 Replies
The second way is indeed better for me, since it doesn't depend on sublists length.
POSTED BY: Ivan Morozov
Posted 11 years ago
How about:
 sumFirstColumn[M_] := Prepend[
   M[[1, 2 ;; -1]],
   Total[M[[All, 1]]]
 lis = {{a, x, y}, {b, x, x}, {c, y, y}, {d, x, y}, {e, y, y}};
 grouped = GatherBy[lis, Rest];
 sumFirstColumn /@ grouped
(* {{a + d, x, y}, {b, x, x}, {c + e, y, y}} *)
POSTED BY: Greg Hurst
Posted 11 years ago
Every time you have:
maybe some stuff,
then a sublist with the right tail,
maybe some more stuff,
then another sublist with the same tail
and maybe some last stuff
replace all that with
the first stuff, the revised sublist, the more stuff and the last stuff
and repeat all that over and over until there are no more changes to make.
In[1]:= {{a,x,y},{b,x,x},{c,y,y},{d,x,y},{e,y,y}}//.{h___,{p_,q_,r_},m___,{s_,q_,r_},t___}:>{h,{p+s,q,r},m,t}

Out[1]= {{a+d,x,y}, {b,x,x}, {c+e,y,y}}
Think of a more efficient method if you list has millions or tens of millions of sublists.
POSTED BY: Bill Simpson
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