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I have two lists, where the number of elements of list 1 = number of sublists of list 2:

list1 = {a,b}; list2 = {{f,g,h,i},{j,k,l,m}}

I would like to add elements of list1 to the last elements of list2, so I did

list3 = MapThread[Append,{list2,list1}]; list3 = list3 /. {x1_,x2_,x3_,x4_,x5_} -> {x1,x2,x3,x4+x5}

It works but I suspect there is a more efficient way with, perhaps, MapAt or some combination of functions that I can't figure out. How would you do it? Thanks in advance.

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2 Answers 2

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With padding:

list2 + Map[PadLeft[{#},4]&, list1]

{{f, g, h, a + i}, {j, k, l, b + m}}

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See code below:

list2[[;; , -1]] = list2[[;; , -1]] + list1; list2

{{f, g, h, a + i}, {j, k, l, b + m}}

EDIT: or the more compact, suggested by @Henrik Schumacher in the comment below:

list2[[;; , -1]] += list1
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    $\begingroup$ Or list2[[;; , -1]] += list1. $\endgroup$ Commented Jul 6, 2018 at 10:24
  • $\begingroup$ True, much nicer :) $\endgroup$ Commented Jul 6, 2018 at 10:24

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