I have nested list with sublists of varying lengths like this:


How can I join the 4th and 5th sublists based on their position to give me:


The closest I have managed to get is using:

FlattenAt[list, {{4}, {5}}]

But this gives:

{{a, b, c, d}, {e, f}, {g, h, i}, j, k, l, {m, n, o}}

Thanks in advance.


6 Answers 6


You could use SubsetMap (available from version 12.0), but since it requires you to return the same size you get, we could trick it by Hold and Nothing:

ReleaseHold @ SubsetMap[{Join @@ #, Hold @ Nothing} &, list, {4, 5}]

(* Out: {{a, b, c, d}, {e, f}, {g, h, i}, {j, k, l}, {m, n, o}} *)
  • $\begingroup$ This is great, works really well. Thanks! $\endgroup$
    – wmv
    Apr 21 at 15:09

Another approach using Delete, Extract and Insert:

Insert[Delete[list, #], Join @@ Extract[list, #], 4] & @@ {{{4}, {5}}}
(*{{a, b, c, d}, {e, f}, {g, h, i}, {j, k, l}, {m, n, o}}*)
spliceParts =  Module[{l = #, p = Flatten @ #2},
  l[[First @ p]] =Flatten @ l[[p]]; l[[Rest @ p]] = Nothing; l] &;


list = {{a, b, c, d}, {e, f}, {g, h, i}, {j}, {k, l}, {m, n, o}};

spliceParts[list, {{4}, {5}}]
{{a, b, c, d}, {e, f}, {g, h, i}, {j, k, l}, {m, n, o}}
spliceParts[list, { {4}, {5}, {2}}]
{{a, b, c, d}, {g, h, i}, {j, k, l, e, f}, {m, n, o}} 

Another way of doing it, albeit not elegant

Join[list[[1 ;; 3]], {Join[list[[4]], list[[5]]]}, {list[[6]]}]



Maybe we can do the 10 ways again, why not?

Hence, one way of doing it

TakeList[Flatten[list], {4, 2, 3, 3, 3}]


  • 1
    $\begingroup$ The ten ways team! :) $\endgroup$ Apr 22 at 16:29
  • 1
    $\begingroup$ @E.Chan-López maybe we should have a banner next to questions with 10 or more answers :) Like level of awesomeness reached or something! $\endgroup$
    – bmf
    Apr 22 at 16:30
  • $\begingroup$ That's an excellent idea, @bmf! $\endgroup$ Apr 22 at 16:31

As ReplacePart "uses rules in the order given", ReplacePart may be used to Join sublists at the first position specified, and then remove all but the joined sublist:

ReplacePart[list, Flatten[{4:> Join[Sequence@@list[[{4,5}]]], Thread[{4,5}:> Nothing]}]]

{{a, b, c, d}, {e, f}, {g, h, i}, {j, k, l}, {m, n, o}}

As a 'pure' function:

ReplacePart[#1, Flatten[{#2[[1]]:> Join[Sequence@@#1[[##2]]], 
    Thread[##2:> Nothing]}]]&@@{list,{4,5}}

{{a, b, c, d}, {e, f}, {g, h, i}, {j, k, l}, {m, n, o}}

ReplacePart[#1, Flatten[{#2[[1]]:> Join[Sequence@@#1[[##2]]], 
    Thread[##2:> Nothing]}]]&@@{list,{5,4,1}

{{e, f}, {g, h, i}, {k, l, j, a, b, c, d}, {m, n, o}}

As a function:

f[lst_List,pos_List]:= ReplacePart[lst, Flatten[{pos[[1]]:> Join[Sequence@@lst[[pos]]], 
                         Thread[pos:> Nothing]}]]


f[list, {4,5}]

{{a, b, c, d}, {e, f}, {g, h, i}, {j, k, l}, {m, n, o}}

f[list, {6,1,3}]

{{e, f}, {j}, {k, l}, {m, n, o, a, b, c, d, g, h, i}}


{m, n, o, k, l, j, g, h, i, e, f, a, b, c, d}


list={{a, b, c, d}, {e, f}, {g, h, i},{j}, {k, l}, {m, n, o}};

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