Modify list elements if last two entries are equal

Question

Suppose I have a list as follows

{{1, 1, 1, 1, 1, 3}, {1, 1, 3, 4, 1, 1}}

and I want to check the last two coordinates are equal or not. If that is the case then I want it to print out T after that list. So the result should be

{{1, 1, 1, 1, 1, 3}, {1, 1, 3, 4, 1, 1,T}}

I am thinking about using "if" but I have no idea how to do it.

Answer 1

I think the most expressive would be to use Replace:

list = {{1, 1, 1, 1, 1, 3}, {1, 1, 3, 4, 1, 1}};
Replace[
  list, 
  {rest___, l_, l_} :> {rest, l, l, T},
  1
]
(* {{1, 1, 1, 1, 1, 3}, {1, 1, 3, 4, 1, 1, T}} *)

Some alternatives:

Replace[list, list : {___, l_, l_} :> Append[list, T], 1]
Replace[list, l_ /; Equal @@ l[[-2 ;;]] :> Append[l, T], 1]
Replace[list, {list : PatternSequence[___, l_, l_]} :> {list, T}, 1]
Map[If[Equal @@ #[[-2 ;;]], Append[#, T], #] &, list]
Map[If[#[[-2]] == #[[-1]], Append[#, T], #] &, list]
Map[If[Equal @@ #[[-2 ;;]], Append[T], # &]@# &, list]

Answer 2

alist = {{1, 1, 1, 1, 1, 3}, {1, 1, 3, 4, 1, 1}}

Join @@@ Transpose[{#, 
      List /@ Equal @@@ #[[All, -2 ;; -1]]}] /. {False -> Nothing, 
    True -> T} & @alist

---

A simpler version of the above:

# ~ Join ~ If[Equal @@ #[[-2 ;; -1]], {T}, {Nothing}] & /@ alist

Description: Take a sublist (#) and Join an item with it. The item will be T if the last two entries are Equal, else Nothing. Map (/@) this over the entire list.

---

Result:

{{1, 1, 1, 1, 1, 3}, {1, 1, 3, 4, 1, 1, T}}

Answer 3

Another way:

 A = {{1, 1, 1, 1, 1, 3}, {1, 1, 3, 4, 1, 1}};
 Map[If[Mean[#[[-Length[#] ;; -(Length[#] - 1)]]] === Last[#], Append[#, T], #] &, A]
 (*{{1, 1, 1, 1, 1, 3}, {1, 1, 3, 4, 1, 1, T}}*)

Answer 4

If your data is always a list of lists and you always want the transformation performed at that second level, then this works (I've named your data theData):

Replace[theData, {head___, t_, t_} :> {head, t, t, "T"}, 1]