Last list item

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Revision as of 17:35, 27 August 2022 by Thundergnat (talk | contribs) (syntax highlighting fixup automation)
Last list item is a draft programming task. It is not yet considered ready to be promoted as a complete task, for reasons that should be found in its talk page.
Task

List = [6, 81, 243, 14, 25, 49, 123, 69, 11]
Find the two smallest items, remove them from the list, find their sum, and add the sum to the end of list.
Repeat until the list contains one element.
Show the steps and last item on this page.
Show the unsorted list in output in case "Without sort"

11l

Translation of: Python
F add_least_reduce(&lis)
   L lis.len > 1
      V el1 = lis.pop(lis.index(min(lis)))
      V el2 = lis.pop(lis.index(min(lis)))
      lis.append(el1 + el2)
      print(‘Interim list: ’lis)
   R lis

V LIST = [6, 81, 243, 14, 25, 49, 123, 69, 11]

print(LIST‘  ==>  ’add_least_reduce(&copy(LIST)))
Output:
Interim list: [81, 243, 14, 25, 49, 123, 69, 17]
Interim list: [81, 243, 25, 49, 123, 69, 31]
Interim list: [81, 243, 49, 123, 69, 56]
Interim list: [81, 243, 123, 69, 105]
Interim list: [243, 123, 105, 150]
Interim list: [243, 150, 228]
Interim list: [243, 378]
Interim list: [621]
[6, 81, 243, 14, 25, 49, 123, 69, 11]  ==>  [621]

Ada

With sorting

with Ada.Text_IO;
with Ada.Containers.Vectors;

procedure Last_List_Item_Sorted is

   package Integer_IO is new Ada.Text_IO.Integer_IO (Integer);

   package Vectors is
     new Ada.Containers.Vectors (Index_Type   => Positive,
                                 Element_Type => Integer);
   package Vector_Sorting is
     new Vectors.Generic_Sorting;
   use Ada.Containers, Vectors, Ada.Text_IO, Integer_IO;

   procedure Put (List : Vector) is
   begin
      Put ("[");
      for V of List loop
         Put (" "); Put (V, Width => 0);
      end loop;
      Put ("]");
   end Put;

   List : Vector := 6 & 81 & 243 & 14 & 25 & 49 & 123 & 69 & 11;

begin
   Default_Width := 0;
   while List.Length >= 2 loop
      Vector_Sorting.Sort (List);
      Put (List);
      declare
         Small_1 : constant Integer := List (List.First_Index + 0);
         Small_2 : constant Integer := List (List.First_Index + 1);
         Sum     : constant Integer := Small_1 + Small_2;
      begin
         List.Delete_First (2);
         Put (". Smallest: ");  Put (Small_1);  Put (" and ");  Put (Small_2);
         Put (". Sum: ");  Put (Sum);  New_Line;
         List.Append (Sum);
      end;
   end loop;
   Put (List); New_Line;
end Last_List_Item_Sorted;
Output:
[ 6 11 14 25 49 69 81 123 243]. Smallest: 6 and 11. Sum: 17
[ 14 17 25 49 69 81 123 243]. Smallest: 14 and 17. Sum: 31
[ 25 31 49 69 81 123 243]. Smallest: 25 and 31. Sum: 56
[ 49 56 69 81 123 243]. Smallest: 49 and 56. Sum: 105
[ 69 81 105 123 243]. Smallest: 69 and 81. Sum: 150
[ 105 123 150 243]. Smallest: 105 and 123. Sum: 228
[ 150 228 243]. Smallest: 150 and 228. Sum: 378
[ 243 378]. Smallest: 243 and 378. Sum: 621
[ 621]

Without sorting

with Ada.Text_IO;
with Ada.Containers.Vectors;

procedure Last_List_Item is

   package Integer_IO is new Ada.Text_IO.Integer_IO (Integer);

   package Element_Vectors is
     new Ada.Containers.Vectors (Index_Type   => Positive,
                                 Element_Type => Integer);
   use Ada.Containers, Element_Vectors, Ada.Text_IO, Integer_IO;

   function Pick_Smallest (List : in out Vector) return Integer is
      Value : Integer := Integer'Last;
   begin
      if List.Is_Empty then
         raise Constraint_Error;
      end if;

      for V of List loop
         Value := Integer'Min (Value, V);
      end loop;
      List.Delete (List.Find_Index (Value));
      return Value;
   end Pick_Smallest;

   procedure Put (List : Vector) is
   begin
      Put ("[");
      for V of List loop
         Put (" "); Put (V, Width => 0);
      end loop;
      Put ("]");
   end Put;

   List : Vector := 6 & 81 & 243 & 14 & 25 & 49 & 123 & 69 & 11;

begin
   Default_Width := 0;
   while List.Length >= 2 loop
      Put (List); Put (". Smallest: ");
      declare
         Small_1 : constant Integer := Pick_Smallest (List);
         Small_2 : constant Integer := Pick_Smallest (List);
         Sum     : constant Integer := Small_1 + Small_2;
      begin
         Put (Small_1);  Put (" and ");  Put (Small_2);
         Put (". Sum: ");  Put (Sum);  New_Line;

         List.Append (Sum);
      end;
   end loop;
   Put (List); New_Line;
end Last_List_Item;
Output:
[ 6 81 243 14 25 49 123 69 11]. Smallest: 6 and 11. Sum: 17
[ 81 243 14 25 49 123 69 17]. Smallest: 14 and 17. Sum: 31
[ 81 243 25 49 123 69 31]. Smallest: 25 and 31. Sum: 56
[ 81 243 49 123 69 56]. Smallest: 49 and 56. Sum: 105
[ 81 243 123 69 105]. Smallest: 69 and 81. Sum: 150
[ 243 123 105 150]. Smallest: 105 and 123. Sum: 228
[ 243 150 228]. Smallest: 150 and 228. Sum: 378
[ 243 378]. Smallest: 243 and 378. Sum: 621
[ 621]

ALGOL 68

With sorting

Translation of: Wren
Library: ALGOL 68-rows
BEGIN # find the last element after repeatedely adding the sum #
      # of the two smallest elements and removing them #
    PR read "rows.incl.a68" PR # row related utilities #

    [ 1 : 9 ]INT a := ( 6, 81, 243, 14, 25, 49, 123, 69, 11 );
    INT a count := UPB a;
    WHILE a count > 1 DO
        QUICKSORT a FROMELEMENT LWB a TOELEMENT a count;
        print( ( "Sorted list:" ) );FOR i TO a count DO print( ( " ", whole( a[ i ], 0 ) ) ) OD;
        INT sum = a[ 1 ] + a[ 2 ];
        print( ( "; two smallest: " , whole( a[ 1 ], 0 )
               , " + ", whole( a[ 2 ], 0 )
               , " = ", whole( sum, 0 )
               , newline
               )
             );
        a[ 1 : a count - 2 ] := a[ 3 : a count ];
        a count     -:= 1  ;
        a[ a count ] := sum
    OD;
    print( ( "Last item is ", whole( a[ 1 ], 0 ), ".", newline ) )
END
Output:
Sorted list: 6 11 14 25 49 69 81 123 243; two smallest: 6 + 11 = 17
Sorted list: 14 17 25 49 69 81 123 243; two smallest: 14 + 17 = 31
Sorted list: 25 31 49 69 81 123 243; two smallest: 25 + 31 = 56
Sorted list: 49 56 69 81 123 243; two smallest: 49 + 56 = 105
Sorted list: 69 81 105 123 243; two smallest: 69 + 81 = 150
Sorted list: 105 123 150 243; two smallest: 105 + 123 = 228
Sorted list: 150 228 243; two smallest: 150 + 228 = 378
Sorted list: 243 378; two smallest: 243 + 378 = 621
Last item is 621.

Without sorting

Translation of the sorted Wren version with the sorting removed.

BEGIN # find the last element after repeatedly adding the sum #
      # of the two smallest elements and removing them #
    [ 1 : 9 ]INT a := ( 6, 81, 243, 14, 25, 49, 123, 69, 11 );
    INT a count := UPB a;
    WHILE a count > 1 DO
        print( ( "List:" ) );FOR i TO a count DO print( ( " ", whole( a[ i ], 0 ) ) ) OD;
        INT s1pos, s2pos;
        IF a[ 1 ] < a[ 2 ]
        THEN s1pos := 1; s2pos := 2
        ELSE s1pos := 2; s2pos := 1
        FI;
        FOR i FROM 3 TO a count DO
            IF   a[ i ] < a[ s1pos ] THEN s2pos := s1pos; s1pos := i
            ELIF a[ i ] < a[ s2pos ] THEN s2pos := i
            FI
        OD;
        INT sum = a[ s1pos ] + a[ s2pos ];
        print( ( "; two smallest: " , whole( a[ s1pos ], 0 ), " @ ", whole( s1pos, 0 )
               , " and ",             whole( a[ s2pos ], 0 ), " @ ", whole( s2pos, 0 )
               , "; sum = ", whole( sum, 0 )
               , newline
               )
             );
        INT m pos := 0;
        FOR i TO a count DO
            IF i /= s1pos AND i /= s2pos THEN a[ m pos +:= 1 ] := a[ i ] FI
        OD;
        a[ m pos + 1 ] := sum;
        a count -:= 1  
    OD;
    print( ( "Last item is ", whole( a[ 1 ], 0 ), ".", newline ) )
END
Output:
List: 6 81 243 14 25 49 123 69 11; two smallest: 6 @ 1 and 11 @ 9; sum = 17
List: 81 243 14 25 49 123 69 17; two smallest: 14 @ 3 and 17 @ 8; sum = 31
List: 81 243 25 49 123 69 31; two smallest: 25 @ 3 and 31 @ 7; sum = 56
List: 81 243 49 123 69 56; two smallest: 49 @ 3 and 56 @ 6; sum = 105
List: 81 243 123 69 105; two smallest: 69 @ 4 and 81 @ 1; sum = 150
List: 243 123 105 150; two smallest: 105 @ 3 and 123 @ 2; sum = 228
List: 243 150 228; two smallest: 150 @ 2 and 228 @ 3; sum = 378
List: 243 378; two smallest: 243 @ 1 and 378 @ 2; sum = 621
Last item is 621.

Applesoft BASIC

 100  DATA 6, 81, 243, 14, 25, 49, 123, 69, 11
 110  FOR L = 0 TO 8
 120      READ L(L)
 130  NEXT 
 140  LET L = L - 1
 150  GOSUB 180"LAST LIST ITEM"
 160  GOSUB 250"PRINT LIST"
 170  END

 180  IF L = 0 THEN  RETURN 
 190  FOR L = L TO 0 STEP 0
 200      GOSUB 340"FIND THE 2 SMALLEST"
 210      LET L = L + 1
 220      LET L(L) = PREVIOUSSMALLEST + SMALLEST
 230  NEXT L
 240  RETURN
 
 250  PRINT 
 260  LET H = 1
 270  FOR I = 0 TO L
 280      IF  NOT H(I) THEN H(I) = H
 290      LET H = H(I) +  LEN ( STR$ (L(I))) + 1
 300      HTAB H(I)
 310      PRINT L(I)" ";
 320  NEXT I
 330  RETURN

 340  GOSUB 250"PRINT LIST"
 350  GOSUB 370"FIND SMALLEST"
 360  LET PREVIOUSSMALLEST = SMALLEST

 370  LET S = 0
 380  FOR I = 0 TO L
 390      LET SMALLEST = L(S)
 400      IF L(I) < SMALLEST THEN S = I
 410  NEXT I
 420  LET SMALLEST = L(S)
 430  LET L = L - 1
 440  IF S > L THEN  RETURN 
 450  FOR I = S TO L
 460      LET L(I) = L(I + 1)
 470      LET H(I) = H(I + 1)
 480  NEXT I
 490  RETURN
Output:
6 81 243 14 25 49 123 69 11 
  81 243 14 25 49 123 69 17 
  81 243    25 49 123 69 31 
  81 243       49 123 69 56 
  81 243          123 69 105 
     243          123    105 150 
     243                 150 228 
     243                 378 
                         621 

AppleScript

on lastListItem(lst)
    set astid to AppleScript's text item delimiters
    set AppleScript's text item delimiters to ", "
    set output to {"Steps:", "{" & lst & "}   ← Original"}
    set len to (count lst)
    if (len > 1) then
        repeat with len from len to 3 by -1
            set {i, j} to {1, 2}
            set {s1, s2} to lst
            if (s2 < s1) then set {i, s1, j, s2} to {j, s2, i, s1}
            repeat with k from 3 to len
                set v to lst's item k
                if (v < s1) then
                    set {i, s1, j, s2} to {k, v, i, s1}
                else if (v < s2) then
                    set {j, s2} to {k, v}
                end if
            end repeat
            tell lst to set {item i, item j} to {missing value, missing value}
            set lst to lst's numbers
            set lst's end to s1 + s2
            set output's end to "{" & lst & "}   ← " & s1 & " + " & s2 & " = " & result
        end repeat
        tell lst to set end of output to "{" & (beginning + end) & "}   ← " & beginning & " + " & end & " = " & (beginning + end)
    end if
    set AppleScript's text item delimiters to linefeed
    set output to output as text
    set AppleScript's text item delimiters to astid
    return output
end lastListItem

lastListItem({6, 81, 243, 14, 25, 49, 123, 69, 11})
Output:
"Steps:
{6, 81, 243, 14, 25, 49, 123, 69, 11}   ← Original
{81, 243, 14, 25, 49, 123, 69, 17}   ← 6 + 11 = 17
{81, 243, 25, 49, 123, 69, 31}   ← 14 + 17 = 31
{81, 243, 49, 123, 69, 56}   ← 25 + 31 = 56
{81, 243, 123, 69, 105}   ← 49 + 56 = 105
{243, 123, 105, 150}   ← 69 + 81 = 150
{243, 150, 228}   ← 105 + 123 = 228
{243, 378}   ← 150 + 228 = 378
{621}   ← 243 + 378 = 621"

AutoHotkey

List := [6, 81, 243, 14, 25, 49, 123, 69, 11]
steps := "Initial List `t`t`t" List2str(List) "`n"
while List.Count() > 1
{
    sum := 0, str := ""
    loop 2
    {
        num := min(List*)
        sum += num 
        removeFromList(List, num)
        str .= (!str ? "2 smallest numbers: " : " + ") num 
    }
    List.Push(sum)
    steps .= str " = " sum "`t" List2str(List) "`n"
}
MsgBox % result := steps
return

removeFromList(ByRef List, num){
    for i, v in List
        if (v = num)
        {
            List.RemoveAt(i)
            break
        }
}
List2str(List){
    for i, v in List
        unsorted_List .= v ", "
    return "[" Trim(unsorted_List, ", ") "]"
}
Output:
Initial List 				[6, 81, 243, 14, 25, 49, 123, 69, 11]
2 smallest numbers: 6 + 11 = 17		[81, 243, 14, 25, 49, 123, 69, 17]
2 smallest numbers: 14 + 17 = 31	[81, 243, 25, 49, 123, 69, 31]
2 smallest numbers: 25 + 31 = 56	[81, 243, 49, 123, 69, 56]
2 smallest numbers: 49 + 56 = 105	[81, 243, 123, 69, 105]
2 smallest numbers: 69 + 81 = 150	[243, 123, 105, 150]
2 smallest numbers: 105 + 123 = 228	[243, 150, 228]
2 smallest numbers: 150 + 228 = 378	[243, 378]
2 smallest numbers: 243 + 378 = 621	[621]

AWK

# syntax: GAWK -f LAST_LIST_ITEM.AWK
BEGIN {
    split("6,81,243,14,25,49,123,69,11",arr1,",")
    PROCINFO["sorted_in"] = "@val_num_asc"
    while (length(arr1) > 1) {
      for (i in arr1) { printf("%s ",arr1[i]) } # show sorted list
      j = 0
      delete arr2
      for (i in arr1) { arr2[++j] = arr1[i] } # copy arr1 into arr2
      sum = arr2[1] + arr2[2]
      printf(": %s+%s=%s\n",arr2[1],arr2[2],sum)
      delete arr2[2]
      arr2[1] = sum
      delete arr1
      for (i in arr2) { arr1[i] = arr2[i] } # copy arr2 into arr1
    }
    printf("sum=%d\n",sum)
    exit(0)
}
Output:
6 11 14 25 49 69 81 123 243 : 6+11=17
14 17 25 49 69 81 123 243 : 14+17=31
25 31 49 69 81 123 243 : 25+31=56
49 56 69 81 123 243 : 49+56=105
69 81 105 123 243 : 69+81=150
105 123 150 243 : 105+123=228
150 228 243 : 150+228=378
243 378 : 243+378=621
sum=621

BQN

With and without sorting

list ← ⟨6, 81, 243, 14, 25, 49, 123, 69, 11⟩

W_sort ← {{i←2↑∧𝕩, (𝕩/˜¬𝕩∊i)∾+´i}⍟(↕≠𝕩)𝕩}

WO_sort ← {{m←⌊´𝕩, n←⌊´𝕩/˜¬m=𝕩, (𝕩/˜¬𝕩∊m‿n)∾m+n}⍟(↕≠𝕩)𝕩}

•Show¨W_sort list
•Show¨WO_sort list
⟨ 6 81 243 14 25 49 123 69 11 ⟩
⟨ 81 243 14 25 49 123 69 17 ⟩
⟨ 81 243 25 49 123 69 31 ⟩
⟨ 81 243 49 123 69 56 ⟩
⟨ 81 243 123 69 105 ⟩
⟨ 243 123 105 150 ⟩
⟨ 243 150 228 ⟩
⟨ 243 378 ⟩
⟨ 621 ⟩
⟨ 6 81 243 14 25 49 123 69 11 ⟩
⟨ 81 243 14 25 49 123 69 17 ⟩
⟨ 81 243 25 49 123 69 31 ⟩
⟨ 81 243 49 123 69 56 ⟩
⟨ 81 243 123 69 105 ⟩
⟨ 243 123 105 150 ⟩
⟨ 243 150 228 ⟩
⟨ 243 378 ⟩
⟨ 621 ⟩

C

With sorting

Translation of: Wren
#include <stdio.h>
#include <stdlib.h>

int compare(const void *a, const void *b) {
    int aa = *(const int *)a;
    int bb = *(const int *)b;
    if (aa < bb) return -1;
    if (aa > bb) return 1;
    return 0;
}

int main() {
    int a[] = {6, 81, 243, 14, 25, 49, 123, 69, 11};
    int isize = sizeof(int);
    int asize = sizeof(a) / isize;
    int i, sum;
    while (asize > 1) {
        qsort(a, asize, isize, compare);
        printf("Sorted list: ");
        for (i = 0; i < asize; ++i) printf("%d ", a[i]);
        printf("\n");
        sum = a[0] + a[1];
        printf("Two smallest: %d + %d = %d\n", a[0], a[1], sum);
        for (i = 2; i < asize; ++i) a[i-2] = a[i];
        a[asize - 2] = sum;
        asize--;
    }
    printf("Last item is %d.\n", a[0]);
    return 0;
}
Output:
Sorted list: 6 11 14 25 49 69 81 123 243 
Two smallest: 6 + 11 = 17
Sorted list: 14 17 25 49 69 81 123 243 
Two smallest: 14 + 17 = 31
Sorted list: 25 31 49 69 81 123 243 
Two smallest: 25 + 31 = 56
Sorted list: 49 56 69 81 123 243 
Two smallest: 49 + 56 = 105
Sorted list: 69 81 105 123 243 
Two smallest: 69 + 81 = 150
Sorted list: 105 123 150 243 
Two smallest: 105 + 123 = 228
Sorted list: 150 228 243 
Two smallest: 150 + 228 = 378
Sorted list: 243 378 
Two smallest: 243 + 378 = 621
Last item is 621.

Without sorting

#include <stdio.h>

int findMin(int a[], int asize, int *pmin) {
    int i, ix = 0;
    *pmin = a[0];
    for (i = 0; i < asize; ++i) {
        if (a[i] < *pmin) {
            ix = i;
            *pmin = a[i];
        }
    }
    return ix;
}

int main() {
    int a[] = {6, 81, 243, 14, 25, 49, 123, 69, 11};
    int isize = sizeof(int);
    int asize = sizeof(a) / isize;
    int i, j, sum, ix, min = 0, s[2];
    while (asize > 1) {
        printf("List: ");
        for (i = 0; i < asize; ++i) printf("%d ", a[i]);
        printf("\n");
        for (i = 0; i < 2; ++i) {
            ix = findMin(a, asize, &min);
            s[i] = min;
            for (j = ix+1; j < asize; ++j) a[j-1] = a[j];
            asize--;
        }
        sum = s[0] + s[1];
        printf("Two smallest: %d + %d = %d\n", s[0], s[1], sum);
        a[asize] = sum;
        asize++;
    }
    printf("Last item is %d.\n", a[0]);
    return 0;
}
Output:
List: 6 81 243 14 25 49 123 69 11 
Two smallest: 6 + 11 = 17
List: 81 243 14 25 49 123 69 17 
Two smallest: 14 + 17 = 31
List: 81 243 25 49 123 69 31 
Two smallest: 25 + 31 = 56
List: 81 243 49 123 69 56 
Two smallest: 49 + 56 = 105
List: 81 243 123 69 105 
Two smallest: 69 + 81 = 150
List: 243 123 105 150 
Two smallest: 105 + 123 = 228
List: 243 150 228 
Two smallest: 150 + 228 = 378
List: 243 378 
Two smallest: 243 + 378 = 621
Last item is 621.

Factor

Works with: Factor version 0.99 2021-06-02
USING: formatting io kernel math math.statistics prettyprint
sequences sequences.extras ;

: list. ( seq -- )
    "List: " write [ bl ] [ pprint ] interleave nl ;

: smallest. ( seq -- )
    first2 2dup + "Two smallest: %d + %d = %d\n" printf ;

: remove-all-first! ( seq elts -- seq' )
    [ swap remove-first! ] each ;

: step ( seq -- seq' )
    dup { 0 1 } kth-smallests tuck remove-all-first! over
    smallest. swap sum suffix! ;
    
V{ 6 81 243 14 25 49 123 69 11 }
[ dup length 1 > ] [ dup list. step ] while
last "Last item is %d.\n" printf
Output:
List: 6 81 243 14 25 49 123 69 11
Two smallest: 6 + 11 = 17
List: 81 243 14 25 49 123 69 17
Two smallest: 14 + 17 = 31
List: 81 243 25 49 123 69 31
Two smallest: 25 + 31 = 56
List: 81 243 49 123 69 56
Two smallest: 49 + 56 = 105
List: 81 243 123 69 105
Two smallest: 69 + 81 = 150
List: 243 123 105 150
Two smallest: 105 + 123 = 228
List: 243 150 228
Two smallest: 150 + 228 = 378
List: 243 378
Two smallest: 243 + 378 = 621
Last item is 621.

FreeBASIC

#define HUGE 99999999

redim as integer list(0 to 8)
list(0) = 6 :list(1) = 81:list(2) = 243:list(3) = 14:list(4) = 25:list(5) = 49:list(6) = 123:list(7) = 69:list(8) = 11 
dim as integer s, si, ss, ssi, i

for i = 0 to ubound(list)
    print list(i);" ";
next i
print
while ubound(list) > 0
    s = HUGE
    ss = HUGE
    for i = 0 to ubound(list)
        if list(i)<s then
            ss = s
            ssi = si
            s = list(i)
            si = i
        elseif list(i)<ss then
            ss = list(i)
            ssi = i
        end if
    next i
    if ss<s then swap s, ss
    for i = si to ubound(list)-1
        list(i) = list(i+1)
    next i
    for i = ssi to ubound(list)-2
        list(i) = list(i+1)
    next i
    list(ubound(list)-1) = s+ss
    redim preserve list(0 to ubound(list)-1)
    for i = 0 to ubound(list)
        print list(i);" ";
    next i
    print
wend
Output:

6  81  243  14  25  49  123  69  11 
81  243  14  25  49  123  69  17 
81  243  25  49  123  69  31 
81  243  49  123  69  56 
81  243  123  69  105 
243  123  105  150 
243  150  228 
243  378 
621

Go

Translation of: Wren

With sorting

package main

import (
    "fmt"
    "sort"
)

func main() {
    a := []int{6, 81, 243, 14, 25, 49, 123, 69, 11}
    for len(a) > 1 {
        sort.Ints(a)
        fmt.Println("Sorted list:", a)
        sum := a[0] + a[1]
        fmt.Printf("Two smallest: %d + %d = %d\n", a[0], a[1], sum)
        a = append(a, sum)
        a = a[2:]
    }
    fmt.Println("Last item is", a[0], "\b.")
}
Output:
Sorted list: [6 11 14 25 49 69 81 123 243]
Two smallest: 6 + 11 = 17
Sorted list: [14 17 25 49 69 81 123 243]
Two smallest: 14 + 17 = 31
Sorted list: [25 31 49 69 81 123 243]
Two smallest: 25 + 31 = 56
Sorted list: [49 56 69 81 123 243]
Two smallest: 49 + 56 = 105
Sorted list: [69 81 105 123 243]
Two smallest: 69 + 81 = 150
Sorted list: [105 123 150 243]
Two smallest: 105 + 123 = 228
Sorted list: [150 228 243]
Two smallest: 150 + 228 = 378
Sorted list: [243 378]
Two smallest: 243 + 378 = 621
Last item is 621.

Without sorting

package main

import "fmt"

func findMin(a []int) (int, int) {
    ix := 0
    min := a[0]
    for i := 1; i < len(a); i++ {
        if a[i] < min {
            ix = i
            min = a[i]
        }
    }
    return min, ix
}

func main() {
    a := []int{6, 81, 243, 14, 25, 49, 123, 69, 11}
    for len(a) > 1 {
        fmt.Println("List:", a)
        var s [2]int
        for i := 0; i < 2; i++ {
            min, ix := findMin(a)
            s[i] = min
            a = append(a[:ix], a[ix+1:]...)
        }
        sum := s[0] + s[1]
        fmt.Printf("Two smallest: %d + %d = %d\n", s[0], s[1], sum)
        a = append(a, sum)
    }
    fmt.Println("Last item is", a[0], "\b.")
}
Output:
List: [6 81 243 14 25 49 123 69 11]
Two smallest: 6 + 11 = 17
List: [81 243 14 25 49 123 69 17]
Two smallest: 14 + 17 = 31
List: [81 243 25 49 123 69 31]
Two smallest: 25 + 31 = 56
List: [81 243 49 123 69 56]
Two smallest: 49 + 56 = 105
List: [81 243 123 69 105]
Two smallest: 69 + 81 = 150
List: [243 123 105 150]
Two smallest: 105 + 123 = 228
List: [243 150 228]
Two smallest: 150 + 228 = 378
List: [243 378]
Two smallest: 243 + 378 = 621
Last item is 621.

J

Here we maintain the unsorted order in the intermediate steps:
   rplc&(' 0';'')"1": ((-. , +/@]) 2 {. /:~)^:a: 81 243 14 25 49 123 69 17
 81 243  14  25  49 123 69 17
 81 243  25  49 123  69 31   
 81 243  49 123  69  56      
 81 243 123  69 105          
243 123 105 150              
243 150 228                  
243 378                      
621
Alternative interpretation:
   ([[:echo,:#~1<#)@((-. , +/@]) 2 {. /:~)^:_] 81 243 14 25 49 123 69 17
81 243 25 49 123 69 31
81 243 49 123 69 56
81 243 123 69 105
243 123 105 150
243 150 228
243 378
621

jq

Adapted from Wren

Works with: jq

Works with gojq, the Go implementation of jq

With sorting

def task_with_sorting:
  foreach range(1; length) as $i ( {a: .};
      .a |= sort
      | .emit = "Sorted list: \(.a)\n"
      | (.a[0] + .a[1]) as $sum
      | .emit += "Two smallest: \(.a[0]) + \(.a[1]) = \($sum)"
      | .a += [$sum]
      | .a |= .[2:] ;
   .emit,
   (select(.a|length==1) | "Last item is \(.a[0]).") );

[6, 81, 243, 14, 25, 49, 123, 69, 11]
| task_with_sorting
Output:
Sorted list: [6,11,14,25,49,69,81,123,243]
Two smallest: 6 + 11 = 17
Sorted list: [14,17,25,49,69,81,123,243]
Two smallest: 14 + 17 = 31
Sorted list: [25,31,49,69,81,123,243]
Two smallest: 25 + 31 = 56
Sorted list: [49,56,69,81,123,243]
Two smallest: 49 + 56 = 105
Sorted list: [69,81,105,123,243]
Two smallest: 69 + 81 = 150
Sorted list: [105,123,150,243]
Two smallest: 105 + 123 = 228
Sorted list: [150,228,243]
Two smallest: 150 + 228 = 378
Sorted list: [243,378]
Two smallest: 243 + 378 = 621
Last item is 621.

Without sorting

def min_index:
  . as $in
  | reduce range(0; length) as $i ( null;
     $in[$i] as $x 
     | if . == null then {ix: 0, min: $x}
       elif $x < .min then {ix: $i, min: $x}
       else . end)
  | .ix;
   
def task_without_sorting:
  # Output: [min, remainder]
  def remove_min:
     min_index as $ix
     | [.[$ix], (.[:$ix] + .[$ix + 1:])];

  foreach range(1; length) as $i ( {a: .};
       .emit = "Unsorted list: \(.a)\n"
     | (.a | remove_min) as [$m1, $x]
     | ($x | remove_min) as [$m2, $y]
     | ($m1 + $m2) as $sum
     | .emit += "Two smallest: \($m1) + \($m2) = \($sum)"
     | .a = $y + [$sum] ;
     .emit,
   (select(.a|length==1) | "Last item is \(.a[0]).") );

[6, 81, 243, 14, 25, 49, 123, 69, 11]
| task_without_sorting
Output:
Unsorted list: [6,81,243,14,25,49,123,69,11]
Two smallest: 6 + 11 = 17
Unsorted list: [81,243,14,25,49,123,69,17]
Two smallest: 14 + 17 = 31
Unsorted list: [81,243,25,49,123,69,31]
Two smallest: 25 + 31 = 56
Unsorted list: [81,243,49,123,69,56]
Two smallest: 49 + 56 = 105
Unsorted list: [81,243,123,69,105]
Two smallest: 69 + 81 = 150
Unsorted list: [243,123,105,150]
Two smallest: 105 + 123 = 228
Unsorted list: [243,150,228]
Two smallest: 150 + 228 = 378
Unsorted list: [243,378]
Two smallest: 243 + 378 = 621
Last item is 621.

Julia

list = [6, 81, 243, 14, 25, 49, 123, 69, 11]

function addleastreduce!(lis)
    while length(lis) > 1
         push!(lis, popat!(lis, last(findmin(lis))) + popat!(lis, last(findmin(lis))))
         println("Interim list: $lis")
    end
    return lis
end

@show list, addleastreduce!(copy(list))
Output:
Interim list: [81, 243, 14, 25, 49, 123, 69, 17]
Interim list: [81, 243, 25, 49, 123, 69, 31]
Interim list: [81, 243, 49, 123, 69, 56]
Interim list: [81, 243, 123, 69, 105]
Interim list: [243, 123, 105, 150]
Interim list: [243, 150, 228]
Interim list: [243, 378]
Interim list: [621]
(list, addleastreduce!(copy(list))) = ([6, 81, 243, 14, 25, 49, 123, 69, 11], [621])

Mathematica/Wolfram Language

With and without sorting

list = {6, 81, 243, 14, 25, 49, 123, 69, 11};

Print[list]
Do[
 poss = Ordering[list, 2];
 extr = Extract[list, List /@ poss];
 extr //= Total;
 list = Delete[list, List /@ poss];
 AppendTo[list, extr];
 Print[list]
 ,
 {Length[list] - 1}
 ]
Output:
{6,81,243,14,25,49,123,69,11}
{81,243,14,25,49,123,69,17}
{81,243,25,49,123,69,31}
{81,243,49,123,69,56}
{81,243,123,69,105}
{243,123,105,150}
{243,150,228}
{243,378}
{621}

Nim

With sorting

We sort in descending order as it is more efficient.

# With sorting.
import algorithm, strformat

proc extractAndAddTwoSmallest(list: var seq[int]) =
  list.sort(Descending)
  stdout.write &"Descending sorted list: {list}"
  let min1 = list.pop()
  let min2 = list.pop()
  echo &"; two smallest: {min1} and {min2}; sum = {min1 + min2}"
  list.add min1 + min2

var list = @[6, 81, 243, 14, 25, 49, 123, 69, 11]

while list.len >= 2:
  list.extractAndAddTwoSmallest()
echo &"Last item is {list[0]}."
Output:
Descending sorted list: @[243, 123, 81, 69, 49, 25, 14, 11, 6]; two smallest: 6 and 11; sum = 17
Descending sorted list: @[243, 123, 81, 69, 49, 25, 17, 14]; two smallest: 14 and 17; sum = 31
Descending sorted list: @[243, 123, 81, 69, 49, 31, 25]; two smallest: 25 and 31; sum = 56
Descending sorted list: @[243, 123, 81, 69, 56, 49]; two smallest: 49 and 56; sum = 105
Descending sorted list: @[243, 123, 105, 81, 69]; two smallest: 69 and 81; sum = 150
Descending sorted list: @[243, 150, 123, 105]; two smallest: 105 and 123; sum = 228
Descending sorted list: @[243, 228, 150]; two smallest: 150 and 228; sum = 378
Descending sorted list: @[378, 243]; two smallest: 243 and 378; sum = 621
Last item is 621.

Without sorting

We could use the function minIndex from module sequtils but it would be less efficient. We used a single loop instead. Please note that we remove the elements using function del which is O(1) rather than function delete which is O(n).

# Without sorting.
import strformat

proc extractAndAddTwoSmallest(list: var seq[int]) =
  var min1, min2 = int.high
  var imin1, imin2 = -1
  for i, val in list:
    if val < min1:
      min2 = min1
      min1 = val
      imin2 = imin1
      imin1 = i
    elif val < min2:
      min2 = val
      imin2 = i
  echo &"List: {list}; two smallest: {min1}@{imin1} and {min2}@{imin2}; sum = {min1 + min2}"
  if imin1 > imin2: swap imin1, imin2   # Make sure "imin2" is the greatest index.
  list.del imin2
  list.del imin1
  list.add min1 + min2

var list = @[6, 81, 243, 14, 25, 49, 123, 69, 11]

while list.len >= 2:
  list.extractAndAddTwoSmallest()
echo &"Last item is {list[0]}."
Output:
List: @[6, 81, 243, 14, 25, 49, 123, 69, 11]; two smallest: 6@0 and 11@8; sum = 17
List: @[69, 81, 243, 14, 25, 49, 123, 17]; two smallest: 14@3 and 17@7; sum = 31
List: @[69, 81, 243, 123, 25, 49, 31]; two smallest: 25@4 and 31@6; sum = 56
List: @[69, 81, 243, 123, 49, 56]; two smallest: 49@4 and 56@5; sum = 105
List: @[69, 81, 243, 123, 105]; two smallest: 69@0 and 81@1; sum = 150
List: @[123, 105, 243, 150]; two smallest: 105@1 and 123@0; sum = 228
List: @[243, 150, 228]; two smallest: 150@1 and 228@2; sum = 378
List: @[243, 378]; two smallest: 243@0 and 378@1; sum = 621
Last item is 621.

Perl

Translation of: Raku
use strict;
use warnings;
use feature 'say';
use List::AllUtils <min firstidx>;

my @list = <6 81 243 14 25 49 123 69 11>;
say " Original @list";
push @list, get(min @list) + get(min @list) and say "@list" while @list > 1;

sub get {
    my($min) = @_;
    splice @list, (firstidx { $min == $_ } @list), 1;
    printf " %3d ", $min;
    $min;
}
Output:
 Original 6 81 243 14 25 49 123 69 11
   6   11 81 243 14 25 49 123 69 17
  14   17 81 243 25 49 123 69 31
  25   31 81 243 49 123 69 56
  49   56 81 243 123 69 105
  69   81 243 123 105 150
 105  123 243 150 228
 150  228 243 378
 243  378 621


Phix

With sorting

with javascript_semantics
sequence list = {6, 81, 243, 14, 25, 49, 123, 69, 11}
while length(list)>1 do
    list = sort(deep_copy(list))
    integer l1 = list[1], l2 = list[2], l12 = l1+l2
    printf(1,"Sorted list: %V, two smallest: %d + %d = %d\n",{list,l1,l2,l12})
    list[1..2] = {l12}
end while
printf(1,"Last item is %d\n",list)
Output:
Sorted list: {6,11,14,25,49,69,81,123,243}, two smallest: 6 + 11 = 17
Sorted list: {14,17,25,49,69,81,123,243}, two smallest: 14 + 17 = 31
Sorted list: {25,31,49,69,81,123,243}, two smallest: 25 + 31 = 56
Sorted list: {49,56,69,81,123,243}, two smallest: 49 + 56 = 105
Sorted list: {69,81,105,123,243}, two smallest: 69 + 81 = 150
Sorted list: {105,123,150,243}, two smallest: 105 + 123 = 228
Sorted list: {150,228,243}, two smallest: 150 + 228 = 378
Sorted list: {243,378}, two smallest: 243 + 378 = 621
Last item is 621

Without sorting

with javascript_semantics
sequence list = {6, 81, 243, 14, 25, 49, 123, 69, 11}
while length(list)>1 do
    printf(1,"List: %V, two smallest: ",{list})
    integer l1, l2 = 0
    for i=1 to 2 do
        l1 = l2
        integer ldx = smallest(list,true)
        l2 = list[ldx]
        list = list[1..ldx-1] & list[ldx+1..$]
    end for
    printf(1,"%d + %d = %d\n",{l1,l2,l1+l2})
    list &= l1+l2
end while
printf(1,"Last item is %d\n",list)
Output:
List: {6,81,243,14,25,49,123,69,11}, two smallest: 6 + 11 = 17
List: {81,243,14,25,49,123,69,17}, two smallest: 14 + 17 = 31
List: {81,243,25,49,123,69,31}, two smallest: 25 + 31 = 56
List: {81,243,49,123,69,56}, two smallest: 49 + 56 = 105
List: {81,243,123,69,105}, two smallest: 69 + 81 = 150
List: {243,123,105,150}, two smallest: 105 + 123 = 228
List: {243,150,228}, two smallest: 150 + 228 = 378
List: {243,378}, two smallest: 243 + 378 = 621
Last item is 621

Python

Translation of: Julia
""" Rosetta code task: Last list item """

def add_least_reduce(lis):
    """ Reduce lis: sum least two elements adding sum to list. Will take len(list) - 1 steps """
    while len(lis) > 1:
        lis.append(lis.pop(lis.index(min(lis))) + lis.pop(lis.index(min(lis))))
        print('Interim list:', lis)
    return lis

LIST = [6, 81, 243, 14, 25, 49, 123, 69, 11]

print(LIST, ' ==> ', add_least_reduce(LIST.copy()))
Output:
Interim list: [81, 243, 14, 25, 49, 123, 69, 17]
Interim list: [81, 243, 25, 49, 123, 69, 31]
Interim list: [81, 243, 49, 123, 69, 56]
Interim list: [81, 243, 123, 69, 105]
Interim list: [243, 123, 105, 150]
Interim list: [243, 150, 228]
Interim list: [243, 378]
Interim list: [621]
[6, 81, 243, 14, 25, 49, 123, 69, 11]  ==>  [621]

Raku

Uses no sorting; does not modify overall list order while processing.

say ' Original ', my @list = 6, 81, 243, 14, 25, 49, 123, 69, 11;

say push @list: get(min @list) + get(min @list) while @list > 1;

sub get ($min) {
    @list.splice: @list.first(* == $min, :k), 1;
    printf " %3d ", $min;
    $min;
}
Output:
 Original [6 81 243 14 25 49 123 69 11]
   6   11 [81 243 14 25 49 123 69 17]
  14   17 [81 243 25 49 123 69 31]
  25   31 [81 243 49 123 69 56]
  49   56 [81 243 123 69 105]
  69   81 [243 123 105 150]
 105  123 [243 150 228]
 150  228 [243 378]
 243  378 [621]

REXX

With sorting

/* REXX */
List = '6 81 243 14 25 49 123 69 11'
Do Until words(list)=1
  list=wordsort(list)
  Say 'Sorted list:' list
  Parse Var list a b c
  Say 'Two smallest:' a '+' b '=' (a+b)
  list=(a+b) c
  End
Say 'Last item:' list
Exit
wordsort: Procedure
/**********************************************************************
* Sort the list of words supplied as argument. Return the sorted list
**********************************************************************/
  Parse Arg wl
  wa.=''
  wa.0=0
  Do While wl<>''
    Parse Var wl w wl
    Do i=1 To wa.0
      If wa.i>w Then Leave
      End
    If i<=wa.0 Then Do
      Do j=wa.0 To i By -1
        ii=j+1
        wa.ii=wa.j
        End
      End
    wa.i=w
    wa.0=wa.0+1
    End
  swl=''
  Do i=1 To wa.0
    swl=swl wa.i
    End
  Return strip(swl)
Output:
Sorted list: 6 11 14 25 49 69 81 123 243
Two smallest: 6 + 11 = 17
Sorted list: 14 17 25 49 69 81 123 243
Two smallest: 14 + 17 = 31
Sorted list: 25 31 49 69 81 123 243
Two smallest: 25 + 31 = 56
Sorted list: 49 56 69 81 123 243
Two smallest: 49 + 56 = 105
Sorted list: 69 81 105 123 243
Two smallest: 69 + 81 = 150
Sorted list: 105 123 150 243
Two smallest: 105 + 123 = 228
Sorted list: 150 228 243
Two smallest: 150 + 228 = 378
Sorted list: 243 378
Two smallest: 243 + 378 = 621
Last item: 621

Without sorting

/* REXX */
List = '6 81 243 14 25 49 123 69 11'
Do Until words(list)=1
  Say 'List:' list
  a=getmin()
  b=getmin()
  Say 'Two smallest:' a '+' b '=' (a+b)
  list=list (a+b)
  End
Say 'Last item:' list
Exit

getmin: Procedure Expose list
/* Return the smallest element of list and remove it from list */
min=1e9
Do i=1 To words(list)
  If word(list,i)<min Then Do
    m=word(list,i)
    min=m
    j=i
    End
  End
list=subword(list,1,j-1) subword(list,j+1)
Return m
Output:
List: 6 81 243 14 25 49 123 69 11
Two smallest: 6 + 11 = 17
List: 81 243 14 25 49 123 69  17
Two smallest: 14 + 17 = 31
List: 81 243 25 49 123 69  31
Two smallest: 25 + 31 = 56
List: 81 243 49 123 69  56
Two smallest: 49 + 56 = 105
List: 81 243 123 69  105
Two smallest: 69 + 81 = 150
List:  243 123 105 150
Two smallest: 105 + 123 = 228
List: 243 150 228
Two smallest: 150 + 228 = 378
List: 243  378
Two smallest: 243 + 378 = 621
Last item:   621


Ring

With sorting

see "working..." + nl

List = [6,81,243,14,25,49,123,69,11]
n = 0

while true 
      n++
      List = sort(List)
      first = List[1]
      second = List[2]
      ind1 = find(List,first)
      ind2 = find(List,second)
      if ind1 < ind2
         del(List,ind2)
         del(List,ind1)
      else
         del(List,ind1)
         del(List,ind2)
      ok
      sum = first + second 
      add(List,sum)
      if len(List) = 1
         exit
      ok
      if n = 1
         see nl + "List = "
         showArray(List)
         see nl
      ok
      showList(first,second,sum,List)
end

see "Last item is: " +List[1] + nl
see "done..." + nl

func showList(first,second,sum,List)
     see "two smallest is = " + first + " + " + second + " = " + sum + nl
     see "List = "
     showArray(List)     

func showArray(array)
     txt = ""
     see "["
     for n = 1 to len(array)
         txt = txt + array[n] + ","
     next
     txt = left(txt,len(txt)-1)
     txt = txt + "]"
     see txt + nl
Output:
working...

List = [14,25,49,69,81,123,243,17]

two smallest is = 6 + 11 = 17
List = [14,25,49,69,81,123,243,17]
two smallest is = 14 + 17 = 31
List = [25,49,69,81,123,243,31]
two smallest is = 25 + 31 = 56
List = [49,69,81,123,243,56]
two smallest is = 49 + 56 = 105
List = [69,81,123,243,105]
two smallest is = 69 + 81 = 150
List = [105,123,243,150]
two smallest is = 105 + 123 = 228
List = [150,243,228]
two smallest is = 150 + 228 = 378
List = [243,378]
Last item is: 621
done...

Without sorting

see "working..." + nl
 
List = [6,81,243,14,25,49,123,69,11]
n = 0
 
while true 
      n++
      if n = 1
         see nl + "List = "
         showArray(List)
         see nl
      ok
      first = min(List)
      ind1 = find(List,first)
      del(List,ind1)
      second = min(List)
      ind2 = find(List,second)
      del(List,ind2)
      sum = first + second 
      add(List,sum)
      if len(List) = 1
         exit
      ok
      showList(first,second,sum,List)
end
 
see "Last item is: " +List[1] + nl
see "done..." + nl
 
func showList(first,second,sum,List)
     see "two smallest is = " + first + " + " + second + " = " + sum + nl
     see "List = "
     showArray(List)     
 
func showArray(array)
     txt = ""
     see "["
     for n = 1 to len(array)
         txt = txt + array[n] + ","
     next
     txt = left(txt,len(txt)-1)
     txt = txt + "]"
     see txt + nl
Output:
working...

List = [6,81,243,14,25,49,123,69,11]

two smallest is = 6 + 11 = 17
List = [81,243,14,25,49,123,69,17]
two smallest is = 14 + 17 = 31
List = [81,243,25,49,123,69,31]
two smallest is = 25 + 31 = 56
List = [81,243,49,123,69,56]
two smallest is = 49 + 56 = 105
List = [81,243,123,69,105]
two smallest is = 69 + 81 = 150
List = [243,123,105,150]
two smallest is = 105 + 123 = 228
List = [243,150,228]
two smallest is = 150 + 228 = 378
List = [243,378]
Last item is: 621
done...

Sidef

With sorting

var k = 2
var list = [6, 81, 243, 14, 25, 49, 123, 69, 11]

while (list.len >= k) {
    var a = list.sort!.shift(k)
    list << a.sum
    say "#{a.map{'%3s' % _}.join(' ')} : #{list}"
}
Output:
  6  11 : [14, 25, 49, 69, 81, 123, 243, 17]
 14  17 : [25, 49, 69, 81, 123, 243, 31]
 25  31 : [49, 69, 81, 123, 243, 56]
 49  56 : [69, 81, 123, 243, 105]
 69  81 : [105, 123, 243, 150]
105 123 : [150, 243, 228]
150 228 : [243, 378]
243 378 : [621]

Without sorting

var k = 2
var list = [6, 81, 243, 14, 25, 49, 123, 69, 11]

while (list.len >= k) {

    var a = gather {
        k.times {
            take(var v = list.min)
            list.delete_first(v)
        }
    }

    list << a.sum
    say "#{a.map{'%3s' % _}.join(' ')} : #{list}"
}
Output:
  6  11 : [81, 243, 14, 25, 49, 123, 69, 17]
 14  17 : [81, 243, 25, 49, 123, 69, 31]
 25  31 : [81, 243, 49, 123, 69, 56]
 49  56 : [81, 243, 123, 69, 105]
 69  81 : [243, 123, 105, 150]
105 123 : [243, 150, 228]
150 228 : [243, 378]
243 378 : [621]

Vlang

Translation of: Go

With sorting

fn main() {
    mut a := [6, 81, 243, 14, 25, 49, 123, 69, 11]
    for a.len > 1 {
        a.sort()
        println("Sorted list: $a")
        sum := a[0] + a[1]
        println("Two smallest: ${a[0]} + ${a[1]} = $sum")
        a << sum
        a = a[2..]
    }
    println("Last item is ${a[0]}.")
}
Output:
Sorted list: [6, 11, 14, 25, 49, 69, 81, 123, 243]
Two smallest: 6 + 11 = 17
Sorted list: [14, 17, 25, 49, 69, 81, 123, 243]
Two smallest: 14 + 17 = 31
Sorted list: [25, 31, 49, 69, 81, 123, 243]
Two smallest: 25 + 31 = 56
Sorted list: [49, 56, 69, 81 123, 243]
Two smallest: 49 + 56 = 105
Sorted list: [69, 81, 105, 123, 243]
Two smallest: 69 + 81 = 150
Sorted list: [105, 123, 150, 243]
Two smallest: 105 + 123, = 228
Sorted list: [150, 228, 243]
Two smallest: 150 + 228, = 378
Sorted list: [243, 378]
Two smallest: 243 + 378 = 621
Last item is 621.

Without sorting

fn find_min(a []int) (int, int) {
    mut ix := 0
    mut min := a[0]
    for i in 1..a.len {
        if a[i] < min {
            ix = i
            min = a[i]
        }
    }
    return min, ix
}
fn main() {
    mut a := [6, 81, 243, 14, 25, 49, 123, 69, 11]
    for a.len > 1 {
        println("List: $a")
        mut s := [2]int{}
        for i in 0..2 {
            min, ix := find_min(a)
            s[i] = min
            a.delete(ix)
        }
        sum := s[0] + s[1]
        println("Two smallest: ${s[0]} + ${s[1]} = $sum")
        a << sum
    }
    println("Last item is ${a[0]}.")
}
Output:
List: [6, 81, 243, 14, 25, 49, 123, 69, 11]
Two smallest: 6 + 11 = 17
List: [81, 243, 14, 25, 49, 123, 69, 17]
Two smallest: 14 + 17 = 31
List: [81, 243, 25, 49, 123, 69, 31]
Two smallest: 25 + 31 = 56
List: [81, 243, 49, 123, 69, 56]
Two smallest: 49 + 56 = 105
List: [81, 243, 123, 69, 105]
Two smallest: 69 + 81 = 150
List: [24,3 123, 105, 150]
Two smallest: 105 + 123 = 228
List: [243, 150, 228]
Two smallest: 150 + 228 = 378
List: [243, 378]
Two smallest: 243 + 378 = 621
Last item is 621.

Wren

With sorting

var a = [6, 81, 243, 14, 25, 49, 123, 69, 11]

while (a.count > 1) {
    a.sort()
    System.print("Sorted list: %(a)")
    var sum = a[0] + a[1]
    System.print("Two smallest: %(a[0]) + %(a[1]) = %(sum)")
    a.add(sum)
    a = a[2..-1]
}

System.print("Last item is %(a[0]).")
Output:
Sorted list: [6, 11, 14, 25, 49, 69, 81, 123, 243]
Two smallest: 6 + 11 = 17
Sorted list: [14, 17, 25, 49, 69, 81, 123, 243]
Two smallest: 14 + 17 = 31
Sorted list: [25, 31, 49, 69, 81, 123, 243]
Two smallest: 25 + 31 = 56
Sorted list: [49, 56, 69, 81, 123, 243]
Two smallest: 49 + 56 = 105
Sorted list: [69, 81, 105, 123, 243]
Two smallest: 69 + 81 = 150
Sorted list: [105, 123, 150, 243]
Two smallest: 105 + 123 = 228
Sorted list: [150, 228, 243]
Two smallest: 150 + 228 = 378
Sorted list: [243, 378]
Two smallest: 243 + 378 = 621
Last item is 621.

Without sorting

var findMin = Fn.new { |a|
    var ix = 0
    var min = a[0]
    for (i in 1...a.count) {
        if (a[i] < min) {
            ix = i
            min = a[i]
        }
    }
    return [min, ix]
}

var a = [6, 81, 243, 14, 25, 49, 123, 69, 11]

while (a.count > 1) {
    System.print("List: %(a)")
    var s = List.filled(2, 0)
    for (i in 0..1) {
        var m = findMin.call(a)
        s[i] = m[0]
        a.removeAt(m[1])
    }
    var sum = s[0] + s[1]
    System.print("Two smallest: %(s[0]) + %(s[1]) = %(sum)")
    a.add(sum)
}

System.print("Last item is %(a[0]).")
Output:
List: [6, 81, 243, 14, 25, 49, 123, 69, 11]
Two smallest: 6 + 11 = 17
List: [81, 243, 14, 25, 49, 123, 69, 17]
Two smallest: 14 + 17 = 31
List: [81, 243, 25, 49, 123, 69, 31]
Two smallest: 25 + 31 = 56
List: [81, 243, 49, 123, 69, 56]
Two smallest: 49 + 56 = 105
List: [81, 243, 123, 69, 105]
Two smallest: 69 + 81 = 150
List: [243, 123, 105, 150]
Two smallest: 105 + 123 = 228
List: [243, 150, 228]
Two smallest: 150 + 228 = 378
List: [243, 378]
Two smallest: 243 + 378 = 621
Last item is 621.

XPL0

int List, End, Sum, Item, Smallest, I, SI;
[List:= [6, 81, 243, 14, 25, 49, 123, 69, 11];
End:= 8;                                \last index
loop    [for I:= 0 to End do            \show List
            [IntOut(0, List(I));  ChOut(0, ^ )];
        CrLf(0);
        if End = 0 then quit;           \done when only one element
        Sum:= 0;                        \find two smallest items
        for Item:= 1 to 2 do
            [Smallest:= -1>>1;
            for I:= 0 to End do
                if List(I) < Smallest then
                    [Smallest:= List(I);  SI:= I];
            Sum:= Sum + Smallest;       \add them
            for I:= SI to End-1 do      \remove them
                List(I):= List(I+1);
            End:= End-1;
            ];
        End:= End+1;                    \insert Sum at End
        List(End):= Sum;
        ];
]
Output:
6 81 243 14 25 49 123 69 11 
81 243 14 25 49 123 69 17 
81 243 25 49 123 69 31 
81 243 49 123 69 56 
81 243 123 69 105 
243 123 105 150 
243 150 228 
243 378 
621