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How to add just the second elements in lists of pairs?


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4














$begingroup$


Say I have two lists of pairs



a = {{1, 2}, {3, 4}};
b = {{5, 6}, {7, 8}};


and I want to add the 2nd element of every pair in b to the 2nd element of every pair in a, so the expected result would be {{1, 8}, {3, 12}}.



How can I do this?



I am a total beginner with Mathematica, so sorry if this is to basic :)










share|improve this question









New contributor



Jim is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.






$endgroup$























    4














    $begingroup$


    Say I have two lists of pairs



    a = {{1, 2}, {3, 4}};
    b = {{5, 6}, {7, 8}};


    and I want to add the 2nd element of every pair in b to the 2nd element of every pair in a, so the expected result would be {{1, 8}, {3, 12}}.



    How can I do this?



    I am a total beginner with Mathematica, so sorry if this is to basic :)










    share|improve this question









    New contributor



    Jim is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
    Check out our Code of Conduct.






    $endgroup$



















      4












      4








      4





      $begingroup$


      Say I have two lists of pairs



      a = {{1, 2}, {3, 4}};
      b = {{5, 6}, {7, 8}};


      and I want to add the 2nd element of every pair in b to the 2nd element of every pair in a, so the expected result would be {{1, 8}, {3, 12}}.



      How can I do this?



      I am a total beginner with Mathematica, so sorry if this is to basic :)










      share|improve this question









      New contributor



      Jim is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
      Check out our Code of Conduct.






      $endgroup$




      Say I have two lists of pairs



      a = {{1, 2}, {3, 4}};
      b = {{5, 6}, {7, 8}};


      and I want to add the 2nd element of every pair in b to the 2nd element of every pair in a, so the expected result would be {{1, 8}, {3, 12}}.



      How can I do this?



      I am a total beginner with Mathematica, so sorry if this is to basic :)







      list-manipulation






      share|improve this question









      New contributor



      Jim is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
      Check out our Code of Conduct.










      share|improve this question









      New contributor



      Jim is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
      Check out our Code of Conduct.








      share|improve this question




      share|improve this question



      share|improve this question








      edited 4 hours ago









      m_goldberg

      91.9k8 gold badges75 silver badges210 bronze badges




      91.9k8 gold badges75 silver badges210 bronze badges






      New contributor



      Jim is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
      Check out our Code of Conduct.








      asked 8 hours ago









      JimJim

      213 bronze badges




      213 bronze badges




      New contributor



      Jim is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
      Check out our Code of Conduct.




      New contributor




      Jim is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
      Check out our Code of Conduct.



























          6 Answers
          6






          active

          oldest

          votes


















          1
















          $begingroup$

          lst1 = {{1, 2}, {2, 4}};

          lst1a = lst1;
          lst1a[[All, 2]] *= 2;
          lst1a



          {{1, 4}, {2, 8}}




          lst2 = {{3, 5}, {10, 2}};

          lst1b = lst1;
          lst1b[[All, 2]] = lst1[[All, 2]] + lst2[[All, 2]];
          lst1b



          {{1, 7}, {2, 6}}







          share|improve this answer










          $endgroup$























            1
















            $begingroup$

            Try this:



            {{1, 2}, {2, 4}} + ({{1, 2}, {2, 4}} /. {x_,y _} -> {0, y})

            (* {{2, 4}, {4, 8}} *)


            or this



            (# + (# /. {x_, y_} -> {0, y})) &[{{1, 2}, {2, 4}}]

            (* {{1, 2}, {4, 8}} *)


            Have fun!






            share|improve this answer










            $endgroup$























              1
















              $begingroup$

              list = {{1, 2}, {2, 4}};

              MapAt[2 # &, {All, 2}] @ list



              {{1, 4}, {2, 8}}







              share|improve this answer










              $endgroup$















              • $begingroup$
                Hi @eldo! It's been a while... :)
                $endgroup$
                – Michael E2
                7 hours ago










              • $begingroup$
                Yes, nice to see you again :}
                $endgroup$
                – eldo
                7 hours ago



















              1
















              $begingroup$

              lst = {{1, 2}, {2, 4}};
              MapThread[{First[#1], Last[#1] + Last[#2]} &, {lst, lst}]



              {{1, 4}, {2, 8}}







              share|improve this answer










              $endgroup$























                1
















                $begingroup$

                I like to do this kind of thing by writing a custom function to perform the the desired operation on two given pairs. I then map this function over the two lists with MapThread. In most cases, by using Mathematica's argument-pattern destructuring, the custom function can be expressed extremely simply. This is one of those cases.



                The custom function is



                f[{x_, y_}, {_, z_}] := {x, y + z}


                Note that the righthand side is a literal expression of the desired result.



                Now let's contrive some test data. I write the generator so that two lists of pairs can easily be given any length.



                With[{n = 3}, {a, b} = Partition[Partition[Range[4 n], 2], n]];
                a
                b



                {{1, 2}, {3, 4}, {5, 6}}
                {{7, 8}, {9, 10}, {11, 12}}



                Now we MapThread the function f over a and b.



                MapThread[f, {a, b}]


                {1, 10}, {3, 14}, {5, 18}}






                share|improve this answer












                $endgroup$























                  0
















                  $begingroup$

                  list1 = {{1, 2}, {2, 4}};


                  Adding the second elements from each pair of the same list:



                  {#[[1]], 2 #[[2]]} & /@ list1



                  {{1, 4}, {2, 8}}




                  Adding the second element from a different list:



                  list2 = {{a, b}, {c, d}};
                  {#[[1, 1]], #[[1, 2]] + #[[2, 2]]} & /@ Transpose[{list1, list2}]



                  {{1, 2 + b}, {2, 4 + d}}







                  share|improve this answer










                  $endgroup$

















                    Your Answer








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                    6 Answers
                    6






                    active

                    oldest

                    votes








                    6 Answers
                    6






                    active

                    oldest

                    votes









                    active

                    oldest

                    votes






                    active

                    oldest

                    votes









                    1
















                    $begingroup$

                    lst1 = {{1, 2}, {2, 4}};

                    lst1a = lst1;
                    lst1a[[All, 2]] *= 2;
                    lst1a



                    {{1, 4}, {2, 8}}




                    lst2 = {{3, 5}, {10, 2}};

                    lst1b = lst1;
                    lst1b[[All, 2]] = lst1[[All, 2]] + lst2[[All, 2]];
                    lst1b



                    {{1, 7}, {2, 6}}







                    share|improve this answer










                    $endgroup$




















                      1
















                      $begingroup$

                      lst1 = {{1, 2}, {2, 4}};

                      lst1a = lst1;
                      lst1a[[All, 2]] *= 2;
                      lst1a



                      {{1, 4}, {2, 8}}




                      lst2 = {{3, 5}, {10, 2}};

                      lst1b = lst1;
                      lst1b[[All, 2]] = lst1[[All, 2]] + lst2[[All, 2]];
                      lst1b



                      {{1, 7}, {2, 6}}







                      share|improve this answer










                      $endgroup$


















                        1














                        1










                        1







                        $begingroup$

                        lst1 = {{1, 2}, {2, 4}};

                        lst1a = lst1;
                        lst1a[[All, 2]] *= 2;
                        lst1a



                        {{1, 4}, {2, 8}}




                        lst2 = {{3, 5}, {10, 2}};

                        lst1b = lst1;
                        lst1b[[All, 2]] = lst1[[All, 2]] + lst2[[All, 2]];
                        lst1b



                        {{1, 7}, {2, 6}}







                        share|improve this answer










                        $endgroup$



                        lst1 = {{1, 2}, {2, 4}};

                        lst1a = lst1;
                        lst1a[[All, 2]] *= 2;
                        lst1a



                        {{1, 4}, {2, 8}}




                        lst2 = {{3, 5}, {10, 2}};

                        lst1b = lst1;
                        lst1b[[All, 2]] = lst1[[All, 2]] + lst2[[All, 2]];
                        lst1b



                        {{1, 7}, {2, 6}}








                        share|improve this answer













                        share|improve this answer




                        share|improve this answer



                        share|improve this answer










                        answered 8 hours ago









                        kglrkglr

                        220k10 gold badges250 silver badges504 bronze badges




                        220k10 gold badges250 silver badges504 bronze badges




























                            1
















                            $begingroup$

                            Try this:



                            {{1, 2}, {2, 4}} + ({{1, 2}, {2, 4}} /. {x_,y _} -> {0, y})

                            (* {{2, 4}, {4, 8}} *)


                            or this



                            (# + (# /. {x_, y_} -> {0, y})) &[{{1, 2}, {2, 4}}]

                            (* {{1, 2}, {4, 8}} *)


                            Have fun!






                            share|improve this answer










                            $endgroup$




















                              1
















                              $begingroup$

                              Try this:



                              {{1, 2}, {2, 4}} + ({{1, 2}, {2, 4}} /. {x_,y _} -> {0, y})

                              (* {{2, 4}, {4, 8}} *)


                              or this



                              (# + (# /. {x_, y_} -> {0, y})) &[{{1, 2}, {2, 4}}]

                              (* {{1, 2}, {4, 8}} *)


                              Have fun!






                              share|improve this answer










                              $endgroup$


















                                1














                                1










                                1







                                $begingroup$

                                Try this:



                                {{1, 2}, {2, 4}} + ({{1, 2}, {2, 4}} /. {x_,y _} -> {0, y})

                                (* {{2, 4}, {4, 8}} *)


                                or this



                                (# + (# /. {x_, y_} -> {0, y})) &[{{1, 2}, {2, 4}}]

                                (* {{1, 2}, {4, 8}} *)


                                Have fun!






                                share|improve this answer










                                $endgroup$



                                Try this:



                                {{1, 2}, {2, 4}} + ({{1, 2}, {2, 4}} /. {x_,y _} -> {0, y})

                                (* {{2, 4}, {4, 8}} *)


                                or this



                                (# + (# /. {x_, y_} -> {0, y})) &[{{1, 2}, {2, 4}}]

                                (* {{1, 2}, {4, 8}} *)


                                Have fun!







                                share|improve this answer













                                share|improve this answer




                                share|improve this answer



                                share|improve this answer










                                answered 7 hours ago









                                Alexei BoulbitchAlexei Boulbitch

                                23k27 silver badges78 bronze badges




                                23k27 silver badges78 bronze badges


























                                    1
















                                    $begingroup$

                                    list = {{1, 2}, {2, 4}};

                                    MapAt[2 # &, {All, 2}] @ list



                                    {{1, 4}, {2, 8}}







                                    share|improve this answer










                                    $endgroup$















                                    • $begingroup$
                                      Hi @eldo! It's been a while... :)
                                      $endgroup$
                                      – Michael E2
                                      7 hours ago










                                    • $begingroup$
                                      Yes, nice to see you again :}
                                      $endgroup$
                                      – eldo
                                      7 hours ago
















                                    1
















                                    $begingroup$

                                    list = {{1, 2}, {2, 4}};

                                    MapAt[2 # &, {All, 2}] @ list



                                    {{1, 4}, {2, 8}}







                                    share|improve this answer










                                    $endgroup$















                                    • $begingroup$
                                      Hi @eldo! It's been a while... :)
                                      $endgroup$
                                      – Michael E2
                                      7 hours ago










                                    • $begingroup$
                                      Yes, nice to see you again :}
                                      $endgroup$
                                      – eldo
                                      7 hours ago














                                    1














                                    1










                                    1







                                    $begingroup$

                                    list = {{1, 2}, {2, 4}};

                                    MapAt[2 # &, {All, 2}] @ list



                                    {{1, 4}, {2, 8}}







                                    share|improve this answer










                                    $endgroup$



                                    list = {{1, 2}, {2, 4}};

                                    MapAt[2 # &, {All, 2}] @ list



                                    {{1, 4}, {2, 8}}








                                    share|improve this answer













                                    share|improve this answer




                                    share|improve this answer



                                    share|improve this answer










                                    answered 7 hours ago









                                    eldoeldo

                                    30.4k2 gold badges28 silver badges85 bronze badges




                                    30.4k2 gold badges28 silver badges85 bronze badges















                                    • $begingroup$
                                      Hi @eldo! It's been a while... :)
                                      $endgroup$
                                      – Michael E2
                                      7 hours ago










                                    • $begingroup$
                                      Yes, nice to see you again :}
                                      $endgroup$
                                      – eldo
                                      7 hours ago


















                                    • $begingroup$
                                      Hi @eldo! It's been a while... :)
                                      $endgroup$
                                      – Michael E2
                                      7 hours ago










                                    • $begingroup$
                                      Yes, nice to see you again :}
                                      $endgroup$
                                      – eldo
                                      7 hours ago
















                                    $begingroup$
                                    Hi @eldo! It's been a while... :)
                                    $endgroup$
                                    – Michael E2
                                    7 hours ago




                                    $begingroup$
                                    Hi @eldo! It's been a while... :)
                                    $endgroup$
                                    – Michael E2
                                    7 hours ago












                                    $begingroup$
                                    Yes, nice to see you again :}
                                    $endgroup$
                                    – eldo
                                    7 hours ago




                                    $begingroup$
                                    Yes, nice to see you again :}
                                    $endgroup$
                                    – eldo
                                    7 hours ago











                                    1
















                                    $begingroup$

                                    lst = {{1, 2}, {2, 4}};
                                    MapThread[{First[#1], Last[#1] + Last[#2]} &, {lst, lst}]



                                    {{1, 4}, {2, 8}}







                                    share|improve this answer










                                    $endgroup$




















                                      1
















                                      $begingroup$

                                      lst = {{1, 2}, {2, 4}};
                                      MapThread[{First[#1], Last[#1] + Last[#2]} &, {lst, lst}]



                                      {{1, 4}, {2, 8}}







                                      share|improve this answer










                                      $endgroup$


















                                        1














                                        1










                                        1







                                        $begingroup$

                                        lst = {{1, 2}, {2, 4}};
                                        MapThread[{First[#1], Last[#1] + Last[#2]} &, {lst, lst}]



                                        {{1, 4}, {2, 8}}







                                        share|improve this answer










                                        $endgroup$



                                        lst = {{1, 2}, {2, 4}};
                                        MapThread[{First[#1], Last[#1] + Last[#2]} &, {lst, lst}]



                                        {{1, 4}, {2, 8}}








                                        share|improve this answer













                                        share|improve this answer




                                        share|improve this answer



                                        share|improve this answer










                                        answered 7 hours ago









                                        Suba ThomasSuba Thomas

                                        4,12611 silver badges20 bronze badges




                                        4,12611 silver badges20 bronze badges


























                                            1
















                                            $begingroup$

                                            I like to do this kind of thing by writing a custom function to perform the the desired operation on two given pairs. I then map this function over the two lists with MapThread. In most cases, by using Mathematica's argument-pattern destructuring, the custom function can be expressed extremely simply. This is one of those cases.



                                            The custom function is



                                            f[{x_, y_}, {_, z_}] := {x, y + z}


                                            Note that the righthand side is a literal expression of the desired result.



                                            Now let's contrive some test data. I write the generator so that two lists of pairs can easily be given any length.



                                            With[{n = 3}, {a, b} = Partition[Partition[Range[4 n], 2], n]];
                                            a
                                            b



                                            {{1, 2}, {3, 4}, {5, 6}}
                                            {{7, 8}, {9, 10}, {11, 12}}



                                            Now we MapThread the function f over a and b.



                                            MapThread[f, {a, b}]


                                            {1, 10}, {3, 14}, {5, 18}}






                                            share|improve this answer












                                            $endgroup$




















                                              1
















                                              $begingroup$

                                              I like to do this kind of thing by writing a custom function to perform the the desired operation on two given pairs. I then map this function over the two lists with MapThread. In most cases, by using Mathematica's argument-pattern destructuring, the custom function can be expressed extremely simply. This is one of those cases.



                                              The custom function is



                                              f[{x_, y_}, {_, z_}] := {x, y + z}


                                              Note that the righthand side is a literal expression of the desired result.



                                              Now let's contrive some test data. I write the generator so that two lists of pairs can easily be given any length.



                                              With[{n = 3}, {a, b} = Partition[Partition[Range[4 n], 2], n]];
                                              a
                                              b



                                              {{1, 2}, {3, 4}, {5, 6}}
                                              {{7, 8}, {9, 10}, {11, 12}}



                                              Now we MapThread the function f over a and b.



                                              MapThread[f, {a, b}]


                                              {1, 10}, {3, 14}, {5, 18}}






                                              share|improve this answer












                                              $endgroup$


















                                                1














                                                1










                                                1







                                                $begingroup$

                                                I like to do this kind of thing by writing a custom function to perform the the desired operation on two given pairs. I then map this function over the two lists with MapThread. In most cases, by using Mathematica's argument-pattern destructuring, the custom function can be expressed extremely simply. This is one of those cases.



                                                The custom function is



                                                f[{x_, y_}, {_, z_}] := {x, y + z}


                                                Note that the righthand side is a literal expression of the desired result.



                                                Now let's contrive some test data. I write the generator so that two lists of pairs can easily be given any length.



                                                With[{n = 3}, {a, b} = Partition[Partition[Range[4 n], 2], n]];
                                                a
                                                b



                                                {{1, 2}, {3, 4}, {5, 6}}
                                                {{7, 8}, {9, 10}, {11, 12}}



                                                Now we MapThread the function f over a and b.



                                                MapThread[f, {a, b}]


                                                {1, 10}, {3, 14}, {5, 18}}






                                                share|improve this answer












                                                $endgroup$



                                                I like to do this kind of thing by writing a custom function to perform the the desired operation on two given pairs. I then map this function over the two lists with MapThread. In most cases, by using Mathematica's argument-pattern destructuring, the custom function can be expressed extremely simply. This is one of those cases.



                                                The custom function is



                                                f[{x_, y_}, {_, z_}] := {x, y + z}


                                                Note that the righthand side is a literal expression of the desired result.



                                                Now let's contrive some test data. I write the generator so that two lists of pairs can easily be given any length.



                                                With[{n = 3}, {a, b} = Partition[Partition[Range[4 n], 2], n]];
                                                a
                                                b



                                                {{1, 2}, {3, 4}, {5, 6}}
                                                {{7, 8}, {9, 10}, {11, 12}}



                                                Now we MapThread the function f over a and b.



                                                MapThread[f, {a, b}]


                                                {1, 10}, {3, 14}, {5, 18}}







                                                share|improve this answer















                                                share|improve this answer




                                                share|improve this answer



                                                share|improve this answer








                                                edited 34 mins ago

























                                                answered 3 hours ago









                                                m_goldbergm_goldberg

                                                91.9k8 gold badges75 silver badges210 bronze badges




                                                91.9k8 gold badges75 silver badges210 bronze badges


























                                                    0
















                                                    $begingroup$

                                                    list1 = {{1, 2}, {2, 4}};


                                                    Adding the second elements from each pair of the same list:



                                                    {#[[1]], 2 #[[2]]} & /@ list1



                                                    {{1, 4}, {2, 8}}




                                                    Adding the second element from a different list:



                                                    list2 = {{a, b}, {c, d}};
                                                    {#[[1, 1]], #[[1, 2]] + #[[2, 2]]} & /@ Transpose[{list1, list2}]



                                                    {{1, 2 + b}, {2, 4 + d}}







                                                    share|improve this answer










                                                    $endgroup$




















                                                      0
















                                                      $begingroup$

                                                      list1 = {{1, 2}, {2, 4}};


                                                      Adding the second elements from each pair of the same list:



                                                      {#[[1]], 2 #[[2]]} & /@ list1



                                                      {{1, 4}, {2, 8}}




                                                      Adding the second element from a different list:



                                                      list2 = {{a, b}, {c, d}};
                                                      {#[[1, 1]], #[[1, 2]] + #[[2, 2]]} & /@ Transpose[{list1, list2}]



                                                      {{1, 2 + b}, {2, 4 + d}}







                                                      share|improve this answer










                                                      $endgroup$


















                                                        0














                                                        0










                                                        0







                                                        $begingroup$

                                                        list1 = {{1, 2}, {2, 4}};


                                                        Adding the second elements from each pair of the same list:



                                                        {#[[1]], 2 #[[2]]} & /@ list1



                                                        {{1, 4}, {2, 8}}




                                                        Adding the second element from a different list:



                                                        list2 = {{a, b}, {c, d}};
                                                        {#[[1, 1]], #[[1, 2]] + #[[2, 2]]} & /@ Transpose[{list1, list2}]



                                                        {{1, 2 + b}, {2, 4 + d}}







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                                                        $endgroup$



                                                        list1 = {{1, 2}, {2, 4}};


                                                        Adding the second elements from each pair of the same list:



                                                        {#[[1]], 2 #[[2]]} & /@ list1



                                                        {{1, 4}, {2, 8}}




                                                        Adding the second element from a different list:



                                                        list2 = {{a, b}, {c, d}};
                                                        {#[[1, 1]], #[[1, 2]] + #[[2, 2]]} & /@ Transpose[{list1, list2}]



                                                        {{1, 2 + b}, {2, 4 + d}}








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                                                        answered 6 hours ago









                                                        MelaGoMelaGo

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