If and summation : why I have the indice of the summation in the final result?












2














Consider the following code :



Sum[If[b != 0, a[m], 0], {m, 1, 4}]

(* 4 If[b != 0, a[m], 0] *)


Why it returns me an a[m] ? m is the indice of the summation, it shouldn't appear in the result ?



Here I expect to have:



If[b != 0, a[1], 0]+If[b != 0, a[2], 0]+If[b != 0, a[3], 0]+If[b != 0, a[4], 0]


I don't understand this behavior



(my example here is to understand this behavior, my specific problem is more complicated but the problem is the same as this one showed here).










share|improve this question





























    2














    Consider the following code :



    Sum[If[b != 0, a[m], 0], {m, 1, 4}]

    (* 4 If[b != 0, a[m], 0] *)


    Why it returns me an a[m] ? m is the indice of the summation, it shouldn't appear in the result ?



    Here I expect to have:



    If[b != 0, a[1], 0]+If[b != 0, a[2], 0]+If[b != 0, a[3], 0]+If[b != 0, a[4], 0]


    I don't understand this behavior



    (my example here is to understand this behavior, my specific problem is more complicated but the problem is the same as this one showed here).










    share|improve this question



























      2












      2








      2







      Consider the following code :



      Sum[If[b != 0, a[m], 0], {m, 1, 4}]

      (* 4 If[b != 0, a[m], 0] *)


      Why it returns me an a[m] ? m is the indice of the summation, it shouldn't appear in the result ?



      Here I expect to have:



      If[b != 0, a[1], 0]+If[b != 0, a[2], 0]+If[b != 0, a[3], 0]+If[b != 0, a[4], 0]


      I don't understand this behavior



      (my example here is to understand this behavior, my specific problem is more complicated but the problem is the same as this one showed here).










      share|improve this question















      Consider the following code :



      Sum[If[b != 0, a[m], 0], {m, 1, 4}]

      (* 4 If[b != 0, a[m], 0] *)


      Why it returns me an a[m] ? m is the indice of the summation, it shouldn't appear in the result ?



      Here I expect to have:



      If[b != 0, a[1], 0]+If[b != 0, a[2], 0]+If[b != 0, a[3], 0]+If[b != 0, a[4], 0]


      I don't understand this behavior



      (my example here is to understand this behavior, my specific problem is more complicated but the problem is the same as this one showed here).







      summation






      share|improve this question















      share|improve this question













      share|improve this question




      share|improve this question








      edited 2 hours ago









      bbgodfrey

      44.2k858109




      44.2k858109










      asked 5 hours ago









      StarBucK

      688211




      688211






















          2 Answers
          2






          active

          oldest

          votes


















          2














          This works:



          Sum[If[b != 0, a[m] // Evaluate, 0], {m, 1, 4}]


          From the documentation:




          If evaluates only the argument determined by the value of the
          condition.




          $ $




          You can use Evaluate to override HoldFirst etc. attributes of built-in
          functions.
           







          share|improve this answer





























            0














            Perhaps what you wanted was



            Sum[If[b =!= 0, a[m], 0], {m, 1, 4}]


            which returns



            a[1] + a[2] + a[3] + a[4]  


            The difference between $texttt{!=}$ and $texttt{=!=}$ is very important here. Because the truth value of $,texttt{b != 0},$ could not be determined, the
            $,texttt{If},$ statement was returned unevaluated.





            share





















              Your Answer





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              2 Answers
              2






              active

              oldest

              votes








              2 Answers
              2






              active

              oldest

              votes









              active

              oldest

              votes






              active

              oldest

              votes









              2














              This works:



              Sum[If[b != 0, a[m] // Evaluate, 0], {m, 1, 4}]


              From the documentation:




              If evaluates only the argument determined by the value of the
              condition.




              $ $




              You can use Evaluate to override HoldFirst etc. attributes of built-in
              functions.
               







              share|improve this answer


























                2














                This works:



                Sum[If[b != 0, a[m] // Evaluate, 0], {m, 1, 4}]


                From the documentation:




                If evaluates only the argument determined by the value of the
                condition.




                $ $




                You can use Evaluate to override HoldFirst etc. attributes of built-in
                functions.
                 







                share|improve this answer
























                  2












                  2








                  2






                  This works:



                  Sum[If[b != 0, a[m] // Evaluate, 0], {m, 1, 4}]


                  From the documentation:




                  If evaluates only the argument determined by the value of the
                  condition.




                  $ $




                  You can use Evaluate to override HoldFirst etc. attributes of built-in
                  functions.
                   







                  share|improve this answer












                  This works:



                  Sum[If[b != 0, a[m] // Evaluate, 0], {m, 1, 4}]


                  From the documentation:




                  If evaluates only the argument determined by the value of the
                  condition.




                  $ $




                  You can use Evaluate to override HoldFirst etc. attributes of built-in
                  functions.
                   








                  share|improve this answer












                  share|improve this answer



                  share|improve this answer










                  answered 2 hours ago









                  Andrew

                  1,8461115




                  1,8461115























                      0














                      Perhaps what you wanted was



                      Sum[If[b =!= 0, a[m], 0], {m, 1, 4}]


                      which returns



                      a[1] + a[2] + a[3] + a[4]  


                      The difference between $texttt{!=}$ and $texttt{=!=}$ is very important here. Because the truth value of $,texttt{b != 0},$ could not be determined, the
                      $,texttt{If},$ statement was returned unevaluated.





                      share


























                        0














                        Perhaps what you wanted was



                        Sum[If[b =!= 0, a[m], 0], {m, 1, 4}]


                        which returns



                        a[1] + a[2] + a[3] + a[4]  


                        The difference between $texttt{!=}$ and $texttt{=!=}$ is very important here. Because the truth value of $,texttt{b != 0},$ could not be determined, the
                        $,texttt{If},$ statement was returned unevaluated.





                        share
























                          0












                          0








                          0






                          Perhaps what you wanted was



                          Sum[If[b =!= 0, a[m], 0], {m, 1, 4}]


                          which returns



                          a[1] + a[2] + a[3] + a[4]  


                          The difference between $texttt{!=}$ and $texttt{=!=}$ is very important here. Because the truth value of $,texttt{b != 0},$ could not be determined, the
                          $,texttt{If},$ statement was returned unevaluated.





                          share












                          Perhaps what you wanted was



                          Sum[If[b =!= 0, a[m], 0], {m, 1, 4}]


                          which returns



                          a[1] + a[2] + a[3] + a[4]  


                          The difference between $texttt{!=}$ and $texttt{=!=}$ is very important here. Because the truth value of $,texttt{b != 0},$ could not be determined, the
                          $,texttt{If},$ statement was returned unevaluated.






                          share











                          share


                          share










                          answered 7 mins ago









                          Somos

                          1507




                          1507






























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