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Function hangs.

2014-03-31
2014-04-04
  • I wrote the following simple function which calculates the cross product of two vectors:

    CP(a,b):= [a[2]*b[3]-a[3]*b[2], a[3]*b[1]-a[1]*b[3], a[1]*b[2]-a[2]*b[1]];
    

    Now everything seems to work fine, but then i tried

    C(b,a);
    

    in the hope, to get the negative value of C(a,b). Nevertheless, wxmaxima hangs.

    So what is going wrong?
    I am running the newest version wxMaxima 13.04.0 available for osx:
    System info
    wxWidgets: 2.8.12
    Unicode Support: yes
    Maxima version: 5.30.0
    Lisp: SBCL 1.0.55.0-abb03f

    Thank you for any information

     
  • Your function works fine on my system:

     (%i1) CP(a,b):= [a[2]*b[3]-a[3]*b[2], a[3]*b[1]-a[1]*b[3], a[1]*b[2]-a[2]*b[1]];
     (%o1)      CP(a, b) := [a  b  - a  b , a  b  - a  b , a  b  - a  b ]
                              2  3    3  2   3  1    1  3   1  2    2  1
     (%i2) a:[a1,a2,a3]; b:[b1,b2,b3];
     (%o2)                            [a1, a2, a3]
     (%o3)                            [b1, b2, b3]
     (%i4) CP(a,b)+CP(b,a);
     (%o4)                              [0, 0, 0]
    

    I'm running Maxima 13.04.2 on an old XP system.

     
  • Can you try

    kill(all);
    CP(a,b):= [a[2]*b[3]-a[3]*b[2], a[3]*b[1]-a[1]*b[3], a[1]*b[2]-a[2]*b[1]];
    CP(a,b);
    

    and then, without defining a and b,

    CP(b,a);
    

    the first function call (CP(a,b)) does work, the second leads to a hanging system in my version of maxima.

    Wolfgang

     
    • Wolfgang Meiners wrote the following on 3/31/2014 5:40 PM:

      kill(all);
      CP(a,b):= [a[2]b[3]-a[3]b[2], a[3]b[1]-a[1]!<-- I removed this symbol

      b[3], a[1]b[2]-a[2]b[1]];
      CP(a,b);

      Yes, CP(b,a) hangs on mine too. As does CP(p,q). I don't see why you
      would expect this to not cause problems, since Maxima does not know if
      the arguments are lists as required. It certainly shouldn't hang. It
      would be better if it displayed an error message. But it also doesn't
      seem like a problem in any reasonable calculation for which the
      arguments to CP are properly defined.

      What is it you're trying to do?

       
      Attachments
      • "What is it you're trying to do?"

        I am trying to understand what happens. I wrote this function - and i am not very experienced in writing programs for wxmaxima. After defining the function, i applied it to some arguments without defining these arguments. There was no problem. The following works on my system:
        kill(all);
        (%o0) done

        CP(a,b):= [a[2]b[3]-a[3]b[2], a[3]b[1]-a[1]b[3], a[1]b[2]-a[2]b[1]];
        (%o1) CP(a,b):=[a[2]b[3]−a[3]b[2],a[3]b[1]−a[1]b[3],a[1]b[2]−a[2]b[1]]

        CP(x,y);
        (%o2) [x[2]y[3]−y[2]x[3],y[1]x[3]−x[1]y[3],x[1]y[2]−y[1]x[2]]

        CP(p,q);
        (%o3) [p[2]q[3]−q[2]p[3],q[1]p[3]−p[1]q[3],p[1]q[2]−q[1]p[2]]

        so there seems to be no need to define vectors before calculating the cross product. But then i tried to verify the antisymmetry of the cross product by writing

        CP(a,b) + CP(b,a)
        

        and then, the program hangs. I can write

        CP(x,y) + CP(y,x);
        (%o5) [0,0,0]
        

        so this seems to be related to the call CP(b,a) where the arguments are changed. I want to understand, what is going wrong in this situation to avoid such errors in future situations.

        In my opinion, the main program should not know about the arguments of the function. The inside world of the function should be opaque to the main program. But this seems not to be the case. So can i use variables a,b in the main program? Obviously not. If i could use them, CP(b,a) should work as well, as CP(a,b) does.

        Wolfgang

         
  • Hello Albrecht,
    thank you for this confirmation

    Wolfgang