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MathGroup Archive 1999

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Re: Help needed

  • To: mathgroup at smc.vnet.net
  • Subject: [mg18856] Re: [mg18801] Help needed
  • From: "Tomas Garza" <tgarza at mail.internet.com.mx>
  • Date: Thu, 22 Jul 1999 22:57:51 -0400
  • Sender: owner-wri-mathgroup at wolfram.com

Bin Liu [binliu at vt.edu] wrote:

> I'm a new Mathematica (3.0) user.  The program is listed below.  The
> problem is that the last command (For loop) gives me no output.  But it
> works if I input a specific number, 1 to 4, to substitute i in
> "Apply[mag,{Watoms[[i]],Gatoms[[i]]}]".
>
> Thanks.
>
> Bin
>
>
> watoms= Table[{{0,0,0},{0.5,-0.5,0.5},{0.5,0.5,0.5},{0,0,1}}]
> Bao = 2.8665;
> Watoms =watoms*Bao;
>
> gatoms=Table[ {{0,0,0},{0.5,-0.5,0.5},{0.5,0.5,0.5},{0,0,1}}]
> Fao = 3.5975
> Gatoms =gatoms*Fao
>
> mag[{a1_,a2_,a3_},{b1_,b2_,b3_}]:=Sqrt[(a1-b1)^2+(a2-b2)^2+(a3-b3)^2]
>
> For[i=1,i<5,i++,Apply[mag,{Watoms[[i]],Gatoms[[i]]}]]

Bin,

Your procedure is OK, except that you have not instructed it what to do with
the results of the calculation. Simply wrap Print around [Apply...] and
you'll get your results nicely printed. Furthermore, you needn't use Apply
in this case. You can instead write mag[Watoms[[i]],Gatoms[[i]]]. But I
think you are not taking full advantage of the power of Mathematica. You are
using procedural programming, instead of functional programming, which I
strongly recommend you to learn. You could then solve your problem in an
elegant, Mathematica-like way as follows:

In[1]:=
mgg[a_List, b_List] := Sqrt[Plus @@ (#^2 & /@ (a - b))]
In[2]:=
MapThread[mgg, {Watoms, Gatoms}]
Out[2]=
{0, 0.633065, 0.633065, 0.731}

Tomas Garza
Mexico City



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