Re: removing non-numeric elements from the table
- To: mathgroup at smc.vnet.net
- Subject: [mg108060] Re: removing non-numeric elements from the table
- From: David Bailey <dave at removedbailey.co.uk>
- Date: Sun, 7 Mar 2010 05:08:21 -0500 (EST)
- References: <hmqj5p$so5$1@smc.vnet.net>
michael partensky wrote:
> Hi. I am building a large table filled with the numerical solutions of an equation.
> Here is an example, where a normal standard variable is being generated.
>
>
> rnd:=Random[];
> Eq:=Solve[1/2 (Erf[z/Sqrt[2]]+1)==2 rnd-1,z];
> tvR=Quiet[Table[z/.Eq[[1]],{20}]];
>
> In[187]:=
> tvR
>
> Out[187]=
> {0.761439, z /. {}[[1]], z /. {}[[1]], 1.07111, 0.031784, z /. {}[[1]], z /.
> {}[[1]], 0.808313, -1.04238, z /. {}[[1]], 0.376982, z /. {}[[1]], 1.06622,
> z /. {}[[1]], -0.546643, -0.367224, 0.28111, -0.0606866, z /. {}[[1]], z /.
> {}[[1]]}
>
> What is a nicer way of removing the non-numeric elements? Clearly, it is
> possible to filter them out using say NumericQ, but I am looking for
> something more elegant (it is apart of a lecture), preferably right while
> the Table is being generated.
>
> Thanks
> Michael
>
>
This question really comes down to style, because as you say, you could
just filter the list with NumericQ.
There are various aspects of this code that I like to change:
1) I would only use Quiet where really necessary, and avoid generating
error messages unnecessarily. Those messages are there to help debug the
code, and if you generate a lot, or just turn them off, you may miss a
some serious errors. The error regarding inverse functions is probably
best suppressed, but the Part error, which your code also generates, is
definitely better avoided. I also suspect that code that generates
suppressed errors, probably runs more slowly than equivalent code that
avoids doing this.
2) Personally, I don't like variables such as rnd defined by delayed
assignment. When you have a lot of code, it can be really hard to
determine which variables are actually functions!
I would avoid fully capitalised variable names such as Eq, which are
reserved for use by Wolfram.
Thus I would probably rewrite your code like this:
eqResults[] :=
Quiet[Solve[1/2 (Erf[z/Sqrt[2]] + 1) == 2 Random[] - 1, z]];
Map[#[[2]] &, Flatten[Table[eqResults[], {20}]]]
Alternatively, you could suppress the inverse functions message
specifically.
David Bailey
http://www.dbaileyconsultancy.co.uk