Re: removing non-numeric elements from the table
- To: mathgroup at smc.vnet.net
- Subject: [mg108063] Re: [mg108016] removing non-numeric elements from the table
- From: Leonid Shifrin <lshifr at gmail.com>
- Date: Sun, 7 Mar 2010 05:08:54 -0500 (EST)
- References: <201003050933.EAA29417@smc.vnet.net>
Hi Michael,
First note that your code could be written a little more compactly using
nested lists of rules:
In[30]:= Quiet[z /. Table[Eq, {20}]]
Out[30]= {z, {-0.658661}, z, {-1.33099}, z, z, {0.95508}, {-1.55336}, \
{-0.453457}, z, z, {-0.976592}, {-0.838431}, {-0.116113}, {-1.07435}, \
{0.031166}, {-0.0701658}, z, z, z}
You can first make a table of solutions and then apply it and eliminate z:
In[31]:= Quiet[DeleteCases[z /. Table[Eq, {20}], z][[All, 1]]]
Out[31]= {-0.0250797, 0.0989299, 1.25444, -1.74394, 1.11036, \
0.569451, 1.70946, -1.48392, 1.45803, -0.878776}
Or, which is the same, eliminate empty lists from the list of solutions
first, and then apply it:
In[32]:= Quiet[Part[z /. DeleteCases[Table[Eq, {20}], {}], All, 1]]
Out[32]= {-1.72183, 1.09259, -0.594554, -0.0791054, -0.60679, \
0.633106, 0.184821, 1.46669, -1.03662, 0.0210264, -0.864565, -0.358463}
If you don't want to even produce the empty solution lists, you can do it
for example as follows:
In[34]:= Quiet[
Part[z /. Table[If[# === {}, Sequence @@ {}, #] &[Eq], {20}], All, 1]]
Out[34]= {-0.630328, 0.411173, -1.34137, -1.04662, -0.781298, \
1.67024, 0.314125, -2.04354, -0.473975, -0.906154, -3.50552, 0.554472}
Note that in the above the pure function is necessary to avoid re-evalution
of Eq in the last branch of If. Not also that with this method you can not
use Sequence[] directly in If, since If is not SequenceHold.
By the way, try to not get into the habit of starting your symbols with a
capital letter, and also using global variables where essentially functions
are needed. This is how I'd rewrite your code:
In[45]:=
Clear[rnd, eq, tvR, solutions];
rnd[] := Random[];
eq[s_] := Solve[1/2 (Erf[s/Sqrt[2]] + 1) == 2 rnd[] - 1, s];
solutions[eqF_, num_Integer?NonNegative] :=
Module[{z},
Quiet[DeleteCases[z /. Table[eqF[z], {num}], z][[All, 1]]]];
In[49]:= tvR = solutions[eq, 20]
Out[49]= {0.588592, 0.296185, -0.914506, 0.473337, 1.97816, 1.48341,
-0.701329, -0.320757}
In the case of <rnd>, the change is purely stylistic - to show to the reader
that you use <rnd> as a function, albeit taking no arguments.
Now you have no chance of accidental collision with some global value that
<z> may have. If the above seems too much of a hassle, at the very least
I'd wrap the code for tvR in Block[{z},...].
Hope this helps.
Regards,
Leonid
On Fri, Mar 5, 2010 at 1:33 AM, michael partensky <partensky at gmail.com>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
>
>
>
- References:
- removing non-numeric elements from the table
- From: michael partensky <partensky@gmail.com>
- removing non-numeric elements from the table