Search All of the Math Forum:

Views expressed in these public forums are not endorsed by NCTM or The Math Forum.

Notice: We are no longer accepting new posts, but the forums will continue to be readable.

Topic: find index in an array
Replies: 4   Last Post: Mar 9, 2010 6:43 AM

 Messages: [ Previous | Next ]
 yehuda ben-shimol Posts: 302 Registered: 2/11/05
Re: find index in an array
Posted: Mar 9, 2010 6:20 AM

a=a = RandomReal[1, {10, 10}];

(*option 1*)
res1 = Position[a, x_ /; 0.3 < x < 0.7]
(*option 2 *)
res2=res2 = Position[a, x_?(0.3 < # < 0.7 &)]

(*option 3*)
you can also define a function

f[x_,xmin_,xmax_]:=cmin<x<xmax
res3=Position[a, x_ /; f[x]]

res1==res2==res3
returns
True

no need for more complicated code (which I usually consider less elegant)

yehuda

On Mon, Mar 8, 2010 at 1:15 PM, Daniel Flatin <dflatin@rcn.com> wrote:

> I work in an environment where another system is the dominant analysis
> tool. In
> porting some code to my preferred work environment, Mathematica, I find
> that I occasionally need to reinvent functionality found in the other
> system. One
> such function is find(). In that system, this function returns all the
> non-zero indices in an array. Usually, this test array is the
> consequence of a logical operation on each element, so that in the that
> system
>
> indx == find(A > 3);
>
> returns all the indices for which elements of A are greater than 3. I
> have replicated this functionality in Mathematica, and I wanted to both
> share it, and maybe get some input in how I could make it more
> efficient or more elegant. One of the ways I learn to program in
> Mathematica is to analyze all the various responses to simple questions
> here, and I am hoping to steer the process here.
>
> Here is my function:
>
> findIndex[ array_?ArrayQ, test_ ] :== Module[
> {n==Length[Dimensions[array]],idx},
> idx == Cases[MapIndexed[If[test[#1],#2]&,array,{n}],{__Integer},n];
> If[n====1,Flatten[idx],idx]
> ]
>
> example:
>
> (* set a *)
>
> a ==
>
> {{{0.08896779137,0.08522648397},{0.1162297255,0.4316697935}},{{0.6409512512,0.3506400003},{0.1156346501,0.9537010025}},{{0.8820963106,0.9962655552},{0.004333293427,0.727745896}}};
>
> (*
>
> get indices *)
>
> indx == findIndex[a, 0.3 < # < 0.7&]
>
> output:
>
> {{1, 2, 2}, {2, 1, 1}, {2, 1, 2}}
>
> and to verify this is a valid result:
>
> Extract[a,indx]
>
> returns
>
> {0.4316697935,0.6409512512,0.3506400003}
>
> as does
>
> Select[Flatten[a], 0.3 < # < 0.7&]
>
> Note that this function is quite a bit like Position[] except that it
> works on results of a logical comparison rather than a pattern.
> Position, on the other hand, has some a feature I view as a virtue. It
> can operate on non-array objects, in fact, it can operate on non-list
> objects.
>
> If any readers has some insight into a more compact, elegant, or
> Mathematica-like approach to this findIndex function, please feel free
> to respond.
>
> Dan
>
>

Date Subject Author
3/9/10 Vivek Joshi
3/9/10 yehuda ben-shimol
3/9/10 lshifr@gmail.com
3/9/10 DrMajorBob