Re: find index in an array
- To: mathgroup at smc.vnet.net
- Subject: [mg108139] Re: find index in an array
- From: Ray Koopman <koopman at sfu.ca>
- Date: Tue, 9 Mar 2010 06:21:46 -0500 (EST)
- References: <hn2m94$40n$1@smc.vnet.net>
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}}};
findIndex2[data_,test_] := Position[data,_?test]
indx2 = findIndex2[a, .3 < # < .7&]
{{1,2,2},{2,1,1},{2,1,2}}
Extract[a,indx2]
{0.43167,0.640951,0.35064}
On Mar 8, 3:15 am, Daniel Flatin <dfla... at 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.
>
> Anyway, thanks for your time, and in advance for your thoughts.
> Dan