
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 > nonzero 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 nonarray objects, in fact, it can operate on nonlist > objects. > > If any readers has some insight into a more compact, elegant, or > Mathematicalike approach to this findIndex function, please feel free > to respond. > > Anyway, thanks for your time, and in advance for your thoughts. > Dan > >

