finding approximate indices in a monotonically increasing array

I need to find the indices of an entry in a monotonically increasing array, where the value of this indices is "ref"(as shown below)

I create my increasing array "t" as follow

dt=1/10e9;
N=13600000;
t=((0:N-1)*dt);    

ref=1.39459e-05;

I've already tried using the find function but it leads to incorrect results or multiple indices or no answers at all.

can anyone suggest any way I can get this working

[Jan]

Please post the find command you have used, when you want us to suggest an improvement.

Answer 1

[Shashank Prasanna]

You can use Nearest Neighbor search. If you have stats toolbox:

f = knnsearch(t',ref)

f =

        139460
  >> t(f)

ans =

                 1.39459e-05

IF you don't have the stats toolbox:

>> tri = delaunayn(t');
f = dsearchn(t',tri,ref)

f =

        139460

Because you have so many points you have to be patient since it takes time.

[Sean de Wolski]

There is so much extra working put into calculating the Delaunay triangulation for all of t when all you need is a simple histogram calculation of one element! I would not recommend this approach.

Foosball?

[Shashank Prasanna]

I agree, i just like using nearest neighbor search, and it always works. lets go.

Answer 2

[Sean de Wolski]

[~,idx] = histc(ref,t);

Use histc() to find the bin containing ref.

[Jan]

HISTC is a fast and efficient C-Mex function, which does not create the temporary vector abs(t - ref) and performs a binary search, which is the optimal strategy in theorie. +1

Answer 3

[Jan]

dt  = 10e-9;
N   = 13600000;
t   = (0:N-1)*dt;
ref = 1.39459e-05;

[value, index] = min(abs(t - ref));

But without doubt, the binary search of histc is smarter than comparing all values in this brute force approach.

Answer 4

[Arsalan]

Hi Guys, thanks for your suggestions, I never knew about the use of some of this commands before. but I found much more efficient way

dt=1/10e9;
N=13600000;
t=((0:N-1)*dt);
ref=1.39459e-05;

index=round(ref/dt);
my_value=t(index);  % should be approximately equal to ref

it works well for values greater then zero, but should work for values less then zero with some manipulation