Large Cell Array Manipulation?

I have a nx3 matrix in the from:

P=

x1 y1 z1
x2 y2 z2
.  .  .
.  .  .
.  .  .
xn yn zn

I have then used the rangesearch() function to find the points within a radius of 4 of each point in turn:

N=rangesearch(P,P,4)

Each cell in N gives the row numbers of these positions which is within the range.

How can I then manipulate N to contain the x,y,z positions of these points?

I have achieved this using:

for n=1:Natoms
    N{n}=P(N{n});
end

however for large n the loop is very time consuming, is there a faster way or a preexisting function?

Answer 1

[Jan]

Is N pre-allocated?

M = cell(1, Natoms);
for n = 1:Natoms
    M{n} = P(N{n});
end

[mfas]

Yes N is pre-allocated, sorry that wasn't clear in the question. Is there a way of avoiding the loop entirely to achieve the same result?

Answer 2

[Cedric]

The loop is probably not the limiting factor. RANGESEARCH will be much slower than the loop, for almost any number of atoms. Try it in the profiler, type:

 profile viewer

in the command window, type the name of your script in the field labeled "Run this code", click on [Start profiling], and you will see the report.

PS: you probably want

 M{n} = P(N{n}, :);  % or N if you update it instead of creating a new cell array.

in your loop.