Finding specific data for a given condition

I have this data set for which column 1 is time and column 2 is the number of atoms, and I'm trying to find variable N, which contains data that start at t>=0. I have done this:

index = find(Data >= 0, 1, 'first');
Ndecay = Data(index:end);

but I only get a row of data that corresponds to the time, rather than two rows, which should correspond to both time and number of atoms. How can I find the number of atoms corresponding to times at t>=0?

Answer 1

[KSSV]

Let data be your m*2 array. To get number of atoms at t = 0, you can use interp1.

t = data(:,1) ;     % time 
N = data(:,2) ;     % Number of atoms 
ti = 0 ;            % N is required here 
Ni = interp1(t,N,ti) 
[ti Ni]

If you want to use find and index.

idx = t==0 ; 
data(idx,:)

[gps]

@KSSV when I use this, I just get [0, 1121600], which is correct, but how can I find all the values for when t >= 0? So, I would have a matrix of values corresponding to t>=0 and the respective number of atoms at each time

[KSSV]

idx = t>=0; data(idx,:)