Sample matrix data and retrieve in Matlab

Question

In Matlab, I want to sample data in such a way that to calculate the length of matrix, and for that, calculate its every 20th row and stores in a array. That what I sampled my data. length(P) for instance which is 251.

Now, I want to check if the Original P index is equal to the sampled Matrix index (obviously the operation is in loop) then merge both same indexes, Which is:

[L]=[0];
for ii=1:length()
if P(ii,:)==  SP{ii}(ii,:) %SP is sample points array
L = [P(ii,:)=; SP{ii}(ii,:);];
end
end

My Problem: I'm unable to sample the data in my accordance, i.e SP= datasample(P,2); and also, couldn't retrive the calculated L very well, may be facing problem of indexes, i.e

if L~=0 l=L(ii,:); end

Answer 1

Sample data after 20th iterations, can be simply in for-loop instead of any built-in function, the code below showing some sketch for only one cell index.

kk = 0;
for ii=1:round(length(P{1})/30)
kk = kk+20;
L{ii} =P{1}(kk,:);
end