I assume that celldata is a structure array
% create some example data
for k=1:10, celldata(k).CN = ceil(4*rand) ; end
% method 1: easy
allCN = [celldata.CN] % comma separated list expansion
tf1 = allCN == 4
index1 = find(tf1)
% method 2: very flexible
fun = @(x) celldata(x).CN == 4 % useful for complicated fields
tf2 = arrayfun(fun, 1:numel(celldata))
index2 = find(tf2)
Both methods can be written as one-liners.
