z = zeros(n,1);
 for i = 1:n
  if Data{i}(1)~='N'
     z(i) = str2double(Data{i});
  else
     z(i) = NaN;
  end
 end

z = nan(n,1); %data matrix
filename = 'largefile.txt';
fid = fopen(filename,'rt');
l = getl(fid);
ct=1;
while ~isnumeric(l)
    z(ct) = str2double(l);
    l = getl(fid);
    ct=ct+1;
end

data = readtable('data.txt','Delimiter','n','Format','%f','ReadVariableNames',false,'TreatAsEmpty','No data');

9.343410
11.54300
6.733000
-135.210
No data
34.23000
0.550001
No data
1.535000
-0.00012
7.244000
9.999999
34.00000
No data

ans =
      9.34341
      11.543
      6.733
      -135.21
      NaN
      34.23
      0.550001
      NaN
      1.535
      -0.00012
      7.244
      9.999999
      34
      NaN