Read data from file: keep separation by empty line in file - matlab

Question

I can make a matrix x and y from data.dat which consists of 2 columns using this code:
load data.dat
x=data(:,1);
y=data(:,2);
What if I want to make matrices x1, x2, ... xn and the same with y,
where x1, x2, are based on the separation by a blank line in the file data.dat:
54.510 1.420
55.294 1.819
55.859 1.935
55.999 2.381
9.017 1.600
9.518 1.916
9.868 2.217
9.896 2.368
10.113 2.533
10.424 2.552
.... ...
Based on this data example, I expect
x1=[54.510;55.294;55.859;55.999]
x2=[9.017;9.518;9.868;9.896;10.113;10.424]
y1=[1.420;1.819;1.935;2.381]
y2=[1.600;1.916;2.217;2.368;2.533;2.552]

Answer 1

You can use fgets to read the data from your file. It is assumed that your file is called test.dat:
(see the comments for explanations on the functionality)
% introduce a counter for all the 'blocks in the file'
block = 1;
% initialize empty vectors for x and y
x = [];
y = [];
% open the file (in this case 'test.dat')
file = fopen('test.dat');
% read the first line of the file
tline = fgets(file);
% continue as long as there are lines in the file
while ischar(tline)
% fgets returs a string, convert to float to process:
q = str2num(tline);
% if q is empty, we have a blank line, otherwise, append the values to
% x and y
if ~isempty(q)
x(end+1) = q(1);
y(end+1) = q(2);
else
% if there is a blank line, write x and y to a cell X and Y
% you need a cell here, because the number of elements per block may
% differ
X{block} = x;
Y{block} = y;
% advance the block counter
block = block + 1;
% clear and re-initialize x and y - this is necessary because the next
% block may contain less values. you do not want artifacts.
clear x y
x = [];
y = [];
end
% read the next line
tline = fgets(file);
% at the end of the file, fgets returns not a string but a number, -1
% in this case, assign x and y to X and Y
% no need to increment block, because you are at the end of the file
if tline == -1
X{block} = x;
Y{block} = y;
end
end
% close the file once all operations are done
fclose(file);
You now have a cell X and a cell Y which contain the data from your file. E.g.
X{1} =
Columns 1 through 2
54.51 55.294
Columns 3 through 4
55.859 55.999
I am aware that you won't get variables called x1, x2, ... xn with this approach. However, I believe that cells X{1}, X{2}, ... X{n} are both easier to generate, populate and handle than individual variables.