MatLab - Automated plotting of differently sized segments of a text file

Question

I have a text file with two columns containing space-delimited numeric data. The data are divided into segments, where each of them contains a variable number of rows and the beginning of the next segment is indicated by a single row of NaN (so the data below would be comprised of four segments):

5.1 [space] 0.0
9.2 [space] 1.4
3.7 [space] 0.6
... [space] ...
... [space] ...
... [space] ...
NaN [space] NaN
9.7 [space] 6.3
1.4 [space] 1.0
... [space] ...
... [space] ...
NaN [space] NaN
8.7 [space] 0.0
5.1 [space] 7.4
3.7 [space] 2.6
... [space] ...
NaN [space] NaN
1.7 [space] 8.4
... [space] ...

and so forth...

Now I'd like to generate a plot displaying a curve for each data segment in the file (by plotting each segments 1st column against its 2nd column).
I know how to do this if

the number of rows in each segment is the same and
the number of segments is known.

How can that be done efficiently when there's a variable segment size and an unknown number of segments?

anandr anandr · Accepted Answer · 2013-05-14T17:18:34

I generated some random data similar to your file. Text file like the one you described can be loaded like this (load correctly handles NaNs):

Data = load(filename, '-ascii');

I know that this code is not perfect, but it works for arbitrary number of segments separated by NaNs.

% Here I generate some random points to test the code
NPoints                     = 100;

Data                        = rand(NPoints,2);
% Insert NaNs in both columns
Data( Data(:,1)>0.9, : )    = NaN;

% Now do some magic
idxNaN                          = find( isnan( Data(:,1) ) );
idxNearNaNs                     = [ idxNaN-1 idxNaN+1 ]';
SegmentStartEnd                 = reshape( [1;idxNearNaNs(:);NPoints] , 2, [] )';
SegmentsOK                      = ( SegmentStartEnd(:,2) - SegmentStartEnd(:,1) ) >= 0;
SegmentStartEnd( ~SegmentsOK,:) = [];

% SegmentStartEnd contain start and end rows of good segments
NumberOfSegments = size(SegmentStartEnd,1);
fprintf('NaNs positions: %s\n', sprintf('%g ',idxNaN));
fprintf('%d segments found\n', NumberOfSegments);

% Now plot the data
figure;
hold all;
for k=1:NumberOfSegments
    SegmentStart    = SegmentStartEnd(k,1);
    SegmentEnd      = SegmentStartEnd(k,2);
    fprintf('Segment #%04d: Rows %04d to %04d\n', k, SegmentStart,SegmentEnd);
    plot( Data(SegmentStart:SegmentEnd,1) , Data(SegmentStart:SegmentEnd,2) );
end

Let me know if you need some explanations.

MatLab - Automated plotting of differently sized segments of a text file

1 Answers