Read one column only of a large file, line by line?
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Louise Wilson on 6 Oct 2020
Answered: Seth Furman on 30 Oct 2020
I have a large table e.g. 7836x72001. I am interested in extracting the first column from the table which is a vector of datenumbers. Would there be a way of doing this quickly rather than waiting to load in the entire file?
I have considered using fopen and fgetl but this is taking a long time.
folder=('Y:\SoundTrap\Boats\PSD Output\Duty cycle data\');
files = dir(fullfile(folder,'*.csv'));
nf = length(files);
for i = 1:nf
filename = files(i).name;
tline=fgetl(fid); %read first line (column headers)
a=1; %relevant row of new table we are adding data to
while ~feof(fid) %read every row until last row reached
tline=fgetl(fid); %read next line
writetable(a,fullfile('L:\PSD output\Boats\Duty cycle data\TVEC',strcat('TVEC_', filename)));
Walter Roberson on 6 Oct 2020
I would suggest textscan with a format that is a %T (or %D as appropriate), followed by %*[\n] to consume the rest of the line.
textscan is much faster than looping yourself. Although you know that you could "do less work" by not processing the rest of the line, your knowledge would be implemented at the m-script level, with the threaded interpreter and Just In Time engine, whereas textscan is compiled into machine code, much lower overhead. And you need to parse the characters for end of line anyways, so textscan in this form is more efficient than one might think.
Walter Roberson on 22 Oct 2020
My understanding is that readtable on text files calls textscan, except possibly for fixed-width text... I don't know how fixed-width text is handled.
More Answers (1)
Seth Furman on 30 Oct 2020
You might also want to consider using a tall table.
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