Read one column only of a large file, line by line?

2 次查看（过去 30 天）
Louise Wilson
Louise Wilson 2020-10-6
回答： Seth Furman 2020-10-30
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);
i=1;
a=1;
for i = 1:nf
filename = files(i).name;
disp(filename);
try
fid=fopen(fullfile(folder,filename));
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
t=tline(1);
output(a)=t;
a=a+1;
end
writetable(a,fullfile('L:\PSD output\Boats\Duty cycle data\TVEC',strcat('TVEC_', filename)));
end

请先登录，再回答此问题。

采纳的回答

Walter Roberson
Walter Roberson 2020-10-6
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.
  3 个评论
显示 1更早的评论
Louise Wilson
Louise Wilson 2020-10-22
(it can take up to 20 mins to read a file using readtable)
Walter Roberson
Walter Roberson 2020-10-22
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.

请先登录，再进行评论。

更多回答（1 个）

Seth Furman
Seth Furman 2020-10-30
You might also want to consider using a tall table.
https://www.mathworks.com/help/matlab/import_export/tall-arrays.html

类别

在 Help Center 和 File Exchange 中查找有关 Text Files 的更多信息

标签

产品


版本

R2020a

Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!

Translated by