how to extract data from an text file?

Question

isamh 2020-2-3

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https://ww2.mathworks.cn/matlabcentral/answers/503428-how-to-extract-data-from-an-text-file

评论： Walter Roberson 2020-2-14

I have two files, one in excel and one in a text file, how would i obtain column data from the text file even though i did it from an excel file.

code is:

filename = 'mct.xlsx'
data = xlsread(filename, 'mct', 'A:D');
Phase1 = data(data(:4)==1,:);

how would i be able to extract this from a text file?

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Answer 1

fred ssemwogerere 2020-2-3

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Hello, there are a number of ways to go about this. Please refer to the following links:

https://mathworks.com/help/matlab/ref/fscanf.html

https://mathworks.com/help/matlab/ref/importdata.html

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isamh 2020-2-3

编辑：isamh 2020-2-3

在 MATLAB Online 中打开

saw this example and had several questions

x = 100*rand(8,1);
fileID = fopen('nums1.txt','w');
fprintf(fileID,'%4.4f\n',x);
fclose(fileID);

what does the 'w' mean?

also, what does ''%4.4f\n'' mean?

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Answer 2

fred ssemwogerere 2020-2-4

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在 MATLAB Online 中打开

% what does the 'w' mean?

'w' gives write access permission to the file

% also, what does ''%4.4f\n'' mean?

This is a formatting operator that prints each input as a fixed point number having field width of 4, and the number of digits after the decimal point of 4, before proceeding to a new line ("\n").

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isamh 2020-2-5

编辑：isamh 2020-2-5

在 MATLAB Online 中打开

fid = fopen('MCT_Drive_Traces1.txt', 'rt');
DATA = cell2mat( textscan(fid, '%f%f%f%f', 'Delimiter', '\t', 'HeaderLines', 4) );
fclose(fid);
%% Phase 1
Phase_1 = DATA(DATA(:,4)==1,:);

hey, i tried the code above and noticed that the matrix DATA doesn't contain all the rows it's missing about one fifth of the rows.

Fixed the problem, there were headers between the numbers at some certian rows, would there be a way to ignore all headers except for the first row?

Walter Roberson 2020-2-5

Sometimes you can use textscan CommentStyle to ignore headers if they always have the same start and stop. Otherwise sometimes you end up looping textscan, especially if you want the blocks to be separated in output.

Another method that can be very useful is to fileread() the entire file as a character vector, and then use a combination of regexp and regexprep to extract parts of it, possibly then passing the resulting characters into textscan.

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Answer 3

isamh 2020-2-5

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will try that out, thanks for everything Walter!

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Answer 4

isamh 2020-2-12

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编辑：isamh 2020-2-12

在 MATLAB Online 中打开

Hey Walter, I tried to ignore any text that appears within the TXT file but kept getting an error message.

Code:

result = [];
fid=fopen('MCT_Data.txt');
tic
while 1
    tline = fgetl(fid);
    if ~ischar(tline), break, end
    celldata = textscan(tline,'%f %f %f %f %f %f');
    matdata = cell2mat(celldata);
    % match fails for text lines, textscan returns empty cells
    result = [result ; matdata];
end
toc
fclose(fid);

the error message is:

Error using textscan.

First input can not be empty. Expected a non-empty character vector or a valid file-id.

so, the two rows contain

R E V D

m k h -

435 345 435 3543

...

the file has alot of numerical data. the headers repeat randomly and are the same every time they repeat.

how would ignore everything but numerical data?

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isamh 2020-2-14

would this: if isempty(tline); continue; end be placed after results? or

got this code from another question and it works but takes some time.

str = fileread('MCT_Data.txt');

nums = cellfun(@str2double, regexp(str, '([\d.,]+)', 'match'));

x = reshape(nums, 4, [])';

Walter Roberson 2020-2-14

在 MATLAB Online 中打开

result = [];
fid=fopen('MCT_Data.txt');
tic
while 1
    tline = fgetl(fid);
    if ~ischar(tline); break; end                     %end of file
    if isempty(tline); continue; end                  %empty line
    celldata = textscan(tline,'%f %f %f %f %f %f');
    matdata = cell2mat(celldata);
    % match fails for text lines, textscan returns empty cells
    result = [result ; matdata];
end
toc
fclose(fid);

What is your file format? It appears that you have text at the beginning of some of the lines, and you are doing this looping so that you can ignore those lines? There are usually easier ways to deal with such files.

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