MATLAB: Are there any examples that demonstrate how to read a comma-delimited ASCII file that contains missing data

asciicommadelimitedfilei/oMATLABread

I have a file called 'myfile.dat' which contains the following data:

 date,time,current,voltage,power
 12/01/60,3:56,1,2,3
 12/01/61,3:57,2,4,6
 12/01/67,3:58,3,,9
 12/01/63,3:59,4,8
 12/01/64,4:00,5,10,12

I am interested in reading the current, voltage, and power values from the file. As you can see, some data is missing so I would like to fill these spots with a zero. I would like to know what would be the best command to read such data in MATLAB.

Best Answer

The TEXTSCAN command can be used to read formatted data into MATLAB. TEXTSCAN reads data from an open text file identified by a file identifier into a cell array. You can use the command as follows:

fid = fopen('myfile.dat')
C = textscan(fid,'%s %s %d %d %d','HeaderLines',1,'Delimiter',',','EmptyValue',0)
current = C{3};
voltage = C{4};
power = C{5};
fclose(fid);

If there is missing data, TEXTSCAN will replace the missing numeric values with 0. This is a configurable property and can be changed using the EmptyValue parameter in TEXTSCAN.

For more general information on how to read data files into MATLAB, see the following link:

https://www.mathworks.com/help/matlab/import_export/recommended-methods-for-importing-data.html

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As mentioned in the documentation of 'textread' for MATLAB, it is recommended to avoid using 'textread' by using 'textscan' instead. The issue seen can be resolved by using 'textscan' instead of 'textread'. Please refer to the attached demo script. The only difference between the scripts is, the attached script uses 'textscan' to read from the file instead of 'textread'.

MATLAB: Does the Import Data wizard not behave as expected when I import files with multiple header rows in MATLAB 7.5 (R2007b)

There is, in general no reason to suppose that headerlines have the same number of delimiters as the data. If the last line has the same number of delimiters, it is treated as column headers. However previous lines usually have very little to do with the data, so they are simply read in as single strings.

To work around this issue, you can use the TEXTSCAN function. The code below illustrates how to do this on the example file given above.

clear all; clc;
fid = fopen('exampledata.txt'); % open file
firstRow = textscan(fid, '%s', 3, 'Delimiter', ','); % read first row of headers
firstRow{1}'
secondRow = textscan(fid, '%s', 3, 'Delimiter', ','); % read second row of headers
secondRow{1}'
temp = textscan(fid, '%n', 3, 'Delimiter', ','); % try to read first line of data
dataRow = 0; % set the number of data lines found so far
while ~isempty(temp{1}) % if the line just read is not empty
      dataRow = dataRow + 1; % increment the number of data lines found
      data(dataRow,:) = temp{1}'; % copy data from line to data matrix
      temp = textscan(fid, '%n', 3, 'Delimiter', ','); % try to read next line of data
end
data
fclose(fid); % close file

Best Answer

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