MATLAB: How to get the vector in a matrix (specific location)

matricesvector

I have matrix A (8×4)

A= [ 1  0  0  0  0  0  0  0  
     2  0  0  0  0  0  0  0
     3  0  0  0  0  0  0  0
     4  5  6  7  8  9  10  11]

How can I get vector of B that start from row 4, column 2 till end?

 B= [ 5  6  7  8  9  10   11]

Best Answer

A= [ 1  0  0  0  0  0  0  0  
     2  0  0  0  0  0  0  0
     3  0  0  0  0  0  0  0
     4  5  6  7  8  9  10  11]
B = A(4,2:size(A,2))

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MATLAB: How To Load Multiple Text Files (specific context)

As dpb suggested use one of the modern file import function such as readtable or readmatrix instead of the old textscan. These can figure the format of your file on their own or if they're struggling a bit have plenty of easy to understand options to help them along. They're also a lot more configurable, particularly if you use detectImportOptions.

For example, your text file is easily decoded with:

spectrum = readtable('1903395U1_04Jun19_154040_0001.Raw8.txt', 'HeaderLines', 8)

or for a neater table:

opts = detectImportOptions('1903395U1_04Jun19_154040_0001.Raw8.txt', 'ExpectedNumVariables', 4);  %only needed once for all the files that follow the same format
spectrum = readtable('1903395U1_04Jun19_154040_0001.Raw8.txt', opts)

detectImportOptions automatically figure out that the header is 8 lines, that the delimiter is ; and that the name of the columns is on the 6th row. I've told it that there is only 4 variables despite the header having 5 names (why is there a 'scope'?).

You can easily wrap that in a loop over all the files. The detectImportOptions is only needed once if all the files follow the same format. You can store the table from each file into a cell array but if your aim is to run statistics across the files then you'd be better off storing it all as one flat table with an additional variable indicating which file the data comes from. After that you can use groupsumarry or similar to compute your statistics all at once.

So the code would be something like:

%Get list of files. You haven't explained how these can be obtained.
filelist = dir('C:\somefolder\*.txt');
%Loop to read all files:
spectra = cell(size(filelist));  %stored in a file array at first
opts = detectImportOptions(fullfile(filelist(1).folder, filelist(1).name, 'ExpectedNumVariables', 4); 
for fileidx = 1:numel(filelist)
    spectrum= readtable(fullfile(filelist(fileidx).folder, filelist(fileidx).name), opts);  %read file
    spectrum.Source = repmat({filelist(fileidx).name}, height(spectrum), 1);  %add a variable indicating the source. Maybe you want to use only part of the filename
    spectra{fileidx} = spectrum;
end
%flatten it all into one table
spectra = vertcat(spectra{:});
%compute some stats, e.g. mean and standard deviation of spectra at each wavelength across the files
groupsumarry(spectra, 'Wave', {'mean', 'std'}, {'Sample', 'Dark', 'Reference'})

Code untested. There might be typos. Read the error messages carefully. Note that I'm using meaningful variable names instead of the utterly useless A.

MATLAB: Standar deviation of a matrix (specific elements)

Two ways, both producing the same value:

M = rand(7);                        % Create Data
diagM = nan(1,size(M,1));
M = M + diag(diagM);
Mmean1 = mean(M(:),'omitnan');
Mmean2 = mean(M(~isnan(M(:))));

Best Answer

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MATLAB: How To Load Multiple Text Files (specific context)

MATLAB: Standar deviation of a matrix (specific elements)

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