I want to open a tab-delimited XY text file in matlab, use the numerical values of the XY columns to create a matrix and plot the signal.

I have a tab-delimited text file with two columns: time and voltage. I'd like to open the file in matlab and put the data into a matrix (n x 2) to make an XY scatter plot. So far, I am able to open and read the file, create the matrix and initialize variables, but I'm struggling to find a way to put the data into the matrix.

Here is what I have:

% pointer to the file

    fid = fopen("AxographSampleData\220812_WT_MO_H134R_EA_1391 015.txt","r");

% first line contains headers

    headers = regexp( fgetl(fid) ,'\t','split');
    timestamp = headers{1};
    membrvolt = headers{2};

%% reads data from file and creates a matrix

% initialize matrix
    nColumns = 2;
    nRows = 40000; 

    data = zeros(nRows,nColumns); 

% set a toggle switch and begin while loop
    toggle = true;
    while toggle
    
        % get a line of tab-delimited data
        aline = regexp( fgetl(fid), '\t','split');
    
        % check whether we are at the end of data
        if strcmpi(aline(1,2),'\s')
            toggle = false;
        else
            % put the data point into the matrix at the appropiate position
           
    
        end
    
    end

I was following an example using a text file with six columns. What they did was to put the value of the 6th column in the matrix using columns 2 and 4 as indexes for data(rows,cols) with the following line of code:

% put the data point into thr matrix at the appropiate position
data(str2double(aline{2}), str2double(aline{4})) = str2double(aline{6}); 

But when I change the code to match the structure of my text file like this:

% put the data point into thr matrix at the appropiate position
data(str2double(aline{1})) = str2double(aline{2})

I get the following error message "Index in position 1 is invalid. Array indices must be positive integers or logical values".

Does anyone know how to fix this error? The text file is attached for reference.

Thanks in advance.

Erwin

Answer 1

[millercommamatt]

fid = fopen("AxographSampleData\220812_WT_MO_H134R_EA_1391 015.txt","r");
C = textscan(fid,'%f %f',"Delimiter",'\t','HeaderLines',1);
fclose(fid);
Sec_voltage_array = [C{1},C{2}];
% check out the textscan documentation in you want to import the seconds
% column as a duration and not a float

[Erwin Arias Hervert]

Thank you. It worked marvelous!

Answer 2

[Les Beckham]

T = readtable('https://www.mathworks.com/matlabcentral/answers/uploaded_files/1230202/220812_WT_MO_H134R_EA_1391%20015.txt')

Warning: Column headers from the file were modified to make them valid MATLAB identifiers before creating variable names for the table. The original column headers are saved in the VariableDescriptions property.
Set 'VariableNamingRule' to 'preserve' to use the original column headers as table variable names.

T = 40000×2 table
    Time_s_    MembraneVoltage_1_V_
    _______    ____________________

      5e-05         -0.075806      
     0.0001         -0.075819      
    0.00015         -0.075813      
     0.0002         -0.075806      
    0.00025           -0.0758      
     0.0003         -0.075813      
    0.00035         -0.075819      
     0.0004         -0.075806      
    0.00045         -0.075819      
     0.0005         -0.075825      
    0.00055         -0.075831      
     0.0006         -0.075838      
    0.00065         -0.075825      
     0.0007         -0.075806      
    0.00075         -0.075831      
     0.0008         -0.075825      

scatter(T.Time_s_, T.MembraneVoltage_1_V_)
grid on
xlabel 'Time (s)'
ylabel 'Membrane Voltage 1 (V)'

In my opinion this looks better as a regular plot instead of a scatter plot.

figure
plot(T.Time_s_, T.MembraneVoltage_1_V_)
grid on
xlabel 'Time (s)'
ylabel 'Membrane Voltage 1 (V)'

[Erwin Arias Hervert]

Thank you, it worked really well. I agree with you, the data looks better as a regular plot.