read and use .txt file

Question

francesco baldi 2021-12-26

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评论： Walter Roberson 2021-12-27

draft.txt

hi,

I was wondering if there's a way to read the .txt file i attached and use it to calculate other parameters. The .txt file reports the transfer function of my system with frequency, amplitude and phase respectively in the first, second and third column. Now, i need to calculate another parameter of the system in function of the transfer function. Assuming, for example, that my transfer function is called H, i need to calculate another parameter G = H*100/(1-H). Is there anyone who can tell me how to do it?

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francesco baldi 2021-12-26

T1_a = readtable(filename, 'VariableNamingRule','preserve') %leggo il file come tabella

V2c_a = cellfun(@(x)sscanf(x, '(%fdB,%f°'), T1_a.Var2, 'Unif',0);

V2m_a = cell2mat(V2c_a')';

T2_a = table('Size',[size(T1_a.Var1,1) 3],'VariableTypes',{'double','double','double'}, 'VariableNames',{'FreqHz','MagndB','PhasDg'});

T2_a.FreqHz = T1_a.Var1;

T2_a.MagndB = V2m_a(:,1);

T2_a.PhasDg = V2m_a(:,2)

that's what i wrote, but i'm not sure if it is usefull honestly.

Star Strider 2021-12-26

Reference — how to use LTSpice values in Matlab and earlier posts by the same OP, how to plot transfer function exported from LTSpice, how to use LTSpice values in Matlab, how to find the transfer function from LTspice, and perhaps others, since I may have missed a few of these prolific posts, all asking essentially the same question.

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Answer 1

Meg Noah 2021-12-26

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There's a lot of extra characters in your text file.

% frequency, amplitude and phase
str = fileread('draft.txt');
str = strrep(str,'{','');
str = strrep(str,'}','');
str = strrep(str,'(','');
str = strrep(str,')','');
str = strrep(str,'°','');
str = strrep(str,'''','');
str = regexprep(str, '\t', ' ');
str = regexprep(str, '  ', ' ');
str = regexprep(str, ' ', ',');
str = regexprep(str, 'dB', '');
objrec = regexp(str, '\r\n|\r|\n', 'split');
% remove empty cells (blank lines)
objrec(strlength(objrec)<1) = [];
nPts = numel(objrec);
frequency = zeros(nPts,1);
amplitude =  zeros(nPts,1);
phase =  zeros(nPts,1);
for iPt = 1:nPts
    v = sscanf(objrec{iPt},'%f,%f,%f,');
    frequency(iPt) = v(1);
    amplitude(iPt) = v(2);
    phase(iPt) = v(3);
end
omega = 2*pi*frequency;
H = 1./(1 + 1i*2*omega);
G = H*100.0./(1-H);
myTable = table(frequency,amplitude,phase,omega,H,G,'VariableNames', ...
    {'frequency','amplitude','phase','omega','H','G'});
writetable(myTable,'myTable.xlsx');

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Answer 2

Walter Roberson 2021-12-26

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It is not clear whether your second column is all negative, or if '(-' is the delimiter, the same way that the line ends in '-)' . The below code assumes that '(-' is the delimiter.

Your file does not have any degree symbols in it.

filename = 'https://www.mathworks.com/matlabcentral/answers/uploaded_files/844820/draft.txt';
S = webread(filename);
data = cell2mat(textscan(S, '%f (-%fdB,%f%*[^\n]'));
whos data
  Name        Size            Bytes  Class     Attributes

  data      301x3              7224  double              

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Walter Roberson 2021-12-27

编辑：Walter Roberson 2021-12-27

在 MATLAB Online 中打开

@Star Strider

Interesting. When I look at the file with Firefox, the line ends with -) . When I readtable() it has a degree sign. When I webread() it has U+65533

"U+FFFD (decimal 65533) is the "replacement character". When a decoder encounters an invalid sequence of bytes, it may (depending on its configuration) substitute for the corrupt sequence and continue. "

XCode and BBEdit both show the character as ∞ U+221E

When I look at the file as a hex dump, the character is U+00B0 which is the degree character.

filename = 'https://www.mathworks.com/matlabcentral/answers/uploaded_files/844820/draft.txt';
SW = webread(filename);
SW(65:75)
ans = 
    '4e+000)
     1'
ans+0
ans = 1×11
          52         101          43          48          48          48       65533          41          13          10          49
ST = readtable(filename);
ST(1:3,:)
ans = 3×2 table
     Var1                              Var2                         
    ______    ______________________________________________________

      1000    {'(-6.02488411418877e+000dB,-1.79940817416164e+000°)'}
    1023.3    {'(-6.02508591811268e+000dB,-1.84129325097736e+000°)'}
    1047.1    {'(-6.02529722271243e+000dB,-1.88415191829931e+000°)'}

Walter Roberson 2021-12-27

在 MATLAB Online 中打开

Let's experiment:

filename = 'https://www.mathworks.com/matlabcentral/answers/uploaded_files/844820/draft.txt';
webopt = weboptions('CharacterEncoding', 'ISO-8859-1');
SW = webread(filename, webopt);
SW(65:75)
ans = 
    '4e+000°)
     1'
ans+0
ans = 1×11
    52   101    43    48    48    48   176    41    13    10    49