MATLAB Answers

Importing Data from Text File Loop

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Nat Person
Nat Person on 31 Dec 2020
Edited: per isakson on 2 Jan 2021
I used Matlab's auto-import feature to generate a script that imports data from a text file
%% Setup the Import Options and import the data
opts = delimitedTextImportOptions("NumVariables", 2);
% Specify range and delimiter
opts.DataLines = [1, Inf];
opts.Delimiter = ",";
% Specify column names and types
opts.VariableNames = ["sam12mark", "dashcolorforcolumn2"];
opts.VariableTypes = ["double", "double"];
% Specify file level properties
opts.ExtraColumnsRule = "ignore";
opts.EmptyLineRule = "read";
% Import the data
N9913A6 = readtable("C:\Users\nperson\Desktop\New folder\Scaled\N9913A_6.16.20 04_HF-000013-T01-SCALED.ep", opts);
%% Clear temporary variables
clear opts
This works perfectly but I need it to cycle through a folder full of files (all '.ep' but with different names)
How would I do that?

Answers (1)

per isakson
per isakson on 31 Dec 2020
Edited: per isakson on 2 Jan 2021
First you need to create an array of file specifications. I prefer to use the function dir()
glob = fullfile( "C:\Users\nperson\Desktop\New folder\Scaled", "N*SCALED.ep" ); % I'm guessing
sad = dir( glob );
Next you need to decide how to store all the tables that will be created. A zillion names like N9913A6 is not a good idea (See TUTORIAL: Why Variables Should Not Be Named Dynamically (eval)). Do you want to keep all the tables? I assume you want that. A structure or a containers.Map are possible (or a cell array, but it's hard to know which file went into cell 17). A structure requires that the field names are legal Matlab names. The keys of a containers.Map can be "any" text.
len = numel( sad );
for jj = 1 : len
name = ... % you decide
S.(name) = readtable( fullfile( sad(jj).folder, sad(jj).name ), opts );
end
In response to comment
The code fragment above creates a scalar structure with a zillions of fields. The field names could be something like N9913A6, which is "easily" created from the file name. However, there is little room for meta-data.
A structure array that allows for meta-data could be something like
S.meta_data_1
S.meta_data_2
S.table
and the loop
for jj = 1 : len
S(jj).meta_data_1 = ...
S(jj).meta_data_2 = ...
S(jj).table = readtable( fullfile( sad(jj).folder, sad(jj).name ), opts );
end
Which of these alternatives is the "better" depends on the context. The question provides little of that.
  1 Comment
Stephen Cobeldick
Stephen Cobeldick on 1 Jan 2021
Alternatives to containers.Map:
  • if names are valid fieldnames, you can use the structure array returned by dir.
  • most efficient and easiest to work with: store the meta-data as data in its own right (i.e. in a variable, rather than forcing it into keys/maps/fieldnames). Doing so will make your code simpler and more versatile.

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