Save multiple columns of multiple csv files?

Question

lil brain 2022-1-20

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1633020-save-multiple-columns-of-multiple-csv-files

评论： lil brain 2022-1-22

采纳的回答： Voss

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Hi,

I have the following code:

[file_list, path_n] = uigetfile('.csv', 'Grab csv', 'Multiselect', 'on')
columns();
if ismatrix(file_list) == 0
    file_list = {file_list}
end
for i = 1:length(file_list)
    filename = file_list{i}
    data_in = readmatrix([path_n, filename]);
    % here I would like to import columns 7-27 for each file
    % but instead of saving only the last iteration of the loop I want to
    % save each in a new variable
    columns(i) = data_in(:,7:27);
end

Every time I run this I get an error that I dont understand.

Unable to perform assignment because the left and right sides have a different number of elements.
Error in test (line 10)
    columns(i) = data_in(:,7:27);

I have figured out how to get the 21 columns saved in columns but it only saves the last iteration. How do I save each iteration?

Help would be much appreciated.

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Stephen23 2022-1-21

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Simpler way to ensure that the output is a cell array of character vectors:

[file_list, path_n] = uigetfile(...);
file_list = cellstr(file_list); % <- easy

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Answer 1

Voss 2022-1-21

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链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1633020-save-multiple-columns-of-multiple-csv-files#answer_878715

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[file_list, path_n] = uigetfile('.csv', 'Grab csv', 'Multiselect', 'on');
if ~iscell(file_list)
    file_list = {file_list};
end
columns = cell(length(file_list),1);
for i = 1:length(file_list)
    filename = file_list{i};
    data_in = readmatrix([path_n, filename]);
    columns{i} = data_in(:,7:27);
end
% if you want to put all the data into one matrix:
columns = cell2mat(columns);

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Voss 2022-1-21

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That's correct: as written, they will all end up in a single matrix. But if you remove the line:

columns = cell2mat(columns);

then they'll be in a cell array where each cell contains a matrix from one file. You can access each file's data like:

columns{1}
columns{2}
columns{end}
% etc.

If all the matrices had the same size, you could put them in a 3D array instead, but a cell array of matrices of different sizes is preferable to multiple matrix variables called column_1, column_2, etc., in MATLAB. Basically, you shouldn't do the column_1, column_2 thing, and here is why: https://www.mathworks.com/matlabcentral/answers/304528-tutorial-why-variables-should-not-be-named-dynamically-eval

lil brain 2022-1-22

Hey Benjamin, wow thanks for the insightful answer and the good directions as well. Very much appreciated! I will have a go at implenting it as a cell array then. I havent had to much experience witht he different formats and so I just kind of used what I knew. Thanks again!

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