Add Variables to workspace from a struct

Question

Jimmy Neutron 2020-11-19

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/653603-add-variables-to-workspace-from-a-struct

评论： Jimmy Neutron 2020-11-24

采纳的回答： Ameer Hamza

I have a struct called "Values" that has two headers - Name and Data.

My final goal is to export the variables as a .mat file that I would be able to use afterwards for other things

The ouput I want to accomplish is that it looks like this in the workspace when the .mat file is loaded into another .m file:

%Name             Value
S1               [3000000x1 double]
S2               [3000000x1 double]
S3               [3000000x1 double]
S4               [3000000x1 double]
S5               [3000000x1 double]

So far I have tried to accomplish this as follows:

Names = {Values(:).Name}';
Data = {Values(:).Data}';
assignin('base',string(Names(:)),cell2mat(Data(:)))

But it give me an error

Error using assignin
Invalid variable name "" in ASSIGNIN. 

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Answer 1

Ameer Hamza 2020-11-19

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/653603-add-variables-to-workspace-from-a-struct#answer_549853

编辑：Ameer Hamza 2020-11-19

It is not a good coding practice to create a variable name like S1, S2, ... https://www.mathworks.com/matlabcentral/answers/304528-tutorial-why-variables-should-not-be-named-dynamically-eval. It usually creates complex, inefficient, hard to debug code, and In the long term, such a code is hard to maintain. Keep the code in an array format. However, if you want to do this, then try something like this.

Names = {Values(:).Name}';
Data = {Values(:).Data}';
    
cellfun(@(x,y) assignin('base', x, y), Names, Data)

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Ameer Hamza 2020-11-19

Yes, that was the mistake in my answer. I have updated it. Can you share your 'Values' struct in a .mat file?

Jimmy Neutron 2020-11-21

Thank you for your reply.Ii have solved my error. It was that my "Values" struct contained boxes with names that had a '.' or a slash in them. When I replaced them with a underscore, everything worked fine :)

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Answer 2

Stephen23 2020-11-19

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/653603-add-variables-to-workspace-from-a-struct#answer_549903

编辑：Stephen23 2020-11-19

"My final goal is to export the variables as a .mat file that I would be able to use afterwards for other things"

Then creating variables names dynamically is entirely the wrong approach:

https://en.wikipedia.org/wiki/XY_problem

The robust and efficient solution is to use the -struct syntax of save, e.g.:

S(1).data = 1:3;
S(1).name = 'S1';
S(2).data = 4:6;
S(2).name = 'S2';
S(3).data = 7:9;
S(3).name = 'S3';
C = [{S.name};{S.data}];
T = struct(C{:});
save('myfile','-struct','T')

Checking the mat-file contents:

whos -file myfile.mat
  Name      Size            Bytes  Class     Attributes

  S1        1x3                24  double              
  S2        1x3                24  double              
  S3        1x3                24  double              

"The ouput I want to accomplish is that it looks like this in the workspace when the .mat file is loaded into another .m file:"

The recommended way to load data from a mat-file is into an output structure:

S = load('myfile')
S = struct with fields:
    S1: [1 2 3]
    S2: [4 5 6]
    S3: [7 8 9]

This avoids a number of bugs that can occur when loading directly into a workspace:

https://www.mathworks.com/matlabcentral/answers/304528-tutorial-why-variables-should-not-be-named-dynamically-eval

You can access the fieldnames dynamically:

https://www.mathworks.com/help/matlab/matlab_prog/generate-field-names-from-variables.html

Note that using numbered variables is a sign that you are doing something wrong. Most likely the data could be designed better so that your code would be simpler and more efficient, e.g. by replacing those pseudo-indices with real indices into an array (which could be a container type, e.g. cell, structure, table, etc.).

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Stephen23 2020-11-21

编辑：Stephen23 2020-11-21

"but I wanted to create a .mat file where once loaded all the variables were displayed right there"

That is exactly what my answer gives you. The difference is that my answer uses a more effiicent approach (i.e. no assignin or eval or the like), but it certainly gives exactly the same mat file (which is why I showed the content of the mat file using whos).

Loading into an output variable is recommended, but does not change the content of the mat file. You can load it into the workspace directly just as you describe.

Jimmy Neutron 2020-11-24