Adding a column to my table

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Austin 2023-9-15

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/2021782-adding-a-column-to-my-table

评论： Austin 2023-9-15

I am still quite new to Matlab, but I have a table where I am trying to add a column of 1 through k with k being the number of rows. The code I am using to set up the table is:

P = 'Filepath';
S = dir(fullfile(P,'*.csv'));
S = natsortfiles(S); 
for k = 1:numel(S)
    F = fullfile(P,S(k).name);
    S(k).data = csvread(F,245,230,[245 230 245 230]);
end

then I am trying to add a column at the end with:

S.time = transpose([1:k]);

but it keeps returning this error: Scalar structure required for this assignment.

How can I fix this?

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Stephen23 2023-9-15

编辑：Stephen23 2023-9-15

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"but I have a table ..."

Where?

The deprecated function CSVREAD returns a numeric matrix, whereas DIR returns a structure array. I do not see anywhere in your code where you would have a table.

"...where I am trying to add a column of 1 through k with k being the number of rows"

The number of rows of what exactly?

Note that k is the loop iterator, which here basically just tells your code to process the 1st, 2nd, 3rd, ... Nth file found by your DIR call. It is therefor unclear what k has to do with "time" or "rows".

If the aim is to add this field to the structure S:

S(1).time = 1;
S(2).time = 2;
S(3).time = 3;
..
S(N).time = N;

then all you are doing is replicating the implicitly defined indices of that structure array.

Note that square brackets are a concatenation operator whereas COLON generates a vector. So with this code:

[1:k]

you generate a vector using the COLON and then concatenate that vector with absolutely nothing else to obtain exactly the same vector. Why do you need to perform a superfluous concatenation of a vector with nothing else to get exactly the same vector?

In any case, lets try to figure out what you want to achieve. Given this structure array:

S(1).name = 'hello.csv';
S(1).date = '2023/09/15';
S(2).name = 'world.csv';
S(1).date = '2023/09/15';

please show the exact output that you want to obtain.

Austin 2023-9-15

Yes I realized that what I had was not a table around 5 minutes after posting this question. I used the struct2table command to convert it to a table and now I am able to manipulate it like I want. The k relating to time is because each file represents one second and therefore the number of the file is the same as the time in seconds that has passed, so I was just reusing that variable from the for loop. Thank you for your help.

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Answer 1

Cris LaPierre 2023-9-15

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https://ww2.mathworks.cn/matlabcentral/answers/2021782-adding-a-column-to-my-table#answer_1311657

编辑：Cris LaPierre 2023-9-15

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You have defined S as a multidimension structure (S(k)), not a table, so in order to add time to your structure, you must now include an index: S(1).time

for k = 1:2
    S(k).data = rand(5);
end
S(1).time = 1:10;
S(1)
ans = struct with fields:
    data: [5×5 double]
    time: [1 2 3 4 5 6 7 8 9 10]
S(2)
ans = struct with fields:
    data: [5×5 double]
    time: []