Dynamically naming new table columns in a loop

I'm inputting large data sets (~300 x 500) and need to act on the data creating lots of new variables. I've got syntax for adding, deleting, moving columns but can't find anything that talks about how to assign values to a column being created in the same line. There are so many columns that need to be acted on in the same way I really need loops. The closest seems to be incorporating eval, but it's not doing what I expect. As a simplified example:

summaryTable = readtable(strcat(path,'/',name,ext)); %import big csv file
    %the imported column names are  ["VarA1"; "VarA2"; "VarB1"; "VarB2"]
    for i = 1:2
        eval(strcat(summaryTable, '.Multiplied', string(i))) = eval(strcat('summaryTable.VarA', string(i))) .* eval(strcat('summaryTable.VarB', string(i)))
    end

The eval statements work on the right, but assigning the new variable doesn't. What's the secret?

[Lisa Lafleur]

I just noticed the left notation is wrong. The 'summaryTable' should be inside the name like

eval(strcat('summaryTable.Multiplied', string(i)))

Answer 1

[Stephen23]

"What's the secret?"

Very simple: don't use EVAL.

It is simpler, much more robust, and much more efficient to add fields using the syntaxes given in the documentation:

https://www.mathworks.com/help/matlab/matlab_prog/access-data-in-a-table.html

For example:

T = readtable(..)
for k = 1:2
    F0 = "Mult"+k;
    F1 = "VarA"+k;
    F2 = "VarB"+k;
    T.(F0) = T.(F1) .* T.(F2);
end

[Lisa Lafleur]

Ahh, the evil eval statement. I wasn't famliar with the

T.(F0) =

notation. This means I need to add a line in my loops to dynamically define my new column name as a string, then I can add it to the table via assigning things to it with the T.(newName) notation. Not a huge deal.

Thanks!

Answer 2

[Voss]

Use dynamic field names (the documentation is for structs, but it works for tables too (evidently)):

VarA1 = [1 2 3].';
VarA2 = [1 2 3].'+3;
VarB1 = [1 2 3].'+6;
VarB2 = [1 2 3].'+9;
summaryTable = table(VarA1,VarA2,VarB1,VarB2);
display(summaryTable);

summaryTable = 3×4 table
    VarA1    VarA2    VarB1    VarB2
    _____    _____    _____    _____

      1        4        7       10  
      2        5        8       11  
      3        6        9       12  

for i = [1 2]
    str = num2str(i);
    summaryTable.(['Multiplied' str]) = summaryTable.(['VarA' str]) .* summaryTable.(['VarB' str]);
end
display(summaryTable);

summaryTable = 3×6 table
    VarA1    VarA2    VarB1    VarB2    Multiplied1    Multiplied2
    _____    _____    _____    _____    ___________    ___________

      1        4        7       10           7             40     
      2        5        8       11          16             55     
      3        6        9       12          27             72     

[Stephen23]

Link to the correct documentation (for tables, not structs):

https://www.mathworks.com/help/matlab/matlab_prog/access-data-in-a-table.html