MATLAB: How to batch convert table columns to categorical

Question

I have a large table (large number of columns and very large number of rows), many of these columns contain simple cell arrays with a very limited number of different options (such as 'ON" and 'OFF' or similar).

I would like to convert all these columns to categorical in an automated way, i.e. extracting column names from

Table.Properties.VariableNames

and not explicitly writing the (large) number of column names in dot notation.

I have seen that

>> ddd2 = categorical(MyTable{:,MyTable.Properties.VariableNames(1)});

works fine (i.e. ddd2 is a correct categorical array), but, at this point, I cannot put this categorical array back in my table. I tried:

>> MyTable{:,MyTable.Properties.VariableNames{1}}= ddd2;
The following error occurred converting from categorical to cell:
Conversion to cell from categorical is not possible.

but it returns the error you see above… On the other hand, if I use dot notation it works fine, i.e.

MyTable.Column1 = ddd2;

yields the desired results. I found a workaround, that works perfectly, but I would like to know if there is a more direct and elegant way to solve this problem…. this is the workaround:

sz0=size(MyTable);
kk=0;
for( i=1:sz0(2) )
    if( iscell(MyTable{:,i}) )
        ddd2 = categorical(MyTable{:,MyTable.Properties.VariableNames(i)});
        eval(sprintf('MyTable.%s = ddd2;',MyTable.Properties.VariableNames{i}));
        kk = kk+1;
    end
end
fprintf( ' %d columns transformed to categorical\n', kk );

it works…

Answer 1

In 18b there is a convenience function to do exactly this:

https://www.mathworks.com/help/releases/R2018b/matlab/ref/convertvars.html

Note, that it may be possible to just create your table with categoricals. How are you creating the table?