How to convert nominal variable to number?

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I have a nominal variable which I would like to convert to double. Just using 'double' function won't work. I found a workaround by first converting the nominal variable to string (then 'str2num' works). I was wondering is there a more elegant 'str2double' style function to convert nominal to double? Also - why str2num works whereas str2double fails to convert appropriately?

double_var = [10,20,30];
nominal_var = nominal(double_var);
back2double_1 = double(nominal_var); % wrong
back2double_2 = str2num(char(nominal_var)); % workaround that works
back2double_3 = str2double(char(nominal_var)); % workaround that doesn't work