All tall tables and arrays belong to the tall class. However, the underlying data type of a tall array can vary.

Create a datastore for the airlinesmall.csv data set. Select a subset of the variables to work with, and treat 'NA' values as missing data so that datastore replaces them with NaN values. Convert the datastore into a tall table.

varnames = {'Year', 'UniqueCarrier'};
ds = tabularTextDatastore('airlinesmall.csv','TreatAsMissing','NA',...
    'SelectedVariableNames',varnames);
tt = tall(ds)

tt =

  Mx2 tall table

    Year    UniqueCarrier
    ____    _____________

    1987       {'PS'}    
    1987       {'PS'}    
    1987       {'PS'}    
    1987       {'PS'}    
    1987       {'PS'}    
    1987       {'PS'}    
    1987       {'PS'}    
    1987       {'PS'}    
     :            :
     :            :

Determine the class of the tall table tt and the first table variable Year.

class(tt)

ans = 
'tall'

class(tt.Year)

ans = 
'tall'

Determine the underlying data types of tt, as well as the Year and UniqueCarrier table variables.

classUnderlying(tt)

ans =

  1x5 tall char array

    'table'

classUnderlying(tt.Year)

ans =

  1x6 tall char array

    'double'

classUnderlying(tt.UniqueCarrier)

ans =

  1x4 tall char array

    'cell'

In some cases, the result returned by classUnderlying is an unevaluated tall array. Unevaluated tall arrays can be evaluated using the gather function to bring the result into memory.