Adding a Column to a Table

Hello everyone, I've just started out with Matlab for uni and am trying to add a column to a table. I ran into the following error message

Error using .

To assign to or create a variable in a table, the number of rows must match the height of the table.

My code is written below:

%Creates a table called 'tab' using the data in the coronavirus-cases.csv
%file.
tab = readtable("coronavirus-cases.csv");
%Creates a row of zeros with the same height as tab.
newTabCol = zeros(height(tab), 1);
%Adds the column and titles it "Seven Day Average".
tab.newTabCol = "Seven Day Average";

What confuses me is that the height of newTabCol matches tab. Both have 153772 rows:

I'm sure I'm missing something basic but any help would be appreciated.

Kind regards,

John

Answer 1

[Dyuman Joshi]

The variable name/title should be used after using the dot notation inside parenthesis and the data to append should be on the right side of the equation.

%Creates a row of zeros with the same height as tab.
newTabCol = zeros(height(tab), 1);

%Adds the column and titles it "Seven Day Average".
tab.("Seven Day Average") = newTabCol;

[John]

Thanks so much Dyuman!

[Dyuman Joshi]

You are welcome!

Also, do check out Image Analst's suggestion as well.

Answer 2

[Image Analyst]

To add a column to a table, use addvars

help addvars

--- help for table/addvars ---

 ADDVARS Add variables to table or timetable.
    T2 = ADDVARS(T1, VAR1, ..., VARN) appends the arrays VAR1,...,VARN as
    variables to the table T1. VAR1,...,VARN can include arrays of any
    type, including tables and timetables. All input arguments must have
    the same number of rows.
 
    T2 = ADDVARS(..., 'Before', LOCATION) 
    T2 = ADDVARS(..., 'After',  LOCATION) inserts the variables either
    before or after the position specified by LOCATION. LOCATION is a
    positive integer, a variable name, or a logical vector containing a
    single true value. ADDVARS(..., 'After', width(T1)) is equivalent to
    the default behavior.
 
    T2 = ADDVARS(..., 'NewVariableNames', NEWNAMES) specifies the names of
    the variables added in T2. NEWNAMES is a string array or cell array
    containing the same number of names as the number of added variables.
 
    Examples:
 
       % Load a table
       load fatalities.mat
 
       % Add per-capita deaths to table
       fatalities = addvars(fatalities, ...
                    fatalities.deaths./fatalities.drivers, ...
                    'NewVariableNames', 'deathsPerThousand', ...
                    'After', 'drivers');
 
    See also REMOVEVARS, MOVEVARS, SPLITVARS, MERGEVARS.

Help for table/addvars is inherited from superclass tabular

    Documentation for table/addvars
       doc table/addvars

    Other uses of addvars

       codistributed/addvars    gpuArray/addvars    tall/addvars
       distributed/addvars      tabular/addvars

[Alessandro Livi]

I read all that, tried it and it didn't work (or I hadn't rotated the vector yet with ' )

What did work is

A = zeros([1 size(app.StimInputTable.Data,1)]);
app.StimInputTable.Data.Var4 = A';

I suppose that

A = zeros([1 size(app.StimInputTable.Data,1)]);
app.StimInputTable.Data = ADDVARS(app.StimInputTable.Data, A'); 

might also work if I get all the app.'s and .Data's and A' in the right place but noone explains about rotating the vector to a column in the Help or in answers copied from Help.

[Walter Roberson]

T = array2table([10 9 8])

T = 1x3 table
    Var1    Var2    Var3
    ____    ____    ____

     10      9       8  

T.GHI = [1 2 3 4]

T = 1x4 table
    Var1    Var2    Var3          GHI       
    ____    ____    ____    ________________

     10      9       8      1    2    3    4

The key here is that the variable to be added might legitimately have multiple columns, so the process of adding a variable cannot automatically flip vectors -- it might be the wrong thing to do.