Is there a simpler way to create an empty Table with a list of VariableNames?

I had to find a long long way around to solve this simple task.

optionsChange = cell2table(cell(0,9));
optionsChange.Properties.VariableNames{'Var1'}='Exp';
optionsChange.Properties.VariableNames{'Var2'}='Strike';
optionsChange.Properties.VariableNames{'Var3'}='Put_Mark';
optionsChange.Properties.VariableNames{'Var4'}='Put_Ask';
optionsChange.Properties.VariableNames{'Var5'}='Put_Bid';
optionsChange.Properties.VariableNames{'Var6'}='Put_Delta';
optionsChange.Properties.VariableNames{'Var7'}='Put_ImplVol';
optionsChange.Properties.VariableNames{'Var8'}='Date';
optionsChange.Properties.VariableNames{'Var9'}='DateNM';

Isn't there a nicer way to do it? Matlab really gets on my nerv after PHP and VBA

Answer 1

[Walter Roberson]

optionsChange  = cell2table(cell(0,9), VariableNames, {'Exp', 'Strike', 'Put_Mark', 'Put_Ask', 'Put_Bid', 'Put_Delta', 'Put_ImplVol', 'Date', 'DateNM'});

[Andrei Makarskiy]

optionsChange  = cell2table(cell(0,9), VariableNames, {'Exp', 'Strike', 'Put_Mark', 'Put_Ask', 'Put_Bid', 'Put_Delta', 'Put_ImplVol', 'Date', 'DateNM'});
I get this error
| Undefined function or variable 'VariableNames'.|

[Walter Roberson]

optionsChange  = cell2table(cell(0,9), 'VariableNames', {'Exp', 'Strike', 'Put_Mark', 'Put_Ask', 'Put_Bid', 'Put_Delta', 'Put_ImplVol', 'Date', 'DateNM'});

[Steven]

This answer is poor quality

Answer 2

[Peter Perkins]

Andrei, Walter's solution works, as would

optionsChange = array2table(zeros(0,9), 'VariableNames',{...});

or even

optionsChange = array2table(zeros(0,9));
optionsChange.Properties.VariableNames = {...};

(Both of those have the advantage that they create 0x1 variables in the table, rather than 0x0, and so if you subsequently do something like

>> optionsChange.Exp(2) = 2
optionsChange = 
    Exp    Strike    Put_Mark    Put_Ask    Put_Bid    Put_Delta    Put_ImplVol    Date    DateNM
    ___    ______    ________    _______    _______    _________    ___________    ____    ______
    0      0         0           0          0          0            0              0       0     
    2      0         0           0          0          0            0              0       0

everything will grow in the vertical direction.)

BUT: based on your variable names, you may not end up what you're ultimately looking for. The problem is that "create an empty table" isn't really fully specified. It's kind of like saying, "create an empty variable". The question left unanswered is, what type of variables do you want in that table? The answer might be "all doubles", but it might not. The above all create nine double variables in the table, but your last two variable names indicate dates. If you're using datenums, you're fine (datenums are are just doubles, but if you want date strings, or datetimes, you'll have to either start out with the right thing, or overwrite the doubles in the table that you want to be dates. For example,

>> vnames = {'Exp', 'Strike', 'Put_Mark', 'Put_Ask', 'Put_Bid', 'Put_Delta', 'Put_ImplVol', 'Date', 'DateNM'};
>> optionsChange = array2table(zeros(0,9), 'VariableNames',vnames);
>> optionsChange.Date = datetime(zeros(0,3)); % simple way to get a 0x1 datetime
>> optionsChange.DateNM = datetime(zeros(0,3));
>> summary(optionsChange)
Variables:
    Exp: 0x1 double
    Strike: 0x1 double
    Put_Mark: 0x1 double
    Put_Ask: 0x1 double
    Put_Bid: 0x1 double
    Put_Delta: 0x1 double
    Put_ImplVol: 0x1 double
    Date: 0x1 datetime
    DateNM: 0x1 datetime

And finally, as is the case with any kind of variable in MATLAB, instead of creating an empty then growing it row by row, you might consider creating a table that's the right size but filled with NaNs and empty strings or whatever. For example,

optionsChange = array2table(nan(0,9));

That may be faster in the long run, because it doesn't have to keep reallocating memory as the table grows. That may or may not be useful or even possible in your case, just a suggestion.

Hope this helps.

[Andrei Makarskiy]

Thanks, Peter. Very informative answer. Let's go on further... Suppose, I created this table optionsChange and filled it with rows of data. Then later on I decided to add a new column (variable). If I do this like, Lets assume I didn't create the Date column from the beginning:

optionsChange.Date = datetime(zeros(0,3));
I get en error:

Error using .... (line 138)
To assign to or create a variable in a table, the number of rows must match the
height of the table.

The point is, do I always have to count the number of rows and create a new column with the same number of rows? It takes time and is very unproductive.

[Peter Perkins]

You might do one of two things. Starting from this table ...

>> optionsChange = array2table(rand(5,2))
optionsChange = 
     Var1       Var2  
    _______    _______
    0.81472    0.09754
    0.90579     0.2785
    0.12699    0.54688
    0.91338    0.95751
    0.63236    0.96489

... you can add a new variable, but it has to be the correct height. You can get that height from the table you're assigning into:

>> optionsChange.Date = NaT(height(optionsChange),1)
optionsChange = 
     Var1       Var2      Date
    _______    _______    ____
    0.81472    0.09754    NaT 
    0.90579     0.2785    NaT 
    0.12699    0.54688    NaT 
    0.91338    0.95751    NaT 
    0.63236    0.96489    NaT

(By the way, it occurred to me that using the NaT function is a simpler way to preallocate a datetime than something like datetime(zeros(0,3))).

Or you can assign to a specific element of a new variable:

>> optionsChange = array2table(rand(5,2))
optionsChange = 
      Var1        Var2  
    ________    ________
     0.70605     0.82346
    0.031833     0.69483
     0.27692      0.3171
    0.046171     0.95022
    0.097132    0.034446
>> optionsChange.Date(5,1) = datetime('now')
optionsChange = 
      Var1        Var2              Date        
    ________    ________    ____________________
     0.70605     0.82346    NaT                 
    0.031833     0.69483    NaT                 
     0.27692      0.3171    NaT                 
    0.046171     0.95022    NaT                 
    0.097132    0.034446    25-Sep-2015 09:02:26

Notice the column number in Date(5,1): that's because Date does not exist and assigning to Date(5) would follow standard MATLAB behavior and create the 5th element of a row vector.

You can even assign off the end, and the other variables will gro, but you will get a warning about that.

>> optionsChange = array2table(rand(5,2));
>> optionsChange.Date(6,1) = datetime('now')
Warning: The assignment added rows to the table, but did not assign values to all of the table's existing variables. Those
variables have been extended with rows containing default values. 
> In table/subsasgnDot (line 285)
  In table/subsasgn (line 67) 
optionsChange = 
     Var1       Var2              Date        
    _______    _______    ____________________
    0.75127    0.95929    NaT                 
     0.2551    0.54722    NaT                 
    0.50596    0.13862    NaT                 
    0.69908    0.14929    NaT                 
     0.8909    0.25751    NaT                 
          0          0    25-Sep-2015 09:03:21

Hope this helps.

[Lachlan Noller]

Just wondering how you would round numbers in a table. I keep getting numbers like -5.2018e+05 but i want them rounded to whole integers (i.e. -520181)

[Walter Roberson]

Lachlan Noller: the easiest thing is to round the numbers before putting them in the table.

[Peter Perkins]

Also, they may already be rounded, and what you're seeing is just the shortg display. Perhaps just set your command window to use the longg format.

Answer 3

[Markus de Ruijter]

I know this is an old question but I usually do the following to initialise a new empty table:

% Make N by 2 matrix of fieldname + value type
variable_names_types = [["id", "double"]; ...
			["x", "double"]; ...
			["y", "double"]; ...
			["time", "double"]; ...
			["type", "string"]; ...
			["description", "string"]];

% Make table using fieldnames & value types from above
my_table = table('Size',[0,size(variable_names_types,1)],... 
	'VariableNames', variable_names_types(:,1),...
	'VariableTypes', variable_names_types(:,2));

I think this is quite readable and allows you to add value types.

[Steven Lord]

FYI the functionality you're using in your answer, the ability to specify 'Size' and 'VariableTypes' in the table constructor, was introduced in release R2018a which is why it wasn't suggested previously.

[Markus de Ruijter]

Ah I see, then my answer might be useful for people using later versions of matlab. Thanks for the addition!