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

Asked by Andrei Makarskiy on 20 Sep 2015
Latest activity Commented on by Peter Perkins
on 21 Jul 2017
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

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2 Answers

Answer by Walter Roberson
on 20 Sep 2015
 Accepted Answer

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

  2 Comments

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

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Answer by Peter Perkins
on 21 Sep 2015

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.

  5 Comments

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

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