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Is it possible to have a table with a numerical index instead of strings?

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Bill Tubbs
Bill Tubbs on 30 Mar 2020
Commented: Peng Li on 31 Mar 2020
I'm new to MATLAB and wanted to make a table containing time-series data where the index (time) is a series of integers.
I tried this but I think it doesn't like the fact that the row names are not strings.
>> N = 5;
>> k_init = 5;
>> u = zeros(N+k_init,1);
>> y = zeros(N+k_init,1);
>> ts = [1-k_init:N]';
>> data = table(u,y,'RowNames',num2cell(ts));
Error using table (line 326)
The RowNames property must be a string array or a cell array, with each element containing one nonempty name.
>> class(num2cell(ts))
ans =
'cell'
It seems to only work with strings:
>> ts = {'-4','-3','-2','-1','0','1','2','3','4','5'};
>> data = table(u, y,'RowNames',ts)
data =
10×2 table
u y
_ _
-4 0 0
-3 0 0
-2 0 0
-1 0 0
0 0 0
1 0 0
2 0 0
3 0 0
4 0 0
5 0 0
What I am ultimately trying to do is access the data using the ts as an index, something like this:
>> k = 2
>> data(k,'y') = 2*data(k-1,'u')
(Similar to a DataFrame in Python)
  1 Comment
Bill Tubbs
Bill Tubbs on 30 Mar 2020
An alternative perhaps: Is there any way to index an iddata object with a timestep value when it doesn't start at 1?
>> data = iddata(y,u,1,'Tstart',-4);
>> data.y(-4)
Error using iddata/subsref (line 49)
Index in position 1 is invalid. Array indices must be positive integers or logical values.
Other than data.u(-4+data.Tstart) obviously.

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Accepted Answer

Peng Li
Peng Li on 30 Mar 2020
Why do you want to make the time as row names? I think it's better to keep it as one of the variable in the table. It is not necessary that each row has a name.
It's easier to index table by dot indexing. e.g., data.u(k) and data.y(k-1) for example. If you make ts as a variable, you can use for example data.u(data.ts == 2) to index variable u in the table where ts is equal to 2.
  5 Comments
Peng Li
Peng Li on 31 Mar 2020
Oh yeah this is correct. So better apply things according to our own understanding of our data.

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More Answers (2)

Fangjun Jiang
Fangjun Jiang on 30 Mar 2020
Keep in mind that in MATLAB, indexing like data(RowIndex, ColIndex) can be numerical or logical. The numerical index is 1-based. It can't be negative numerical value. It can't be zero like in C.
Your value is -4 to 5. There is an easy way to specify an offset.
The other thing you might be able to use is cellstr(num2str((-4:5)'))
  2 Comments
Bill Tubbs
Bill Tubbs on 30 Mar 2020
Given that this seems to be impossible, I think the best solution might be to give up and just maintain two seperate variables, one for the actual timestep (k=-4:5) and another for the index (i=1:10).
In other words:
>> k = 2
>> i = k - k_start;
>> data(i,'y') = 2*data(i-1,'u')
However, this only works here were the timestep is 1.

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