How can I select multiple columns in a table using variable names and wildcards?

665 vues (au cours des 30 derniers jours)
Hi all,
I am trying to index and extract a series of variables from a table i've created based on the VariableNames. I would rather index using the VariableNames as opposed to numerical or logical index due to the fact that there might be different numbers of trials for the variables.
Using the below code I can call one variable at a time. The '_1' represents the trial number which could go up to '_20'.
C1vC2Trials = C1MPVNormVariables(:,'C1MPVNorm_1');
However when trying to use the * wildcard to call all trials for that variable I get an error.
Error: Unrecognized variable name 'C1MPVNorm_*'.
Is this possible to do with tables or is there a easy work around to find the number of VariableNames in the table which correspond to the ones I wish to extract?
Thanks in advance!

Réponse acceptée

Cedric
Cedric le 4 Oct 2017
Modifié(e) : Cedric le 4 Oct 2017
I am not using tables, so you should probably wait for a better answer, but here is one way that I could see. Say we have the table:
T =
5×4 table
Age Height Weight BloodPressure
___ ______ ______ _____________
Smith 38 71 176 124 93
Johnson 43 69 163 109 77
Williams 38 64 131 125 83
Jones 40 67 133 117 75
Brown 49 64 119 122 80
and we want all variables that contain the string 'ei' (pattern):
>> hasMatch = ~cellfun('isempty', regexp(T.Properties.VariableNames, 'ei', 'once')) ;
>> T(:, T.Properties.VariableNames(hasMatch))
ans =
5×2 table
Height Weight
______ ______
Smith 71 176
Johnson 69 163
Williams 64 131
Jones 67 133
Brown 64 119
Here we are using regular expressions, so you must use regexp types of "wildcards".
  4 commentaires
BenRaanan
BenRaanan le 14 Fév 2018
I believe the preferred syntax is:
>> T(:, contains(T.Properties.VariableNames, 'ei'))
Peter Perkins
Peter Perkins le 15 Fév 2018
Yes, in recent versions of MATLAB you should be able to avoid using regexp in most cases, by using things like contains and startsWith instead. In older versions, you can often use strfind, depending on how complicated the pattern is.
Even with regexp, I think you can skip using cellfun and just make a vectorized call to regexp.

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Plus de réponses (2)

Roman Romanovski
Roman Romanovski le 6 Mai 2019
T(:,ismember(T.Properties.VariableNames, {""list of variables""}))
This will get you there for the variable names part. Above already answered wild card.
> T(:, contains(T.Properties.VariableNames, 'C1MPVNorm_'))
  5 commentaires
Peter Perkins
Peter Perkins le 14 Mai 2019
Tables have variables, not columns. "Variables", because the things in a table can themselves have multiple columns. So yes, dot subscripting extracts exactly one variable.
The syntax to extract multiple variables is T{:,ListOfVars}.
Zzz
Zzz le 6 Mai 2021
The correct syntax is T(:, {'col_name_1', 'col_name_2'}) to get the data like a table.

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Chenye Shen
Chenye Shen le 22 Oct 2021
  2 commentaires
Peter Perkins
Peter Perkins le 21 Sep 2023
To expand on the link that Chenye has provided: new features are added all the time. In particular, tables have supported patters for variable names since ... I actualy don't recall exactly when. The doc in that link show an example, but for the record:
t = table([1;2;3],[4;5;6],[7;8;9],VariableNames=["x1" "x2" "y"])
t = 3×3 table
x1 x2 y __ __ _ 1 4 7 2 5 8 3 6 9
t(:,"x"+digitsPattern)
ans = 3×2 table
x1 x2 __ __ 1 4 2 5 3 6
Also, of course, use "strings", not 'cell arrays of char vectors'!
Steven Lord
Steven Lord le 21 Sep 2023
The ability to use pattern objects to index into table rows, variables, and properties was introduced in release R2022a.

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