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Advantages of Using Tables

Conveniently Store Mixed-Type Data in Single Container

You can use the table data type to collect mixed-type data and metadata properties, such as variable name, row names, descriptions, and variable units, in a single container. Tables are suitable for column-oriented or tabular data that is often stored as columns in a text file or in a spreadsheet. For example, you can use a table to store experimental data, with rows representing different observations and columns representing different measured variables.

Tables consist of rows and column-oriented variables. Each variable in a table can have a different data type and a different size, but each variable must have the same number of rows.

For example, load sample patients data.

load patients

Then, combine the workspace variables, Systolic and Diastolic into a single BloodPressure variable and convert the workspace variable, Gender, from a cell array of character vectors to a categorical array.

BloodPressure = [Systolic Diastolic];
Gender = categorical(Gender);

  Name                 Size            Bytes  Class          Attributes

  Age                100x1               800  double                   
  BloodPressure      100x2              1600  double                   
  Gender             100x1               346  categorical              
  Smoker             100x1               100  logical                  

The variables Age, BloodPressure, Gender, and Smoker have varying data types and are candidates to store in a table since they all have the same number of rows, 100.

Now, create a table from the variables and display the first five rows.

T = table(Gender,Age,Smoker,BloodPressure);
ans=5x4 table
    Gender    Age    Smoker    BloodPressure
    ______    ___    ______    _____________

    Male      38     true      124     93   
    Male      43     false     109     77   
    Female    38     false     125     83   
    Female    40     false     117     75   
    Female    49     false     122     80   

The table displays in a tabular format with the variable names at the top.

Each variable in a table is a single data type. If you add a new row to the table, MATLAB® forces consistency of the data type between the new data and the corresponding table variables. For example, if you try to add information for a new patient where the first column contains the patient's age instead of gender, as in the expression T(end+1,:) = {37,{'Female'},true,[130 84]}, then you receive the error:

Invalid RHS for assignment to a categorical array.

The error occurs because MATLAB® cannot assign numeric data, 37, to the categorical array, Gender.

For comparison of tables with structures, consider the structure array, StructArray, that is equivalent to the table, T.

StructArray = table2struct(T)
StructArray = 100x1 struct array with fields:

Structure arrays organize records using named fields. Each field's value can have a different data type or size. Now, display the named fields for the first element of StructArray.

ans = struct with fields:
           Gender: Male
              Age: 38
           Smoker: 1
    BloodPressure: [124 93]

Fields in a structure array are analogous to variables in a table. However, unlike with tables, you cannot enforce homogeneity within a field. For example, you can have some values of S.Gender that are categorical array elements, Male or Female, others that are character vectors, 'Male' or 'Female', and others that are integers, 0 or 1.

Now consider the same data stored in a scalar structure, with four fields each containing one variable from the table.

ScalarStruct = struct(...
ScalarStruct = struct with fields:
           Gender: [100x1 categorical]
              Age: [100x1 double]
           Smoker: [100x1 logical]
    BloodPressure: [100x2 double]

Unlike with tables, you cannot enforce that the data is rectangular. For example, the field ScalarStruct.Age can be a different length than the other fields.

A table allows you to maintain the rectangular structure (like a structure array) and enforce homogeneity of variables (like fields in a scalar structure). Although cell arrays do not have named fields, they have many of the same disadvantages as structure arrays and scalar structures. If you have rectangular data that is homogeneous in each variable, consider using a table. Then you can use numeric or named indexing, and you can use table properties to store metadata.

Access Data Using Numeric or Named Indexing

You can index into a table using parentheses, curly braces, or dot indexing. Parentheses allow you to select a subset of the data in a table and preserve the table container. Curly braces and dot indexing allow you to extract data from a table. Within each table indexing method, you can specify the rows or variables to access by name or by numeric index.

Consider the sample table from above. Each row in the table, T, represents a different patient. The workspace variable, LastName, contains unique identifiers for the 100 rows. Add row names to the table by setting the RowNames property to LastName and display the first five rows of the updated table.

T.Properties.RowNames = LastName;
ans=5x4 table
                Gender    Age    Smoker    BloodPressure
                ______    ___    ______    _____________

    Smith       Male      38     true      124     93   
    Johnson     Male      43     false     109     77   
    Williams    Female    38     false     125     83   
    Jones       Female    40     false     117     75   
    Brown       Female    49     false     122     80   

In addition to labeling the data, you can use row and variable names to access data in the table. For example, use named indexing to display the age and blood pressure of the patients Williams and Brown.

ans=2x2 table
                Age    BloodPressure
                ___    _____________

    Williams    38     125     83   
    Brown       49     122     80   

Now, use numeric indexing to return an equivalent subtable. Return the third and fifth row from the second and fourth variables.

ans=2x2 table
                Age    BloodPressure
                ___    _____________

    Williams    38     125     83   
    Brown       49     122     80   

With cell arrays or structures, you do not have the same flexibility to use named or numeric indexing.

  • With a cell array, you must use strcmp to find desired named data, and then you can index into the array.

  • With a scalar structure or structure array, it is not possible to refer to a field by number. Furthermore, with a scalar structure, you cannot easily select a subset of variables or a subset of observations. With a structure array, you can select a subset of observations, but you cannot select a subset of variables.

  • With a table, you can access data by named index or by numeric index. Furthermore, you can easily select a subset of variables and a subset of rows.

For more information on table indexing, see Access Data in a Table.

Use Table Properties to Store Metadata

In addition to storing data, tables have properties to store metadata, such as variable names, row names, descriptions, and variable units. You can access a property using T.Properties.PropName, where T is the name of the table and PropName is one of the table properties.

For example, add a table description, variable descriptions, and variable units for Age.

T.Properties.Description = 'Simulated Patient Data';

T.Properties.VariableDescriptions = ...
    {'Male or Female' ...
    '' ...
    'true or false' ...

T.Properties.VariableUnits{'Age'} = 'Yrs';

Individual empty character vectors within the cell array for VariableDescriptions indicate that the corresponding variable does not have a description. For more information, see the Properties section of table.

To print a table summary, use the summary function.

Description:  Simulated Patient Data


    Gender: 100x1 categorical

        Description:  Male or Female

            Female    53      
            Male      47      

    Age: 100x1 double

        Units:  Yrs

            Min       25   
            Median    39   
            Max       50   

    Smoker: 100x1 logical

        Description:  true or false

            True     34      
            False    66      

    BloodPressure: 100x2 double

        Description:  Systolic/Diastolic
                      BloodPressure_1    BloodPressure_2
                      _______________    _______________

            Min       109                  68           
            Median    122                81.5           
            Max       138                  99           

Structures and cell arrays do not have properties for storing metadata.

See Also


Related Topics

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