Advantages of Using Categorical Arrays
In this section...
“Natural Representation of Categorical Data” on page 8-34
“Mathematical Ordering for Character Vectors” on page 8-34
“Reduce Memory Requirements” on page 8-34
Natural Representation of Categorical Data
categorical is a data type to store data with values from a finite set of discrete categories. One
common alternative to using categorical arrays is to use character arrays or cell arrays of character
vectors. To compare values in character arrays and cell arrays of character vectors, you must use
strcmp which can be cumbersome. With categorical arrays, you can use the logical operator eq (==)
to compare elements in the same way that you compare numeric arrays. The other common
alternative to using categorical arrays is to store categorical data using integers in numeric arrays.
Using numeric arrays loses all the useful descriptive information from the category names, and also
tends to suggest that the integer values have their usual numeric meaning, which, for categorical
data, they do not.
Mathematical Ordering for Character Vectors
Categorical arrays are convenient and memory efficient containers for nonnumeric data with values
from a finite set of discrete categories. They are especially useful when the categories have a
meaningful mathematical ordering, such as an array with entries from the discrete set of categories
{'small','medium','large'} where small < medium < large.
An ordering other than alphabetical order is not possible with character arrays or cell arrays of
character vectors. Thus, inequality comparisons, such as greater and less than, are not possible. With
categorical arrays, you can use relational operations to test for equality and perform element-wise
comparisons that have a meaningful mathematical ordering.
Reduce Memory Requirements
This example shows how to compare the memory required to store data as a cell array of character
vectors versus a categorical array. Categorical arrays have categories that are defined as character
vectors, which can be costly to store and manipulate in a cell array of character vectors or char
array. Categorical arrays store only one copy of each category name, often reducing the amount of
memory required to store the array.
Create a sample cell array of character vectors.
state = [repmat({'MA'},25,1);repmat({'NY'},25,1);...
repmat({'CA'},50,1);...
repmat({'MA'},25,1);repmat({'NY'},25,1)];
Display information about the variable state.
whos state
8 Categorical Arrays
8-34
In this section...
“Natural Representation of Categorical Data” on page 8-34
“Mathematical Ordering for Character Vectors” on page 8-34
“Reduce Memory Requirements” on page 8-34
Natural Representation of Categorical Data
categorical is a data type to store data with values from a finite set of discrete categories. One
common alternative to using categorical arrays is to use character arrays or cell arrays of character
vectors. To compare values in character arrays and cell arrays of character vectors, you must use
strcmp which can be cumbersome. With categorical arrays, you can use the logical operator eq (==)
to compare elements in the same way that you compare numeric arrays. The other common
alternative to using categorical arrays is to store categorical data using integers in numeric arrays.
Using numeric arrays loses all the useful descriptive information from the category names, and also
tends to suggest that the integer values have their usual numeric meaning, which, for categorical
data, they do not.
Mathematical Ordering for Character Vectors
Categorical arrays are convenient and memory efficient containers for nonnumeric data with values
from a finite set of discrete categories. They are especially useful when the categories have a
meaningful mathematical ordering, such as an array with entries from the discrete set of categories
{'small','medium','large'} where small < medium < large.
An ordering other than alphabetical order is not possible with character arrays or cell arrays of
character vectors. Thus, inequality comparisons, such as greater and less than, are not possible. With
categorical arrays, you can use relational operations to test for equality and perform element-wise
comparisons that have a meaningful mathematical ordering.
Reduce Memory Requirements
This example shows how to compare the memory required to store data as a cell array of character
vectors versus a categorical array. Categorical arrays have categories that are defined as character
vectors, which can be costly to store and manipulate in a cell array of character vectors or char
array. Categorical arrays store only one copy of each category name, often reducing the amount of
memory required to store the array.
Create a sample cell array of character vectors.
state = [repmat({'MA'},25,1);repmat({'NY'},25,1);...
repmat({'CA'},50,1);...
repmat({'MA'},25,1);repmat({'NY'},25,1)];
Display information about the variable state.
whos state
8 Categorical Arrays
8-34