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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 strings. To compare strings in character arrays and cell arrays of strings, you must use strcmp which can be cumbersome. With categorical arrays, you can use the logical operator eq (==) to compare strings 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.
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 strings. 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 of strings that have a meaningful mathematical ordering.
This example shows how to compare the memory required to store data as a cell array of strings versus a categorical array. Categorical arrays have categories that are defined as strings, which can be costly to store and manipulate in a cell array of strings 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 strings.
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
Name Size Bytes Class Attributes state 150x1 17400 cell
The variable state is a cell array of strings requiring 17,400 bytes of memory.
Convert state to a categorical array.
state = categorical(state);
Display the discrete categories in the variable state.
categories(state)
ans = 'CA' 'MA' 'NY'
state contains 150 elements, but only three distinct categories.
Display information about the variable state.
whos state
Name Size Bytes Class Attributes state 150x1 754 categorical
There is a significant reduction in the memory required to store the variable.