reordercats

Reorder categories in categorical array

Syntax

B = reordercats(A)

B = reordercats(A,neworder)

Description

B = reordercats(A) reorders the categories in the categorical array, A, to be in alphanumeric order.

The order of the categories is used by functions such as summary and histogram. If the categorical array is ordinal, the order of the categories defines their mathematical ordering. The first category specified is the smallest and the last category is the largest.

B = reordercats(A,neworder) puts the categories in the order specified by neworder.

Examples

Alphabetize Categories of Nonordinal Categorical Array

Create two categorical arrays, X and Y.

X = categorical({'frog';'cat';'cat';'ant';'frog'})

X = 5x1 categorical
     frog 
     cat 
     cat 
     ant 
     frog 

Y = categorical({'deer';'bear';'eagle';'deer'})

Y = 4x1 categorical
     deer 
     bear 
     eagle 
     deer 

X is a 5-by-1 categorical array. The categories of X are the sorted unique values from the array: {'ant';'cat';'frog'}.

Y is a 4-by-1 categorical array. The categories of Y are the sorted unique values from the array: {'bear';'deer';'eagle'}.

Concatenate X and Y into a single categorical array, A.

A = [X;Y]

A = 9x1 categorical
     frog 
     cat 
     cat 
     ant 
     frog 
     deer 
     bear 
     eagle 
     deer 

vertcat appends the values from Y to the values from X.

List the categories of the categorical array, A.

acats = categories(A)

acats = 6x1 cell
    {'ant'  }
    {'cat'  }
    {'frog' }
    {'bear' }
    {'deer' }
    {'eagle'}

vertcat appends the categories of Y to the categories from X. The categories of A are not in alphabetical order.

Reorder the categories of A into alphabetical order.

B = reordercats(A)

B = 9x1 categorical
     frog 
     cat 
     cat 
     ant 
     frog 
     deer 
     bear 
     eagle 
     deer 

The output categorical array, B, has the same elements in the same order as the input categorical array, A.

List the categories of the categorical array, B.

bcats = categories(B)

bcats = 6x1 cell
    {'ant'  }
    {'bear' }
    {'cat'  }
    {'deer' }
    {'eagle'}
    {'frog' }

The categories of B are in alphabetical order.

Reorder Categories in Nonordinal Categorical Array

Create a categorical array containing the color of various items.

A = categorical({'red';'green';'blue';'red';'green';'red';'blue';'blue'})

A = 8x1 categorical
     red 
     green 
     blue 
     red 
     green 
     red 
     blue 
     blue 

A is an 8-by-1 categorical array.

Display the categories of A.

categories(A)

ans = 3x1 cell
    {'blue' }
    {'green'}
    {'red'  }

The categories of A are in alphabetical order and have no mathematical meaning.

Reorder the categories to match the order commonly used for colors.

B = reordercats(A,{'red','green','blue'})

B = 8x1 categorical
     red 
     green 
     blue 
     red 
     green 
     red 
     blue 
     blue 

B contains the same values as A.

Display the categories of B.

categories(B)

ans = 3x1 cell
    {'red'  }
    {'green'}
    {'blue' }

B is not ordinal and the order of the categories has no mathematical meaning. Although the categories appear in the order of the color spectrum, relational operations, such as greater than and less than, have no meaning.

Reorder Categories in Ordinal Categorical Array

Create an ordinal categorical array, A, containing modes of transportation. Order the categories based on the average price of travel.

A = categorical({'plane';'car'; 'train';'car';'plane';'car'},...
    {'car','train','plane'},'Ordinal',true)

A = 6x1 categorical
     plane 
     car 
     train 
     car 
     plane 
     car 

A is a 6-by-1 ordinal categorical array.

Display the categories of A.

categories(A)

ans = 3x1 cell
    {'car'  }
    {'train'}
    {'plane'}

Since A is ordinal, car < train < plane.

Reorder the categories to reflect a decrease in the cost of train travel.

B = reordercats(A,{'train','car','plane'})

B = 6x1 categorical
     plane 
     car 
     train 
     car 
     plane 
     car 

B contains the same values as A.

Display the categories of B.

categories(B)

ans = 3x1 cell
    {'train'}
    {'car'  }
    {'plane'}

The mathematical ordering of the categories is now train < car < plane. The results from relational operations, min, and max reflect the new category ordering.

Reorder Categories with Numeric Vector

Create a categorical array, A, containing modes of transportation.

A = categorical({'plane';'car';'train';'car';'car';'plane';'car'})

A = 7x1 categorical
     plane 
     car 
     train 
     car 
     car 
     plane 
     car 

Display the categories of A.

categories(A)

ans = 3x1 cell
    {'car'  }
    {'plane'}
    {'train'}

Reorder categories from least to most frequent occurrence in A.

B = countcats(A);
[C,neworder] = sort(B);
neworder

neworder = 3×1

     3
     2
     1

D = reordercats(A,neworder);
categories(D)

ans = 3x1 cell
    {'train'}
    {'plane'}
    {'car'  }

Because countcats counts the occurrences of each category, neworder describes how to reorder the categories—not the elements—of A.

Specify Categories by Using Pattern

Create a categorical array. This array has many different categories that stand for "yes" and "no".

C = categorical(["Y" "Yes" "Yeah" "N" "No" "Nope"])

C = 1x6 categorical
     Y      Yes      Yeah      N      No      Nope 

List the categories in order. By default, the sort order of these categories is alphabetical order, because MATLAB® stores characters as Unicode®.

categories(C)

ans = 6x1 cell
    {'N'   }
    {'No'  }
    {'Nope'}
    {'Y'   }
    {'Yeah'}
    {'Yes' }

You can match multiple category names by using a pattern. For example, to specify category names that start with a Y, you can use a wildcard pattern. To create a wildcard pattern, use the wildcardPattern function.

Reorder the categories. Change the sort order so that the categories that start with Y come before the categories that start with N.

C = reordercats(C,["Y"+wildcardPattern,"N"+wildcardPattern])

C = 1x6 categorical
     Y      Yes      Yeah      N      No      Nope 

List the categories in their new order.

categories(C)

ans = 6x1 cell
    {'Y'   }
    {'Yeah'}
    {'Yes' }
    {'N'   }
    {'No'  }
    {'Nope'}

Input Arguments

A — Input array

categorical array

Input array, specified as a categorical array. If A is an ordinal categorical array, a reordering of the categories changes the mathematical meaning. Consequently, the relational operators, such as greater than and less than, might return different results.

neworder — New category order for B

string array | cell array of character vectors | numeric vector | pattern array

New category order for B, specified as a string array, cell array of character vectors, numeric vector, or pattern array. neworder must be a permutation of categories(A).

Tips

• To convert the categorical array, B, to an ordinal categorical array, use B = categorical(B,'Ordinal',true). You can specify the order of the categories with B = categorical(B,valueset,'Ordinal',true), where the order of the values in valueset defines the category order.

Extended Capabilities

Tall Arrays

Calculate with arrays that have more rows than fit in memory.

Usage notes and limitations:

• The neworder input argument does not support pattern expressions.

For more information, see Tall Arrays.

C/C++ Code Generation

Generate C and C++ code using MATLAB® Coder™.

Usage notes and limitations:

• The neworder input argument does not support pattern expressions.

For more information, see Code Generation for Categorical Arrays (MATLAB Coder).

Thread-Based Environment

Run code in the background using MATLAB® backgroundPool or accelerate code with Parallel Computing Toolbox™ ThreadPool.

This function fully supports thread-based environments. For more information, see Run MATLAB Functions in Thread-Based Environment.

Distributed Arrays

Partition large arrays across the combined memory of your cluster using Parallel Computing Toolbox™.

This function fully supports distributed arrays. For more information, see Run MATLAB Functions with Distributed Arrays (Parallel Computing Toolbox).

Version History

Introduced in R2013b