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.

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.

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.

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.