reordercats

When you create a categorical array, the categories always have a certain order. But if you add categories later, then the new set of categories might be out of order. To put categories in alphanumeric order, use the reordercats function.

For example, concatenate two categorical arrays. Then reorder the categories of the result.

First create the arrays.

X = categorical(["Frog" "Cat" "Cat" "Ant" "Frog"])

X = 1x5 categorical
     Frog      Cat      Cat      Ant      Frog 

Y = categorical(["Deer" "Bear" "Eagle" "Deer"])

Y = 1x4 categorical
     Deer      Bear      Eagle      Deer 

The categories of each array are in alphanumeric order.

Xcats = categories(X)

Xcats = 3x1 cell
    {'Ant' }
    {'Cat' }
    {'Frog'}

Ycats = categories(Y)

Ycats = 3x1 cell
    {'Bear' }
    {'Deer' }
    {'Eagle'}

Then concatenate X and Y into one categorical array.

A = [X Y]

A = 1x9 categorical
     Frog      Cat      Cat      Ant      Frog      Deer      Bear      Eagle      Deer 

List the categories. The combined set of categories is out of alphanumeric order. Concatenation simply appends one set of categories to the end of the other set.

Acats = categories(A)

Acats = 6x1 cell
    {'Ant'  }
    {'Cat'  }
    {'Frog' }
    {'Bear' }
    {'Deer' }
    {'Eagle'}

Reorder the categories. The output categorical array has the same elements as the input array.

B = reordercats(A)

B = 1x9 categorical
     Frog      Cat      Cat      Ant      Frog      Deer      Bear      Eagle      Deer 

List the categories. Now the combined set of categories is in alphanumeric order.

Bcats = categories(B)

Bcats = 6x1 cell
    {'Ant'  }
    {'Bear' }
    {'Cat'  }
    {'Deer' }
    {'Eagle'}
    {'Frog' }

Create a categorical array.

A = categorical(["red" "green" "blue" "red" "green" "red" "blue" "blue"])

A = 1x8 categorical
     red      green      blue      red      green      red      blue      blue 

Display the categories. They are in alphanumeric order.

categories(A)

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

Reorder the categories.

B = reordercats(A,["red" "green" "blue"])

B = 1x8 categorical
     red      green      blue      red      green      red      blue      blue 

Display the categories. They are now in the RGB order commonly used for the color spectrum.

categories(B)

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

Because the array is not an ordinal categorical array, the order of the categories has no mathematical meaning. So, while 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 that has modes of transportation. Order the categories based on the average cost of travel by each mode of transportation.

A = categorical(["plane" "car" "train" "car" "plane" "car"], ...
                ["car" "train" "plane"], ...
                Ordinal=true)

A = 1x6 categorical
     plane      car      train      car      plane      car 

Display the categories. Because the array A is ordinal, car < train < plane.

categories(A)

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

For example, any element whose category is plane or train is greater than the category car.

A(A > "car")

ans = 1x3 categorical
     plane      train      plane 

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

B = reordercats(A,["train" "car" "plane"])

B = 1x6 categorical
     plane      car      train      car      plane      car 

Display the categories. The mathematical ordering of the categories is now train < car < plane.

categories(B)

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

For example, train is no longer greater than car. Results from relational operations, min, and max reflect the new category ordering.

B(B > "car")

ans = 1x2 categorical
     plane      plane 

Create a categorical array that has modes of transportation.

A = categorical(["plane" "car" "train" "car" "car" "plane" "car"])

A = 1x7 categorical
     plane      car      train      car      car      plane      car 

Display the categories.

categories(A)

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

Count the number of times each category occurs in the array by using the countcats function.

B = countcats(A)

B = 1×3

     4     2     1

Create an order that goes from the category that occurs least frequently to the category that occurs most. To specify that order as a numeric vector, use the second output from the sort function. The output neworder describes how to reorder the categories—not the elements—of the categorical array.

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

neworder = 1×3

     3     2     1

Reorder categories from least to most frequent occurrence in the array.

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

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

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'}