reordercats

Reorder categories in categorical array

Description

example

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.

example

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

Examples

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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.

Input Arguments

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Categorical array, specified as a vector, matrix, or multidimensional 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.

New category order for B, specified as a cell array of character vectors, a string array, or a numeric vector. 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.