Add and Drop Category Levels

This example shows how to add and drop levels from a nominal or ordinal array.

Note

The nominal and ordinal array data types are not recommended. To represent ordered and unordered discrete, nonnumeric data, use the Categorical Arrays data type instead.

• Load sample data.

• Create an ordinal array.

• Check for undefined categories.

• Identify elements in undefined categories.

• Add a new category.

• Reorder category levels.

• Compare elements.

• Explore categories.

• Drop a category.

Load sample data.

load('examgrades')

The array grades contains exam scores from 0 to 100 on five exams for a sample of 120 students.

Create an ordinal array.

Assign letter grades to each student for each test using these categories.

Grade Range	Letter Grade
100	A+
90–99	A
80–89	B
70–79	C
60–69	D

letter = ordinal(grades,{'D','C','B','A','A+'},[],...
                 [60,70,80,90,100,100]);
getlevels(letter)

ans = 

     D      C      B      A      A+ 

There are five grade categories, in the specified order D < C < B < A < A+.

Check for undefined categories.

Check whether or not there are any exam scores that do not fall into the five letter categories.

any(isundefined(letter))

ans =

     1     0     1     1     0

Recall that there are five exam scores for each student. The previous command returns a logical value for each of the five exams, indicating whether there are any scores that are <undefined>. There are scores for the first, third, and fourth exams that are <undefined>, that is, missing a category level.

Identify elements in undefined categories.

You can find the exam scores that do not have a letter grade using the isundefined logical condition.

grades(isundefined(letter))

ans =

    55
    59
    58
    59
    54
    57
    56
    59
    59
    50
    59
    52

The exam scores that are in the 50s do not have a letter grade.

Add a new category.

Put all scores that are <undefined> into a new category labeled D-.

letter(isundefined(letter)) = 'D-';
getlevels(letter)

Warning: Categorical level 'D-' being added. 
> In categorical.subsasgn at 55

ans = 

     D      C      B      A      A+      D- 

The ordinal variable, letter, has a new category added to the end.

Reorder category levels.

Reorder the categories so that D- < D.

letter = reorderlevels(letter,{'D-','D','C','B','A','A+'});
getlevels(letter)

ans = 

     D-      D      C      B      A      A+ 

Compare elements.

Now that all exam scores have a letter grade, count how many students received a higher letter grade on the second test than on the first test.

sum(letter(:,2) > letter(:,1))

ans =

    32

Thirty-two students improved their letter grade between the first two exams.

Explore categories.

Count the number of A+ scores in each of the five exams.

sum(letter=='A+')

ans =

     0     0     0     0     0

There are no A+ scores on any of the five exams.

Drop a category.

Drop the category A+ from the ordinal variable, letter.

letter = droplevels(letter,'A+');
getlevels(letter)

ans = 

     D-      D      C      B      A 

Category A+ is no longer in the ordinal variable, letter.