Create Categorical Array from Cell Array of Character Vectors

You can use the categorical function to create a categorical array from a numeric array, logical array, string array, cell array of character vectors, or an existing categorical array.

Create a 1-by-11 cell array of character vectors containing state names from New England.

state = {'MA','ME','CT','VT','ME','NH','VT','MA','NH','CT','RI'};

Convert the cell array, state, to a categorical array that has no mathematical order.

state = categorical(state)

state = 1x11 categorical
  Columns 1 through 9

     MA      ME      CT      VT      ME      NH      VT      MA      NH 

  Columns 10 through 11

     CT      RI 

class(state)

ans = 
'categorical'

List the discrete categories in the variable state.

categories(state)

ans = 6x1 cell
    {'CT'}
    {'MA'}
    {'ME'}
    {'NH'}
    {'RI'}
    {'VT'}

The categories are listed in alphabetical order.

Create Ordinal Categorical Array from Cell Array of Character Vectors

Create a 1-by-8 cell array of character vectors containing the sizes of eight objects.

AllSizes = {'medium','large','small','small','medium',...
            'large','medium','small'};

The cell array, AllSizes, has three distinct values: 'large', 'medium', and 'small'. With the cell array of character vectors, there is no convenient way to indicate that small < medium < large.

Convert the cell array, AllSizes, to an ordinal categorical array. Use valueset to specify the values small, medium, and large, which define the categories. For an ordinal categorical array, the first category specified is the smallest and the last category is the largest.

valueset = {'small','medium','large'};
sizeOrd = categorical(AllSizes,valueset,'Ordinal',true)

sizeOrd = 1x8 categorical
  Columns 1 through 6

     medium      large      small      small      medium      large 

  Columns 7 through 8

     medium      small 

class(sizeOrd)

ans = 
'categorical'

The order of the values in the categorical array, sizeOrd, remains unchanged.

List the discrete categories in the categorical variable, sizeOrd.

categories(sizeOrd)

ans = 3x1 cell
    {'small' }
    {'medium'}
    {'large' }

The categories are listed in the specified order to match the mathematical ordering small < medium < large.

Create Ordinal Categorical Array by Binning Numeric Data

Create a vector of 100 random numbers between zero and 50.

x = rand(100,1)*50;

Use the discretize function to create a categorical array by binning the values of x. Put all values between zero and 15 in the first bin, all the values between 15 and 35 in the second bin, and all the values between 35 and 50 in the third bin. Each bin includes the left endpoint, but does not include the right endpoint.

catnames = {'small','medium','large'};
binnedData = discretize(x,[0 15 35 50],'categorical',catnames);

binnedData is a 100-by-1 ordinal categorical array with three categories, such that small < medium < large.

Use the summary function to print the number of elements in each category.

summary(binnedData)

     small       30 
     medium      35 
     large       35 

Create Categorical Array from String Array

Starting in R2016b, you can create string arrays with the string function and convert them to categorical array.

Create a string array that contains names of planets.

str = string({'Earth','Jupiter','Neptune','Jupiter','Mars','Earth'})

str = 1x6 string
    "Earth"    "Jupiter"    "Neptune"    "Jupiter"    "Mars"    "Earth"

Convert str to a categorical array.

planets = categorical(str)

planets = 1x6 categorical
     Earth      Jupiter      Neptune      Jupiter      Mars      Earth 

Add missing elements to str and convert it to a categorical array. Where str has missing values, planets has undefined values.

str(8) = 'Mars'

str = 1x8 string
  Columns 1 through 6

    "Earth"    "Jupiter"    "Neptune"    "Jupiter"    "Mars"    "Earth"

  Columns 7 through 8

    <missing>    "Mars"

planets = categorical(str)

planets = 1x8 categorical
  Columns 1 through 6

     Earth      Jupiter      Neptune      Jupiter      Mars      Earth 

  Columns 7 through 8

     <undefined>      Mars