nominal

(Not Recommended) Arrays for nominal data

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.

Description

Nominal data are discrete, nonnumeric values that do not have a natural ordering. nominal array objects provide efficient storage and convenient manipulation of such data, while also maintaining meaningful labels for the values.

You can manipulate nominal arrays like ordinary numeric arrays, by subscripting, concatenating, and reshaping. Use nominal arrays as grouping variables when the elements indicate the group to which an observation belongs.

Creation

Syntax

B = nominal(X)

B = nominal(X,labels)

B = nominal(X,labels,levels)

B = nominal(X,labels,[],edges)

Description

B = nominal(X) creates a nominal array B from the array X. nominal creates the levels of B from the sorted unique values in X, and creates default labels for the levels.

B = nominal(X,labels) labels the levels in B according to labels.

B = nominal(X,labels,levels) creates a nominal array with possible levels defined by levels.

B = nominal(X,labels,[],edges) creates a nominal array by binning the numeric array X with bin edges given by the numeric vector edges.

Input Arguments

X — Input array

numeric | logical | character | string | categorical | cell array of character vectors

Input array to convert to nominal, specified as a numeric, logical, character, string, or categorical array, or a cell array of character vectors. The levels of the resulting nominal array correspond to the sorted unique values in X.

labels — Labels for discrete levels

character array | string array | cell array of character vectors

Labels for the discrete levels, specified as a character array, string array, or cell array of character vectors. By default, nominal assigns labels to the levels in B in order of the sorted unique values in X.

You can include duplicate labels in labels to merge multiple values in X into a single level in B.

Data Types: char | string | cell

levels — Possible nominal levels

vector

Possible nominal levels for the output nominal array, specified as a vector whose values can be compared to those in X using the equality operator. nominal assigns labels to each level from the corresponding elements of labels. If X contains any values not present in levels, the levels of the corresponding elements of B are undefined.

edges — Bin edges

numeric vector

Bin edges used to create the nominal array by binning a numeric array, specified as a numeric vector. The uppermost bin includes values equal to the rightmost edge. nominal assigns labels to each level in the resulting nominal array from the corresponding elements of labels. When you specify the edges input argument, it must have one more element than labels.

Output Arguments

B — Nominal array

nominal array object

Nominal array, returned as a nominal array object.

By default, an element of B is undefined if the corresponding element of X is NaN (when X is numeric), an empty character vector (when X is a character), an empty or missing string (when X is a string), or undefined (when X is categorical). nominal treats such elements as undefined or missing and does not include entries for them among the possible levels. To create an explicit level for such elements instead of treating them as undefined, use the levels input argument and include NaN, the empty character vector, the empty or missing string, or an undefined element.

Object Functions

addlevels	(Not Recommended) Add levels to nominal or ordinal arrays
droplevels	(Not Recommended) Drop levels from a nominal or ordinal array
getlabels	(Not Recommended) Access nominal or ordinal array labels
getlevels	(Not Recommended) Access nominal or ordinal array levels
islevel	(Not Recommended) Determine if levels are in nominal or ordinal array
levelcounts	(Not Recommended) Element counts by level of a nominal or ordinal array
mergelevels	(Not Recommended) Merge levels of nominal or ordinal arrays
reorderlevels	(Not Recommended) Reorder levels of nominal or ordinal arrays
setlabels	(Not Recommended) Assign labels to levels of nominal or ordinal arrays

The following is a partial list of the many other MATLAB^® array functions you can use with nominal arrays.

double	Double-precision arrays
histogram	Histogram plot
isequal	Determine array equality
isundefined	Find undefined elements in categorical array
pie	Legacy pie chart
summary	Data summary
times	Multiplication

Examples

Create and Label Nominal Arrays

Create nominal arrays from a cell array of character vectors and from integer data. Provide explicit labels.

Create a nominal array from a cell array of character vectors with values 'r', 'g', and 'b'. Label these levels 'red', 'green', and 'blue', respectively. nominal assigns the labels according to the sorted (alphabetical) order of the elements in X.

X = {'r' 'b' 'g';'g' 'r' 'b';'b' 'r' 'g'}

X = 3x3 cell
    {'r'}    {'b'}    {'g'}
    {'g'}    {'r'}    {'b'}
    {'b'}    {'r'}    {'g'}

labels = {'blue','green','red'};
B = nominal(X,labels)

B = 3x3 nominal
     red        blue      green 
     green      red       blue  
     blue       red       green 

Create a nominal array from integer data with values 1 to 4, merging odd and even values into two nominal levels with the labels 'odd' and 'even'. Merge the values by duplicating the labels.

X = randi([1 4],5,2)

X = 5×2

     4     1
     4     2
     1     3
     4     4
     3     4

labels = {'odd','even','odd','even'};
B = nominal(X,labels)

B = 5x2 nominal
     even      odd  
     even      even 
     odd       odd  
     even      even 
     odd       even 

Create and Manipulate Nominal Arrays

Create a nominal array from data in a cell array.

X = {'r','b','g';'g','r','b';'b','r','g'};
labels = {'blue','green','red'};
colors = nominal(X,labels)

colors = 3x3 nominal
     red        blue      green 
     green      red       blue  
     blue       red       green 

Identify the elements in colors that are members of the level 'red'. A value of 1 in the resulting array indicates that the corresponding element of colors is a member of 'red'.

colors == 'red'

ans = 3x3 logical array

   1   0   0
   0   1   0
   0   1   0

Identify the elements in colors that are members of either the level 'red' or 'blue'.

ismember(colors,{'red','blue'})

ans = 3x3 logical array

   1   1   0
   0   1   1
   1   1   0

Merge the elements of the 'red' and 'blue' levels into a new level labeled 'purple'.

colors = mergelevels(colors,{'red','blue'},'purple')

colors = 3x3 nominal
     purple      purple      green  
     green       purple      purple 
     purple      purple      green  

Display the levels of colors.

getlevels(colors)

ans = 1x2 nominal
     purple      green 

Summarize the number of elements in each level. By default, summary returns counts for each column of the input array.

summary(colors)

     purple      2      3      1 
     green       1      0      2 

Create a pie chart for the data in colors.

pie(colors)

Version History

Introduced in R2007a