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Classify observations using naive Bayes classifier

`[`

also returns the Posterior Probability (`label`

,`Posterior`

,`Cost`

]
= predict(`Mdl`

,`X`

)`Posterior`

) and predicted
(expected) Misclassification Cost (`Cost`

) corresponding to
the observations (rows) in `Mdl.X`

. For each observation in
`X`

, the predicted class label corresponds to the minimum
expected classification cost among all classes.

Load the `fisheriris`

data set. Create `X`

as a numeric matrix that contains four petal measurements for 150 irises. Create `Y`

as a cell array of character vectors that contains the corresponding iris species.

load fisheriris X = meas; Y = species; rng('default') % for reproducibility

Randomly partition observations into a training set and a test set with stratification, using the class information in `Y`

. Specify a 30% holdout sample for testing.

`cv = cvpartition(Y,'HoldOut',0.30);`

Extract the training and test indices.

trainInds = training(cv); testInds = test(cv);

Specify the training and test data sets.

XTrain = X(trainInds,:); YTrain = Y(trainInds); XTest = X(testInds,:); YTest = Y(testInds);

Train a naive Bayes classifier using the predictors `XTrain`

and class labels `YTrain`

. A recommended practice is to specify the class names. `fitcnb`

assumes that each predictor is conditionally and normally distributed.

Mdl = fitcnb(XTrain,YTrain,'ClassNames',{'setosa','versicolor','virginica'})

Mdl = ClassificationNaiveBayes ResponseName: 'Y' CategoricalPredictors: [] ClassNames: {'setosa' 'versicolor' 'virginica'} ScoreTransform: 'none' NumObservations: 105 DistributionNames: {'normal' 'normal' 'normal' 'normal'} DistributionParameters: {3x4 cell} Properties, Methods

`Mdl`

is a trained `ClassificationNaiveBayes`

classifier.

Predict the test sample labels.

idx = randsample(sum(testInds),10); label = predict(Mdl,XTest);

Display the results for a random set of 10 observations in the test sample.

table(YTest(idx),label(idx),'VariableNames',... {'TrueLabel','PredictedLabel'})

`ans=`*10×2 table*
TrueLabel PredictedLabel
______________ ______________
{'virginica' } {'virginica' }
{'versicolor'} {'versicolor'}
{'versicolor'} {'versicolor'}
{'virginica' } {'virginica' }
{'setosa' } {'setosa' }
{'virginica' } {'virginica' }
{'setosa' } {'setosa' }
{'versicolor'} {'versicolor'}
{'versicolor'} {'virginica' }
{'versicolor'} {'versicolor'}

Create a confusion chart from the true labels `YTest`

and the predicted labels `label`

.

cm = confusionchart(YTest,label);

Estimate posterior probabilities and misclassification costs for new observations using a naive Bayes classifier. Classify new observations using a memory-efficient pretrained classifier.

Load the `fisheriris`

data set. Create `X`

as a numeric matrix that contains four petal measurements for 150 irises. Create `Y`

as a cell array of character vectors that contains the corresponding iris species.

load fisheriris X = meas; Y = species; rng('default') % for reproducibility

Partition the data set into two sets: one contains the training set, and the other contains new, unobserved data. Reserve 10 observations for the new data set.

n = size(X,1); newInds = randsample(n,10); inds = ~ismember(1:n,newInds); XNew = X(newInds,:); YNew = Y(newInds);

Train a naive Bayes classifier using the predictors `X`

and class labels `Y`

. A recommended practice is to specify the class names. `fitcnb`

assumes that each predictor is conditionally and normally distributed.

Mdl = fitcnb(X(inds,:),Y(inds),... 'ClassNames',{'setosa','versicolor','virginica'});

`Mdl`

is a trained `ClassificationNaiveBayes`

classifier.

Conserve memory by reducing the size of the trained naive Bayes classifier.

CMdl = compact(Mdl); whos('Mdl','CMdl')

Name Size Bytes Class Attributes CMdl 1x1 5406 classreg.learning.classif.CompactClassificationNaiveBayes Mdl 1x1 12731 ClassificationNaiveBayes

`CMdl`

is a `CompactClassificationNaiveBayes`

classifier. It uses less memory than `Mdl`

because `Mdl`

stores the data.

Display the class names of `CMdl`

using dot notation.

CMdl.ClassNames

`ans = `*3x1 cell*
{'setosa' }
{'versicolor'}
{'virginica' }

Predict the labels. Estimate the posterior probabilities and expected class misclassification costs.

[labels,PostProbs,MisClassCost] = predict(CMdl,XNew);

Compare the true labels with the predicted labels.

table(YNew,labels,PostProbs,MisClassCost,'VariableNames',... {'TrueLabels','PredictedLabels',... 'PosteriorProbabilities','MisclassificationCosts'})

`ans=`*10×4 table*
TrueLabels PredictedLabels PosteriorProbabilities MisclassificationCosts
______________ _______________ _________________________________________ ______________________________________
{'virginica' } {'virginica' } 4.0832e-268 4.6422e-09 1 1 1 4.6422e-09
{'setosa' } {'setosa' } 1 3.0706e-18 4.6719e-25 3.0706e-18 1 1
{'virginica' } {'virginica' } 1.0007e-246 5.8758e-10 1 1 1 5.8758e-10
{'versicolor'} {'versicolor'} 1.2022e-61 0.99995 4.9859e-05 1 4.9859e-05 0.99995
{'virginica' } {'virginica' } 2.687e-226 1.7905e-08 1 1 1 1.7905e-08
{'versicolor'} {'versicolor'} 3.3431e-76 0.99971 0.00028983 1 0.00028983 0.99971
{'virginica' } {'virginica' } 4.05e-166 0.0028527 0.99715 1 0.99715 0.0028527
{'setosa' } {'setosa' } 1 1.1272e-14 2.0308e-23 1.1272e-14 1 1
{'virginica' } {'virginica' } 1.3292e-228 8.3604e-10 1 1 1 8.3604e-10
{'setosa' } {'setosa' } 1 4.5023e-17 2.1724e-24 4.5023e-17 1 1

`PostProbs`

and `MisClassCost`

are `10`

-by-`3`

numeric matrices, where each row corresponds to a new observation and each column corresponds to a class. The order of the columns corresponds to the order of `CMdl.ClassNames`

.

Load the `fisheriris`

data set. Create `X`

as a numeric matrix that contains petal length and width measurements for 150 irises. Create `Y`

as a cell array of character vectors that contains the corresponding iris species.

```
load fisheriris
X = meas(:,3:4);
Y = species;
```

Train a naive Bayes classifier using the predictors `X`

and class labels `Y`

. A recommended practice is to specify the class names. `fitcnb`

assumes that each predictor is conditionally and normally distributed.

Mdl = fitcnb(X,Y,'ClassNames',{'setosa','versicolor','virginica'});

`Mdl`

is a trained `ClassificationNaiveBayes`

classifier.

Define a grid of values in the observed predictor space.

xMax = max(X); xMin = min(X); h = 0.01; [x1Grid,x2Grid] = meshgrid(xMin(1):h:xMax(1),xMin(2):h:xMax(2));

Predict the posterior probabilities for each instance in the grid.

[~,PosteriorRegion] = predict(Mdl,[x1Grid(:),x2Grid(:)]);

Plot the posterior probability regions and the training data.

h = scatter(x1Grid(:),x2Grid(:),1,PosteriorRegion); h.MarkerEdgeAlpha = 0.3;

Plot the data.

hold on gh = gscatter(X(:,1),X(:,2),Y,'k','dx*'); title 'Iris Petal Measurements and Posterior Probabilities'; xlabel 'Petal length (cm)'; ylabel 'Petal width (cm)'; axis tight legend(gh,'Location','Best') hold off

`Mdl`

— Naive Bayes classification model`ClassificationNaiveBayes`

model object | `CompactClassificationNaiveBayes`

model objectNaive Bayes classification model, specified as a `ClassificationNaiveBayes`

model object or `CompactClassificationNaiveBayes`

model object returned by `fitcnb`

or `compact`

,
respectively.

`X`

— Predictor data to be classifiednumeric matrix | table

Predictor data to be classified, specified as a numeric matrix or table.

Each row of `X`

corresponds to one observation, and
each column corresponds to one variable.

For a numeric matrix:

The variables that make up the columns of

`X`

must have the same order as the predictor variables that trained`Mdl`

.If you train

`Mdl`

using a table (for example,`Tbl`

), then`X`

can be a numeric matrix if`Tbl`

contains only numeric predictor variables. To treat numeric predictors in`Tbl`

as categorical during training, identify categorical predictors using the`'CategoricalPredictors'`

name-value pair argument of`fitcnb`

. If`Tbl`

contains heterogeneous predictor variables (for example, numeric and categorical data types) and`X`

is a numeric matrix, then`predict`

throws an error.

For a table:

`predict`

does not support multicolumn variables or cell arrays other than cell arrays of character vectors.If you train

`Mdl`

using a table (for example,`Tbl`

), then all predictor variables in`X`

must have the same variable names and data types as the variables that trained`Mdl`

(stored in`Mdl.PredictorNames`

). However, the column order of`X`

does not need to correspond to the column order of`Tbl`

.`Tbl`

and`X`

can contain additional variables (response variables, observation weights, and so on), but`predict`

ignores them.If you train

`Mdl`

using a numeric matrix, then the predictor names in`Mdl.PredictorNames`

must be the same as the corresponding predictor variable names in`X`

. To specify predictor names during training, use the '`PredictorNames`

' name-value pair argument of`fitcnb`

. All predictor variables in`X`

must be numeric vectors.`X`

can contain additional variables (response variables, observation weights, and so on), but`predict`

ignores them.

**Data Types: **`table`

| `double`

| `single`

**Notes:**

If

`Mdl.DistributionNames`

is`'mn'`

, then the software returns`NaN`

s corresponding to rows of`X`

that contain at least one`NaN`

.If

`Mdl.DistributionNames`

is not`'mn'`

, then the software ignores`NaN`

values when estimating misclassification costs and posterior probabilities. Specifically, the software computes the conditional density of the predictors given the class by leaving out the factors corresponding to missing predictor values.For predictor distribution specified as

`'mvmn'`

, if`X`

contains levels that are not represented in the training data (that is, not in`Mdl.CategoricalLevels`

for that predictor), then the conditional density of the predictors given the class is 0. For those observations, the software returns the corresponding value of`Posterior`

as a`NaN`

. The software determines the class label for such observations using the class prior probability stored in`Mdl.Prior`

.

`label`

— Predicted class labelscategorical vector | character array | logical vector | numeric vector | cell array of character vectors

Predicted class labels, returned as a categorical vector, character array, logical or numeric vector, or cell array of character vectors.

The predicted class labels have the following:

Same data type as the observed class labels (

`Mdl.Y`

). (The software treats string arrays as cell arrays of character vectors.)Length equal to the number of rows of

`Mdl.X`

.Class yielding the lowest expected misclassification cost (

`Cost`

).

`Posterior`

— Class posterior probabilitynumeric matrix

Class Posterior Probability, returned as a numeric matrix.
`Posterior`

has rows equal to the number of rows of
`Mdl.X`

and columns equal to the number of distinct classes in the
training data (`size(Mdl.ClassNames,1)`

).

`Posterior(j,k)`

is the predicted posterior probability of class
`k`

(in class `Mdl.ClassNames(k)`

) given the
observation in row `j`

of `Mdl.X`

.

`Cost`

— Expected misclassification costsnumeric matrix

Expected Misclassification Cost, returned as a numeric matrix.
`Cost`

has rows equal to the number of rows of
`Mdl.X`

and columns equal to the number of distinct classes in the
training data (`size(Mdl.ClassNames,1)`

).

`Cost(j,k)`

is the expected misclassification cost of the observation in row
`j`

of `Mdl.X`

predicted into class
`k`

(in class `Mdl.ClassNames(k)`

).

A *misclassification cost* is
the relative severity of a classifier labeling an observation into
the wrong class.

There are two types of misclassification costs: true and expected.
Let *K* be the number of classes.

*True misclassification cost*— A*K*-by-*K*matrix, where element (*i*,*j*) indicates the misclassification cost of predicting an observation into class*j*if its true class is*i*. The software stores the misclassification cost in the property`Mdl.Cost`

, and uses it in computations. By default,`Mdl.Cost(i,j)`

= 1 if`i`

≠`j`

, and`Mdl.Cost(i,j)`

= 0 if`i`

=`j`

. In other words, the cost is`0`

for correct classification and`1`

for any incorrect classification.*Expected misclassification cost*— A*K*-dimensional vector, where element*k*is the weighted average misclassification cost of classifying an observation into class*k*, weighted by the class posterior probabilities.$${c}_{k}={\displaystyle \sum _{j=1}^{K}\widehat{P}}\left(Y=j|{x}_{1},\mathrm{...},{x}_{P}\right)Cos{t}_{jk}.$$

In other words, the software classifies observations to the class corresponding with the lowest expected misclassification cost.

The *posterior probability* is
the probability that an observation belongs in a particular class,
given the data.

For naive Bayes, the posterior probability that a classification
is *k* for a given observation (*x*_{1},...,*x _{P}*)
is

$$\widehat{P}\left(Y=k|{x}_{1},\mathrm{..},{x}_{P}\right)=\frac{P\left({X}_{1},\mathrm{...},{X}_{P}|y=k\right)\pi \left(Y=k\right)}{P\left({X}_{1},\mathrm{...},{X}_{P}\right)},$$

where:

$$P\left({X}_{1},\mathrm{...},{X}_{P}|y=k\right)$$ is the conditional joint density of the predictors given they are in class

*k*.`Mdl.DistributionNames`

stores the distribution names of the predictors.*π*(*Y*=*k*) is the class prior probability distribution.`Mdl.Prior`

stores the prior distribution.$$P\left({X}_{1},\mathrm{..},{X}_{P}\right)$$ is the joint density of the predictors. The classes are discrete, so $$P({X}_{1},\mathrm{...},{X}_{P})={\displaystyle \sum _{k=1}^{K}P}({X}_{1},\mathrm{...},{X}_{P}|y=k)\pi (Y=k).$$

The *prior
probability* of a class is the assumed relative frequency with which observations
from that class occur in a population.

Calculate with arrays that have more rows than fit in memory.

This function fully supports tall arrays. You can use models trained on either in-memory or tall data with this function.

For more information, see Tall Arrays.

Generate C and C++ code using MATLAB® Coder™.

Usage notes and limitations:

Use

`saveLearnerForCoder`

,`loadLearnerForCoder`

, and`codegen`

(MATLAB Coder) to generate code for the`predict`

function. Save a trained model by using`saveLearnerForCoder`

. Define an entry-point function that loads the saved model by using`loadLearnerForCoder`

and calls the`predict`

function. Then use`codegen`

to generate code for the entry-point function.To generate single-precision C/C++ code for

`predict`

, specify the name-value argument`'DataType','single'`

when you call the`loadLearnerForCoder`

function.This table contains notes about the arguments of

`predict`

. Arguments not included in this table are fully supported.Argument Notes and Limitations `Mdl`

For the usage notes and limitations of the model object, see Code Generation of the

`CompactClassificationNaiveBayes`

object.`X`

`X`

must be a single-precision or double-precision matrix or a table containing numeric variables, categorical variables, or both.The number of rows, or observations, in

`X`

can be a variable size, but the number of columns in`X`

must be fixed.If you want to specify

`X`

as a table, then your model must be trained using a table, and your entry-point function for prediction must:Accept data as arrays.

Create a table from the data input arguments and specifies the variable names in the table.

Pass the table to

`predict`

.

For an example of this table workflow, see Generate Code to Classify Data in Table. For more information on using tables in code generation, see Code Generation for Tables (MATLAB Coder) and Table Limitations for Code Generation (MATLAB Coder).

For more information, see Introduction to Code Generation.

`ClassificationNaiveBayes`

| `CompactClassificationNaiveBayes`

| `loss`

| `resubPredict`

| `fitcnb`

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