Add SelectKBest docs

README.md

@@ -6,7 +6,7 @@
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 <a href="http://www.yegor256.com/2016/10/23/award-2017.html">
@@ -89,6 +89,7 @@ Example scripts are available in a separate repository [php-ai/php-ml-examples](
     * [Stratified Random Split](http://php-ml.readthedocs.io/en/latest/machine-learning/cross-validation/stratified-random-split/)
 * Feature Selection
     * [Variance Threshold](http://php-ml.readthedocs.io/en/latest/machine-learning/feature-selection/variance-threshold/)
+    * [SelectKBest](http://php-ml.readthedocs.io/en/latest/machine-learning/feature-selection/selectkbest/)
 * Preprocessing
     * [Normalization](http://php-ml.readthedocs.io/en/latest/machine-learning/preprocessing/normalization/)
     * [Imputation missing values](http://php-ml.readthedocs.io/en/latest/machine-learning/preprocessing/imputation-missing-values/)

docs/index.md

@@ -6,7 +6,7 @@
 [![Documentation Status](https://readthedocs.org/projects/php-ml/badge/?version=master)](http://php-ml.readthedocs.org/)
 [![Total Downloads](https://poser.pugx.org/php-ai/php-ml/downloads.svg)](https://packagist.org/packages/php-ai/php-ml)
 [![License](https://poser.pugx.org/php-ai/php-ml/license.svg)](https://packagist.org/packages/php-ai/php-ml)
-[![Coverage Status](https://coveralls.io/repos/github/php-ai/php-ml/badge.svg?branch=coveralls)](https://coveralls.io/github/php-ai/php-ml?branch=coveralls)
+[![Coverage Status](https://coveralls.io/repos/github/php-ai/php-ml/badge.svg?branch=master)](https://coveralls.io/github/php-ai/php-ml?branch=master)
 [![Scrutinizer Code Quality](https://scrutinizer-ci.com/g/php-ai/php-ml/badges/quality-score.png?b=master)](https://scrutinizer-ci.com/g/php-ai/php-ml/?branch=master)
 
 <a href="http://www.yegor256.com/2016/10/23/award-2017.html">
@@ -78,6 +78,7 @@ Example scripts are available in a separate repository [php-ai/php-ml-examples](
     * [Stratified Random Split](machine-learning/cross-validation/stratified-random-split.md)
 * Feature Selection
     * [Variance Threshold](machine-learning/feature-selection/variance-threshold.md)
+    * [SelectKBest](machine-learning/feature-selection/selectkbest.md)
 * Preprocessing
     * [Normalization](machine-learning/preprocessing/normalization.md)
     * [Imputation missing values](machine-learning/preprocessing/imputation-missing-values.md)

docs/machine-learning/feature-selection/selectkbest.md

@@ -0,0 +1,96 @@
+# SelectKBest
+
+`SelectKBest` - select features according to the k highest scores.
+
+## Constructor Parameters
+
+* $k (int) - number of top features to select, rest will be removed (default: 10)
+* $scoringFunction (ScoringFunction) - function that take samples and targets and return array with scores (default: ANOVAFValue)
+
+```php
+use Phpml\FeatureSelection\SelectKBest;
+
+$transformer = new SelectKBest(2);
+```
+
+## Example of use
+
+As an example we can perform feature selection on Iris dataset to retrieve only the two best features as follows:
+
+```php
+use Phpml\FeatureSelection\SelectKBest;
+use Phpml\Dataset\Demo\IrisDataset;
+
+$dataset = new IrisDataset();
+$selector = new SelectKBest(2);
+$selector->fit($samples = $dataset->getSamples(), $dataset->getTargets());
+$selector->transform($samples);
+
+/*
+$samples[0] = [1.4, 0.2]; 
+*/
+```
+
+## Scores
+
+You can get a array with the calculated score for each feature. 
+A higher value means that a given feature is better suited for learning.
+Of course, the rating depends on the scoring function used.
+
+```
+use Phpml\FeatureSelection\SelectKBest;
+use Phpml\Dataset\Demo\IrisDataset;
+
+$dataset = new IrisDataset();
+$selector = new SelectKBest(2);
+$selector->fit($samples = $dataset->getSamples(), $dataset->getTargets());
+$selector->scores();
+
+/*
+..array(4) {
+  [0]=>
+  float(119.26450218451)
+  [1]=>
+  float(47.364461402997)
+  [2]=>
+  float(1179.0343277002)
+  [3]=>
+  float(959.32440572573)
+} 
+*/
+```
+
+## Scoring function
+
+Available scoring functions:
+
+For classification:
+ - **ANOVAFValue**
+   The one-way ANOVA tests the null hypothesis that 2 or more groups have the same population mean.
+   The test is applied to samples from two or more groups, possibly with differing sizes.
+
+For regression:
+ - **UnivariateLinearRegression**  
+   Quick linear model for testing the effect of a single regressor, sequentially for many regressors.
+   This is done in 2 steps:
+     - 1. The cross correlation between each regressor and the target is computed, that is, ((X[:, i] - mean(X[:, i])) * (y - mean_y)) / (std(X[:, i]) *std(y)).
+     - 2. It is converted to an F score 
+
+## Pipeline
+
+`SelectKBest` implements `Transformer` interface so it can be used as part of pipeline:
+
+```php
+use Phpml\FeatureSelection\SelectKBest;
+use Phpml\Classification\SVC;
+use Phpml\FeatureExtraction\TfIdfTransformer;
+use Phpml\Pipeline;
+
+$transformers = [
+    new TfIdfTransformer(),
+    new SelectKBest(3)
+];
+$estimator = new SVC();
+
+$pipeline = new Pipeline($transformers, $estimator);
+```

mkdocs.yml

@@ -27,6 +27,7 @@ pages:
       - Stratified Random Split: machine-learning/cross-validation/stratified-random-split.md
     - Feature Selection:
       - VarianceThreshold: machine-learning/feature-selection/variance-threshold.md
+      - SelectKBest: machine-learning/feature-selection/selectkbest.md
     - Preprocessing:
       - Normalization: machine-learning/preprocessing/normalization.md
       - Imputation missing values: machine-learning/preprocessing/imputation-missing-values.md

src/FeatureSelection/ScoringFunction/UnivariateLinearRegression.php

@@ -16,7 +16,7 @@
  *
  * 1. The cross correlation between each regressor and the target is computed,
  * that is, ((X[:, i] - mean(X[:, i])) * (y - mean_y)) / (std(X[:, i]) *std(y)).
- * 2. It is converted to an F score then to a p-value.
+ * 2. It is converted to an F score.
  *
  * Ported from scikit-learn f_regression function (http://scikit-learn.org/stable/modules/generated/sklearn.feature_selection.f_regression.html#sklearn.feature_selection.f_regression)
  */

tests/PipelineTest.php

@@ -7,7 +7,6 @@
 use Phpml\Classification\SVC;
 use Phpml\FeatureExtraction\TfIdfTransformer;
 use Phpml\FeatureExtraction\TokenCountVectorizer;
-use Phpml\FeatureSelection\ScoringFunction\ANOVAFValue;
 use Phpml\FeatureSelection\SelectKBest;
 use Phpml\ModelManager;
 use Phpml\Pipeline;
@@ -108,7 +107,7 @@ public function testPipelineTransformers(): void
         $this->assertEquals($expected, $predicted);
     }
 
-    public function testPipelineTransformersWithTargets() : void
+    public function testPipelineTransformersWithTargets(): void
     {
         $samples = [[1, 2, 1], [1, 3, 4], [5, 2, 1], [1, 3, 3], [1, 3, 4], [0, 3, 5]];
         $targets = ['a', 'a', 'a', 'b', 'b', 'b'];