Implement first regression scoring function UnivariateLinearRegression

src/FeatureSelection/ScoringFunction/UnivariateLinearRegression.php

@@ -0,0 +1,81 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Phpml\FeatureSelection\ScoringFunction;
+
+use Phpml\FeatureSelection\ScoringFunction;
+use Phpml\Math\Matrix;
+use Phpml\Math\Statistic\Mean;
+
+/**
+ * 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 then to a p-value.
+ *
+ * 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)
+ */
+final class UnivariateLinearRegression implements ScoringFunction
+{
+    /**
+     * @var bool
+     */
+    private $center;
+
+    /**
+     * @param bool $center - if true samples and targets will be centered
+     */
+    public function __construct(bool $center = true)
+    {
+        $this->center = $center;
+    }
+
+    public function score(array $samples, array $targets): array
+    {
+        if ($this->center) {
+            $this->centerTargets($targets);
+            $this->centerSamples($samples);
+        }
+
+        $correlations = [];
+        foreach ($samples[0] as $index => $feature) {
+            $featureColumn = array_column($samples, $index);
+            $correlations[$index] =
+                (Matrix::dot($targets, $featureColumn)[0] / (new Matrix($featureColumn, false))->transpose()->frobeniusNorm())
+                / (new Matrix($targets, false))->frobeniusNorm();
+        }
+
+        $degreesOfFreedom = count($targets) - ($this->center ? 2 : 1);
+
+        return array_map(function (float $correlation) use ($degreesOfFreedom): float {
+            return $correlation ** 2 / (1 - $correlation ** 2) * $degreesOfFreedom;
+        }, $correlations);
+    }
+
+    private function centerTargets(&$targets): void
+    {
+        $mean = Mean::arithmetic($targets);
+        foreach ($targets as &$target) {
+            $target -= $mean;
+        }
+    }
+
+    private function centerSamples(&$samples): void
+    {
+        $means = [];
+        foreach ($samples[0] as $index => $feature) {
+            $means[$index] = Mean::arithmetic(array_column($samples, $index));
+        }
+
+        foreach ($samples as &$sample) {
+            foreach ($sample as $index => &$feature) {
+                $feature -= $means[$index];
+            }
+        }
+    }
+}

src/Math/Matrix.php

@@ -236,6 +236,29 @@ public function isSingular(): bool
         return $this->getDeterminant() == 0;
     }
 
+    /**
+     * Frobenius norm (Hilbert–Schmidt norm, Euclidean norm) (‖A‖F)
+     * Square root of the sum of the square of all elements.
+     *
+     * https://en.wikipedia.org/wiki/Matrix_norm#Frobenius_norm
+     *
+     *          _____________
+     *         /ᵐ   ⁿ
+     * ‖A‖F = √ Σ   Σ  |aᵢⱼ|²
+     *         ᵢ₌₁ ᵢ₌₁
+     */
+    public function frobeniusNorm(): float
+    {
+        $squareSum = 0;
+        for ($i = 0; $i < $this->rows; ++$i) {
+            for ($j = 0; $j < $this->columns; ++$j) {
+                $squareSum += ($this->matrix[$i][$j]) ** 2;
+            }
+        }
+
+        return sqrt($squareSum);
+    }
+
     /**
      * Returns the transpose of given array
      */
@@ -259,7 +282,7 @@ public static function dot(array $array1, array $array2): array
     /**
      * Element-wise addition or substraction depending on the given sign parameter
      */
-    protected function _add(self $other, int $sign = 1): self
+    private function _add(self $other, int $sign = 1): self
     {
         $a1 = $this->toArray();
         $a2 = $other->toArray();
@@ -277,7 +300,7 @@ protected function _add(self $other, int $sign = 1): self
     /**
      * Returns diagonal identity matrix of the same size of this matrix
      */
-    protected function getIdentity(): self
+    private function getIdentity(): self
     {
         $array = array_fill(0, $this->rows, array_fill(0, $this->columns, 0));
         for ($i = 0; $i < $this->rows; ++$i) {

tests/FeatureSelection/ScoringFunction/UnivariateLinearRegressionTest.php

@@ -0,0 +1,29 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Phpml\Tests\FeatureSelection\ScoringFunction;
+
+use Phpml\FeatureSelection\ScoringFunction\UnivariateLinearRegression;
+use PHPUnit\Framework\TestCase;
+
+final class UnivariateLinearRegressionTest extends TestCase
+{
+    public function testRegressionScore(): void
+    {
+        $samples = [[73676, 1996], [77006, 1998], [10565, 2000], [146088, 1995], [15000, 2001], [65940, 2000], [9300, 2000], [93739, 1996], [153260, 1994], [17764, 2002], [57000, 1998], [15000, 2000]];
+        $targets = [2000, 2750, 15500, 960, 4400, 8800, 7100, 2550, 1025, 5900, 4600, 4400];
+
+        $function = new UnivariateLinearRegression();
+        self::assertEquals([6.97286, 6.48558], $function->score($samples, $targets), '', 0.0001);
+    }
+
+    public function testRegressionScoreWithoutCenter(): void
+    {
+        $samples = [[73676, 1996], [77006, 1998], [10565, 2000], [146088, 1995], [15000, 2001], [65940, 2000], [9300, 2000], [93739, 1996], [153260, 1994], [17764, 2002], [57000, 1998], [15000, 2000]];
+        $targets = [2000, 2750, 15500, 960, 4400, 8800, 7100, 2550, 1025, 5900, 4600, 4400];
+
+        $function = new UnivariateLinearRegression(false);
+        self::assertEquals([1.74450, 18.08347], $function->score($samples, $targets), '', 0.0001);
+    }
+}

tests/FeatureSelection/SelectKBestTest.php

@@ -8,6 +8,7 @@
 use Phpml\Exception\InvalidArgumentException;
 use Phpml\Exception\InvalidOperationException;
 use Phpml\FeatureSelection\ScoringFunction\ANOVAFValue;
+use Phpml\FeatureSelection\ScoringFunction\UnivariateLinearRegression;
 use Phpml\FeatureSelection\SelectKBest;
 use PHPUnit\Framework\TestCase;
 
@@ -45,6 +46,21 @@ public function testSelectKBestWithIrisDataset(): void
         self::assertEquals(2, count($samples[0]));
     }
 
+    public function testSelectKBestWithRegressionScoring(): void
+    {
+        $samples = [[73676, 1996, 2], [77006, 1998, 5], [10565, 2000, 4], [146088, 1995, 2], [15000, 2001, 2], [65940, 2000, 2], [9300, 2000, 2], [93739, 1996, 2], [153260, 1994, 2], [17764, 2002, 2], [57000, 1998, 2], [15000, 2000, 2]];
+        $targets = [2000, 2750, 15500, 960, 4400, 8800, 7100, 2550, 1025, 5900, 4600, 4400];
+
+        $selector = new SelectKBest(new UnivariateLinearRegression(), 2);
+        $selector->fit($samples, $targets);
+        $selector->transform($samples);
+
+        self::assertEquals(
+            [[73676, 1996], [77006, 1998], [10565, 2000], [146088, 1995], [15000, 2001], [65940, 2000], [9300, 2000], [93739, 1996], [153260, 1994], [17764, 2002], [57000, 1998], [15000, 2000]],
+            $samples
+        );
+    }
+
     public function testThrowExceptionOnEmptyTargets(): void
     {
         $this->expectException(InvalidArgumentException::class);

tests/Math/MatrixTest.php

@@ -251,4 +251,55 @@ public function testDot(): void
         $dot = [6, 12];
         $this->assertEquals($dot, Matrix::dot($matrix2, $matrix1));
     }
+
+    /**
+     * @dataProvider dataProviderForFrobeniusNorm
+     */
+    public function testFrobeniusNorm(array $matrix, float $norm): void
+    {
+        $matrix = new Matrix($matrix);
+
+        $this->assertEquals($norm, $matrix->frobeniusNorm(), '', 0.0001);
+    }
+
+    public function dataProviderForFrobeniusNorm()
+    {
+        return [
+            [
+                [
+                    [1, -7],
+                    [2, 3],
+                ], 7.93725,
+            ],
+            [
+                [
+                    [1, 2, 3],
+                    [2, 3, 4],
+                    [3, 4, 5],
+                ], 9.643651,
+            ],
+            [
+                [
+                    [1, 5, 3, 9],
+                    [2, 3, 4, 12],
+                    [4, 2, 5, 11],
+                ], 21.330729,
+            ],
+            [
+                [
+                    [1, 5, 3],
+                    [2, 3, 4],
+                    [4, 2, 5],
+                    [6, 6, 3],
+                ], 13.784049,
+            ],
+            [
+                [
+                    [5, -4, 2],
+                    [-1, 2, 3],
+                    [-2, 1, 0],
+                ], 8,
+            ],
+        ];
+    }
 }