StepFunction implements UnivariateFunction { public double value(double x) { int index = Arrays.binarySearch(abscissa, x); double fx = 0; if (index < -1) { fx = ordinate[-index-2]; } else if (index >= 0) { fx = ordinate[index]; } else { fx = ordinate[0]; } return fx; } StepFunction(double[] x,
                        double[] y); double value(double x);  }
@Test public void testSomeValues() { final double[] x = { -2, -0.5, 0, 1.9, 7.4, 21.3 }; final double[] y = { 4, -1, -5.5, 0.4, 5.8, 51.2 }; final UnivariateFunction f = new StepFunction(x, y); Assert.assertEquals(4, f.value(Double.NEGATIVE_INFINITY), EPS); Assert.assertEquals(4, f.value(-10), EPS); Assert.assertEquals(-1, f.value(-0.4), EPS); Assert.assertEquals(-5.5, f.value(0), EPS); Assert.assertEquals(0.4, f.value(2), EPS); Assert.assertEquals(5.8, f.value(10), EPS); Assert.assertEquals(51.2, f.value(30), EPS); Assert.assertEquals(51.2, f.value(Double.POSITIVE_INFINITY), EPS); }
@Test public void testEndpointBehavior() { final double[] x = {0, 1, 2, 3}; final double[] xp = {-8, 1, 2, 3}; final double[] y = {1, 2, 3, 4}; final UnivariateFunction f = new StepFunction(x, y); final UnivariateFunction fp = new StepFunction(xp, y); Assert.assertEquals(f.value(-8), fp.value(-8), EPS); Assert.assertEquals(f.value(-10), fp.value(-10), EPS); Assert.assertEquals(f.value(0), fp.value(0), EPS); Assert.assertEquals(f.value(0.5), fp.value(0.5), EPS); for (int i = 0; i < x.length; i++) { Assert.assertEquals(y[i], f.value(x[i]), EPS); if (i > 0) { Assert.assertEquals(y[i - 1], f.value(x[i] - 0.5), EPS); } else { Assert.assertEquals(y[0], f.value(x[i] - 0.5), EPS); } } }
@Test public void testHeaviside() { final UnivariateFunction h = new StepFunction(new double[] {-1, 0}, new double[] {0, 1}); Assert.assertEquals(0, h.value(Double.NEGATIVE_INFINITY), 0); Assert.assertEquals(0, h.value(-Double.MAX_VALUE), 0); Assert.assertEquals(0, h.value(-2), 0); Assert.assertEquals(0, h.value(-Double.MIN_VALUE), 0); Assert.assertEquals(1, h.value(0), 0); Assert.assertEquals(1, h.value(2), 0); Assert.assertEquals(1, h.value(Double.POSITIVE_INFINITY), 0); }