Interpolation Bug

[Owen Arnold]

From Michael Wedel via mantid help:

Hi,

I was using Kernel::Interpolation and I think I discovered a small bug in Interpolation::value(), line 54. This line covers the case of values above the upper interpolation interval limit.

Currently the line reads:

return m_y[N-1]+(at-m_y[N-1])*(m_y[N-1]-m_y[N-2])/(m_x[N-1]-m_x[N-2]);

---

I think it should read:

return m_y[N-1]+(at-m_x[N-1])*(m_y[N-1]-m_y[N-2])/(m_x[N-1]-m_x[N-2]);

---

Making this change breaks InterpolationTest::test1():

Line 31: TS_ASSERT_DELTA( interpolation.value(3000), 260400.0 ,0.000000001);

The test should then be adjusted like this:

Line 31: TS_ASSERT_DELTA( interpolation.value(3000), 28000.0 ,0.000000001); Line 52: TS_ASSERT_DELTA( interpolation.value(3000), 28000.0 ,0.000000001);

I think this is just a typo, since subtracting a y-value from an x-value would not make much sense and in all other cases in that method, x values are subtracted from other x-values. Tomorrow I could write the changes and open a pull-request on github.

[Owen Arnold]

Merge branch 'feature/9222_interpolation_defect'

Full changeset: ​d1d28a9a33437c2588cc138ee175b78c754e2aa8

[Stuart Campbell]

This ticket has been transferred to github issue ​10065