Interpolation Bug

[OwenArnold]

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.

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Keywords: PatchCandidate

[OwenArnold]

This issue was originally trac ticket 9222