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*Subject*: Re: INT_TABULATED Errors*From*: landsman(at)my-deja.com*Date*: Sun, 23 Jan 2000 21:35:10 GMT*Newsgroups*: comp.lang.idl-pvwave*Organization*: Deja.com - Before you buy.*References*: <3888997E.7EBB8BE1@fsl.noaa.gov>*Xref*: news.doit.wisc.edu comp.lang.idl-pvwave:18117

In article <3888997E.7EBB8BE1@fsl.noaa.gov>, Andy Loughe <loughe@fsl.noaa.gov> wrote: > > ; Compute the area under the curve using INT_TABULATED > ; > ; Example 1 > x = [0.0, 0.3333, 0.985, 1.0] > y = [0.0, 0.529, 0.894, 1.0] > plot, x, y, psym=-2, title='Example 1' > print, ' Too small ==> ', INT_TABULATED(x, y) > ; > ; Example 2 > x = [0.0, 0.10, 0.79, 1.0] > y = [0.0, 0.67, 0.96, 1.0] > plot, x, y, psym=-2, title='Example 2' > print, ' Too large ==> ', INT_TABULATED(x, y) Your example has two characteristics that make them unsuitable for INT_TABULATED. First they have abrupt changes in slope, and so INT_TABULATED, which uses cubic spline interpolation, will poorly approximate the function. Second, they have a very irregularly spaced grid, and INT_TABULATED computes the integral using a regular grid with the same spacing. (I would have rewritten INT_TABULATED to make sure the regular grid spacing is at least as fine as the smallest separation in the irregular input grid.) Anyway, for a non-smooth curve I recommend using a simple trapezoidal integration, such as tsum.pro available at http://idlastro.gsfc.nasa.gov/ftp/pro/math/tsum.pro Wayne Landsman landsman@mpb.gsfc.nasa.gov Sent via Deja.com http://www.deja.com/ Before you buy.

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