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88 EXCEL: NUMERICAL METHODS
Figure 5-12. Four bracketing x values required
to perform cubic interpolation at x = 33.3%.
(folder 'Chapter 05 Interpolation', workbook 'Interpolation I', sheet Cubic Interpolation')
The Lagrange fourth-order polynomial is cumbersome to use in a worksheet
function, but convenient to use in the form of a custom function. A compact and
elegant implementation of cubic interpolation in the form of an Excel 4.0 Macro
Language custom function was provided by Orvis'. A slightly modified version,
in VBA, is provided here (Figure 5-13). The syntax of the custom function is
InterpC(/ookup-value, known-x's, knownj's). The argument lookup-value is
the value of the independent variable for which you want the interpolated y
value; known-x's and known-y's are the arrays of independent and dependent
variables, respectively, that comprise the table. The table must be sorted in
ascending order of known-x 8.
* William J. Orvis, Excel 4for Scientists and Engineers, Sybex Inc., Alameda, CA, 1993.
