Name

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    POLINT

Purpose

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    Interpolate a set of N points by fitting a polynomial of degree N-1

Explanation

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    Adapted from algorithm in Numerical Recipes, Press et al. (1992),
    Section 3.1.

Calling Sequence

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    POLINT, xa, ya, x, y, [ dy ]

Inputs

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    XA - X Numeric vector, all values must be distinct. The number of
          values in XA should rarely exceed 10 (i.e. a 9th order polynomial)
    YA - Y Numeric vector, same number of elements
    X - Numeric scalar specifying value to be interpolated

Output

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    Y - Scalar, interpolated value in (XA,YA) corresponding to X

Optional Output

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    DY - Error estimate on Y, scalar

Example

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    Find sin(2.5) by polynomial interpolation on sin(indgen(10))
              IDL> xa = indgen(10)
              IDL> ya = sin( xa )
              IDL> polint, xa, ya, 2.5, y ,dy
           
    The above method gives y = .5988 & dy = 3.1e-4 a close
    approximation to the actual sin(2.5) = .5985

Method

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    Uses Neville's algorithm to iteratively build up the correct
    polynomial, with each iteration containing one higher order.

Revision History

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    Written W. Landsman January, 1992
    Converted to IDL V5.0 W. Landsman September 1997