Given the arrays X and Y, which tabulate a function (with the Xi in ascending order), and given the array Y2, which is the output from SPL_INIT, and given an input value of X2, the SPL_INTERP function returns a cubic-spline interpolated value for the given value of XI.
Examples
To create a spline interpolation over a tabulated set of data, [Xi, Yi], first create the tabulated data. In this example, Xi will be in the range [0.0, 2π] and Yi in the range [sin(0.0), sin(2π)].
X = (FINDGEN(21)/20.0) * 2.0 * !PI
Y = SIN(X)
Y2 = SPL_INIT(X, Y)
X2= FINDGEN(11)/11.0 * !PI
result = SPL_INTERP(X, Y, Y2, X2)
PRINT, result
IDL prints:
0.00000 0.281733 0.540638 0.755739 0.909613 0.989796
0.989796 0.909613 0.755739 0.540638 0.281733
The exact solution vector is sin(X2).
To interpolate a line in the XY plane, see SPLINE_P.
Syntax
Result = SPL_INTERP( X, Y, Y2, X2 [, /DOUBLE] )
Return Value
Returns either single- or double-precision floating result of the same structure as X2.
Arguments
X
An input array that specifies the tabulated points in ascending order.
Y
An input array that specifies the values of the tabulate function corresponding to Xi.
Y2
The output from SPL_INIT for the specified X and Y.
X2
The input value for which an interpolated value is desired. X can be scalar or an array of values. The result of SPL_INTERP will have the same structure.
Keywords
DOUBLE
Set this keyword to force the computation to be done in double-precision arithmetic.
Version History
Resources and References
SPL_INTERP is based on the routine splint described in section 3.3 of Numerical Recipes in C: The Art of Scientific Computing (Second Edition), published by Cambridge University Press, and is used by permission.
See Also
SPL_INIT, SPLINE, SPLINE_P