SPL_INTERP

Given the arrays X and Y, which tabulate a function (with the X_i 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 X_I.

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.

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 X_i.

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.

Examples

To create a spline interpolation over a tabulated set of data, [X_i, Y_i], first create the tabulated data. In this example, X_i will be in the range [0.0, 2p] and Y_i in the range [sin(0.0), sin(2p)].

X = (FINDGEN(21)/20.0) * 2.0 * !PI 
Y = SIN(X) 
 
; Calculate interpolating cubic spline: 
Y2 = SPL_INIT(X, Y) 
 
; Define the X values P at which we desire interpolated Y values: 
X2= FINDGEN(11)/11.0 * !PI 
 
; Calculate the interpolated Y values corresponding to X2[i]: 
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.

Version History

Introduced: 4.0

See Also

SPL_INIT, SPLINE, SPLINE_P