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### SPL_INTERP

SPL_INTERP

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 * !PIY = 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.

## 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

 4 Introduced

## 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.