Routines (alphabetical) > Routines: S > 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 * !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.

## 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

4.0 | 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.

## See Also

SPL_INIT, SPLINE, SPLINE_P