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LINTERP

LINTERP

## Purpose

Linearly interpolate tabulated 1-d data from one grid to a new one.

## Explanation

The results of LINTERP are numerically equivalent to the IDL intrinsic
INTERPOL() function, but note the following:
(1) LINTERP is a procedure rather than a function
(2) INTERPOL() extrapolates beyond the end points whereas LINTERP
truncates to the endpoints (or uses the MISSING keyword)
(3) LINTERP (unlike INTERPOL) uses the intrinsic INTERPOLATE function
and thus may have a speed advantage
(4) LINTERP always converts the new grid vector to floating point
(because INTERPOLATE does this) whereas INTERPOL() will
keep double precision if supplied.

## Calling Sequence

LINTERP, Xtab, Ytab, Xint, Yint, [MISSING =, /NoInterp ]

## Input Parameters

Xtab - Vector containing the current independent variable grid.
Must be monotonic increasing or decreasing
Ytab - Vector containing the current dependent variable values at
the XTAB grid points.
Xint - Scalar or vector containing the new independent variable grid
points for which interpolated value(s) of the dependent
variable are sought. Note that -- due to a limitation of the
intrinsic INTERPOLATE() function -- Xint is always converted to
floating point internally.

## Output Parameters

Yint - Scalar or vector with the interpolated value(s) of the
dependent variable at the XINT grid points.
YINT is double precision if XTAB or YTAB are double,
otherwise YINT is REAL*4

## Optional Input Keyword

MISSING - Scalar specifying YINT value(s) to be assigned, when Xint
value(s) are outside of the range of Xtab. Default is to
truncate the out of range YINT value(s) to the nearest value
of YTAB. See the help for the INTERPOLATE function.
/NoINTERP - If supplied then LINTERP returns the YTAB value(s)
associated with the closest XTAB value(s)rather than
interpolating.

## Example

To linearly interpolate from a spectrum wavelength-flux pair
Wave, Flux to another wavelength grid defined as:
WGrid = [1540., 1541., 1542., 1543., 1544, 1545.]

IDL> LINTERP, Wave, Flux, WGrid, FGrid
FGRID will be a 6 element vector containing the values of Flux
linearly interpolated onto the WGrid wavelength scale

## Procedure

Uses TABINV to calculate the effective index of the values
in Xint in the table Xtab. The resulting index is used
with the intrinsic INTERPOLATE function to find the corresponding
Yint value in Ytab. Unless the MISSING keyword is supplied, out
of range Yint values are truncated to the nearest value of Ytab.

## Procedures Called

TABINV, ZPARCHECK

## Modification History

Adapted from the IUE RDAF, W. Landsman October, 1988
Modified to use the new INTERPOLATE function June, 1992
Modified to always return REAL*4 October, 1992