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LINTERP

LINTERP

Name


      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) Prior to V8.2.3 LINTERP converted the new grid vector to floating point
                (because INTERPOLATE does this) whereas INTERPOL() and post-V8.2.3
                LINTERP will keep double precision if supplied.
      Use QUADTERP for quadratic interpolation.

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 float

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
      Added MISSING keyword August, 1993
      Converted to IDL V5.0 W. Landsman September 1997
      Added NoInterp keyword W. Landsman July 1999
      Work for unsigned, 64 bit integers W. Landsman October 2001
      Call INTERPOLATE with /DOUBLE if V8.2.3 W. Landsman Feb 2015



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