VDIFF is the jVarfun module of jLab.

 VDIFF	Length-preserving first central difference.
    DX=VDIFF(X,DIM) differentiates X along dimension DIM using the first 
    central difference; DX is the same size as X.                                 
    [D1,D2,...,DN]=VDIFF(X1,X2,...,XN,DIM) for multiple input variables 
    also works. 
    VDIFF(X1,X2,...,DIM); with no output arguments overwrites the
    original input variables.
    DXDT=VDIFF(DT,...) optionally uses scalar timestep DT to approximate
    a time derivative, i.e. DXDT equals DX divided by DT.
    First and last points
    The first and last points must be treated differently, as the central 
    difference is not defined there.  Three different methods can be used.
    VDIFF(...,STR) specifies which method to use.
         'endpoint'  uses the first forwards / first backwards difference
                     at the first and last point, respectively.  
         'periodic'  treats the array as being periodic along dimension DIM,
                     so that the central difference is defined at endpoints.
         'nans'      fills in the first and last values with NANs.
    The default behavior is 'endpoint'.
    'vdiff --t' runs some tests.
    Usage:  x=vdiff(x,dim);
    This is part of JLAB --- type 'help jlab' for more information
    (C) 2000--2015 J.M. Lilly --- type 'help jlab_license' for details    

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