[Scilab-users] Insufficient documentation of interp1

[Samuel Gougeon]

Le 10/01/2021 à 08:18, Federico Miyara a écrit :
>
> Dear All,
>
> I find that the extrapolation option for interp1 is not thoroughly 
> documented. In the Arguments section it states, for extrapolation,
>
>     (optional) string, or real value defining the yp(j) components for
>     xp(j) values outside [x1,xn] interval.
>
>
> In the Description section the only string listed is "extrap" which says:
>
>     the extrapolation is performed by the defined method
>
>
> But which is the defined method? 

<method>, used for interpolation.


> If one selects the method "spline" it is suggested to refer to 
> interp(), which has a specific argument (called out_mode) with several 
> possibilities for extrapolation. These possibilities aren't accessible 
> from the interp1() function, so it should be completely clear in its 
> documantation which one is used.


Right. This is true also for interp(), for which the default 
extrapolation behavior is not documented.


>
> Besides, the argument extrapolation is optional but it is not clear 
> what happens if it is not used. I suppose it is what appears under "by 
> default",


Sure


> but again it says
>
>     the extrapolation is performed by the defined method (for spline
>     method)
>
>
> But, as already mentioned, tha spline method has more than one 
> extrapolation strategy, which can be selected from interp but not from 
> interp1.


It is the same remark as above.

> Finally, at least for spline it seems to be no difference between 
> setting the extrapolation argument to "extrap" or not using it.

According to the current documentation,

interp1(x, y, xp, "spline", "extrap")  // is equivalent to
interp1(x, y, xp, "spline", "spline")

// while

interp1(x, y, xp, "spline")  // is as well equivalent to
interp1(x, y, xp, "spline", "spline")

So yes, same results are expected.

But i find as well the "by default" explanation uselessly complicated: From
/  by default ://
//       the extrapolation is performed by the defined method (for 
spline method), and by nan for//
//       linear and nearest methods. yp=interp1(x,y,xp,method)//
/it could rather be something like
/  by default ://
//       the extrapolation is performed by spline when the interpolation 
is done by spline, and by nan for//
//       linear and nearest interpolation methods.//

/Samuel Gougeon

/
/

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