[R-sig-Geo] Better print method for Spatial*DataFrames?

[Roger Bivand]

On Sat, 29 May 2010, Barry Rowlingson wrote:

> On Sat, May 29, 2010 at 10:06 AM, Roger Bivand <Roger.Bivand at nhh.no> wrote:
>
>
>> In other software systems (octave, Stata, ...), one can turn on and off a
>> more/less screen-by-screen displayer (not scrolling upwards, just chunking),
>> but I'm not aware of an equivalent in R/S. I'm not sure how head() and
>> tail() work in R,
>
> They don't seem to work very well at all for Spatial*DataFrames. If I
> add coordinates to meuse to get a SpatialPointsDataFrame and
> head(that) I get all the 'rows' but with only the cadmium
> measurements. It's slicing it the wrong way. Odd.
>
>> and personally use str() by default. If I need to access
>> the coordinates of a particular line or polygon, I print() just that list
>> element (Line or Polygon object).
>>
>> I can see what you mean, but feel that users will benefit much more by using
>> str(), which is a real gem!
>
> str is great if you need to know the str-ucture of an R object. But
> it doesn't even align the values so you can see across rows of your
> data, which is what I'd like print to do (by analogy with
> print.data.frame).
>
> Currently if I print a SpatialPolygonsDataFrame I get the structure.
> Print methods should do better than that - you're almost suggesting
> not having, for example, a print method for data frames and that we'd
> be better off having what print.default(anyDataFrame) gives us.
>
> So my proposal is that print of a SpatialPolygonsDataFrame class
> should print like a data frame but with some indicator of the geometry
> at the start of the row, such as POLYGON(...) - literally with dots,
> there's no need to spell it out. Similarly for Lines.
>
> Another suggestion is for head() and tail methods on
> Spatial*DataFrame objects - I think just subscripting [1:n,] from the
> object and returning would do it. I think currently head and tail
> treat these objects as lists and the results are not pretty.

Right, because they see S4 objects as lists with no components, only with 
attributes. str() does have support for S4 objects. They would need to be 
wrapped around an S4 show/print method, with the output captured, as in 
capture.output(). Would it make sense to have the default print/show for 
Spatial* be str() with max.level= set, and for Spatial*DataFrame be the 
print method for the data slot prepended with some text (perhaps POINT, 
MULTILINESTRING, MULTIPOLYGON, PIXEL, CELL, or better an abbreviation)?

One would do this by cbind()ing the text in front of the as(, 
"data.frame"), I think, as a "geometry" variable.

Roger

>
> Barry
>

-- 
Roger Bivand
Economic Geography Section, Department of Economics, Norwegian School of
Economics and Business Administration, Helleveien 30, N-5045 Bergen,
Norway. voice: +47 55 95 93 55; fax +47 55 95 95 43
e-mail: Roger.Bivand at nhh.no