[R] {grid} plain units with non NULL data arguments

[Prof Brian Ripley]

On Tue, 14 Aug 2007, Wolfram Fischer wrote:

> In help(unit) I read:
>
>     The 'data' argument must be a list when the 'unit.length()'
>     is greater than 1.  For example, 'unit(rep(1, 3), c("npc",
>     "strwidth", "inches"), data=list(NULL, "my string", NULL))'.
>
> In the newest R-versions it is not anymore allowed to let strings
> in the data-argument for plain units, otherwise one gets the
> following error:
>    Non-NULL value supplied for plain unit
>
> I have some labels. Between them I wanted to set a distance of 1.5 lines.
> (I wanted to use that for a grid.layout for a legend:
> The space is for the symbols.)
>
>    labels <- c( 'xxxx:', 'a', 'bb', 'ccc', 'dddd', 'eeeee' )
>    n <- length( labels )
>    s <- as.list( c( labels[1], rep( labels[-1], each=2 ) ) )
>    u <- unit( data=s, x=c( 1, rep( c( 1.5, 1 ), n-1 ) ),
>        units=c( 'strwidth', rep( c( 'lines', 'strwidth' ), n-1 ) ) )
>
> How can I insert the NULL values into the list ``s''?
>
> To fill every second element of s with NULL, I tried:
>    s[ 2 * ( 1 : length( labels[-1] ) ) ] <- NULL
> But this deletes every second element.

A value of list(NULL) is correct for inserting NULLs into lists.
(More generally to substitute in a list you need a list value.)

> The following would work:
>    s[ 2 * ( 1 : length( labels[-1] ) ) ] <- NA
> But unit() does not accept NAs.

More to the point, it does not accept logical vectors as NULL values.

-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
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