Re: [Groff] fetching a register's numeric value

[Ted Harding]

On 31-Aug-2012 06:25:26 Dave Kemper wrote:
> Hi,
> Is there a way to fetch the numeric value of a register regardless
> of what output format is currently associated with the register?
> 
> The only method I've been able to figure out is:
> 
> .ds reigning_format \g%
> .af % 0
> .nr page_number \n[%]
> .af % \*[reigning_format]
> 
> to save the format, switch it to numeric, fetch the value, and then
> switch the format back to its saved status. Then I can use
> \n[page_number] when I need the numeric version of the page number
> (in calculations,for instance).  This works, but is not very elegant.
> 
> The existence of a better mechanism is implied by the .af documentation:
> "Changing the output format of a read-only register causes an error.
> It is necessary to first copy the register's value to a writeable
> register, then apply the `af' request to this other register."
> Unfortunately, no hint is provided on how one might copy the register's
> value in a case where the register's output format cannot be changed.
> The intuitive solution -- using .nr -- fails when the output format is
> "i", generating the message:
> 
>    warning: numeric expression expected (got `i')
> 
> (or "got `v'" or "got `x'", depending on the register's current value)

Given that when groff enounters (e.g.) \n% it substitutes at that
point the value of \n% in its current ouitput format, I think that
your "not very elegantg" solution (or equivalent) is the way to do it.

However, you can sweep the inelegance under the carpet by defining
a macro. For example, the following:

.nr % 9
\n% \g%               \" To display default format (Arabic)
.br
.af % i
\n% \g%               \" To display new format (l.c. Roman)
.br
.de get%              \" Defines macro "get%"
.ds rf \\g%
.af % 1
.nr \\$1 \\n%
.af % \\*[rf]
..
.get% X
\n[X] \g[X] \n% \g%   \" To display final and original formats


produces the result:

9 0
ix i
9 0 ix i

And you could extend the macro to get the value from some register
other than \n%, using a second argument, e.g.

.de getnum
.ds rf \\g[\\$1]
.af \\$1 1
.nr \\$2 \\n[\\$1]
.af \\$1 \\*[rf]
..

so that ".getnum reg1 reg2" would put the numerical value of reg1
into reg2. Thus

.getnum % Y
\n[Y] \g[Y] \n% \g%

produces the same result

9 0 ix i

as before.

Ted.

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E-Mail: (Ted Harding) <address@hidden>
Date: 31-Aug-2012  Time: 10:10:01
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