@Section
   @Title { Dealing with tab characters in programs }
   @Tag { tabs }
@Begin
@PP
Tab characters provide a convenient way to indent and align parts of
programs. @RawIndex { programs }
programs.tab.characters @SubIndex { tab characters }
tab.characters.programs @Index { tab characters in programs }
computer programs.  With care, this alignment can be preserved in the final
print even with varying-width fonts.
@PP
The distance between two tab stops in the program text is by default taken
to be 8 characters, which is standard for Unix.  This can be changed with
programs. @RawIndex { programs }
programs.tabin @SubIndex { @Code "tabin" option }
tabin.programs @Index { @Code "tabin" option (programs) }
the @Code "tabin" option.  For example,
@ID @Code "@CP tabin { 4 }"
informs Lout that tab stops occur every 4 characters in the program
text.  All the symbols ({@Code "@CP"}, {@Code "@Eiffel"}, etc.) and
their setup files have this option and the next; but to save repetition
we will stick with C for the rest of this section.
@PP
The distance between two tab stops on the printed page is quite a different
thing, and it is determined by the value of the @Code "tabout" option, which
programs. @RawIndex { programs }
programs.tabout @SubIndex { @Code "tabout" option }
tabout.programs @Index { @Code "tabout" option (programs) }
must be a Lout length.  For example,
@ID @Code "@CP tabout { 0.5i }"
requests that tab stops be placed at half-inch intervals.  In other
words, a distance of one tab stop in the program text will be equivalent to a
distance of half an inch on the printed page.  For example,
@ID @Code "@CP style { varying } tabout { 3f } numbered { Yes }"
might produce the following, where tab characters in the program text
have been used for indenting and also to align the comments:
@ID @OneRow @CP style { varying } tabout { 3f } numbered { Yes } {
struct tnode {			/* the basic node */
	char *word;		/* points to the text */
	int count;		/* number of occurrences */
	struct tnode *left;	/* left child */
	struct tnode *right;	/* right child */
};
}
We've used @Code "numbered { Yes }" to demonstrate that the features
for dealing with tabs work even with line numbers.  The value {@Code "3f"}
means three times the current font size, and it is the default value of
@Code "tabout" for the @Code { varying } and @Code { symbol } styles
(Section {@NumberOf cpsetup}).  In a 12 point font this is 36 points,
or half an inch.
@PP
If @Code "tabout" is too small, there is a danger that the
alignment might fail.  For example,
@ID @Code "@CP style { varying } tabout { 0.2i }"
produces
@ID @OneRow @CP style { varying } tabout { 0.2i } {
struct tnode {			/* the basic node */
	char *word;		/* points to the text */
	int count;		/* number of occurrences */
	struct tnode *left;	/* left child */
	struct tnode *right;	/* right child */
};
}
given the same C text as the previous example.  The problem here is that
we are asking for @CP { /* } to appear four tab stops or 0.8 inches
from the left edge, and yet the material to its left on the line is
wider than this.  This causes @CP { /* } to be shifted further to the
right than expected, and the alignment is lost.  The only solution is
to increase {@Code "tabout"}.
@PP
When typesetting computer program texts independently of any document,
there are @Code "-t" and @Code "-T" options to the @Code "prg2lout"
program equivalent to @Code "tabin" and @Code "tabout" respectively.  For
example, @Code "-T0.5i" produces a half-inch tab width.
@End @Section