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@Section
@Title { Dealing with tab characters in programs }
@Tag { tabs }
@Begin
@PP
Tab characters provide a convenient way to indent and align parts of
tab.c @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
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
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 }"
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 } {
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 */
};
}
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
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