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(Formatting Strings): Treat - and 0 as flag characters.

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Chong Yidong 2008-02-26 18:44:55 +00:00
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lispref/strings.texi
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@ -822,58 +822,80 @@ operation} error.
@cindex field width
@cindex padding
A specification can have a @dfn{width}, which is a signed decimal
number between the @samp{%} and the specification character. If the
printed representation of the object contains fewer characters than
this width, @code{format} extends it with padding. The padding goes
on the left if the width is positive (or starts with zero) and on the
right if the width is negative. The padding character is normally a
space, but it's @samp{0} if the width starts with a zero.
Some of these conventions are ignored for specification characters
for which they do not make sense. That is, @samp{%s}, @samp{%S} and
@samp{%c} accept a width starting with 0, but still pad with
@emph{spaces} on the left. Also, @samp{%%} accepts a width, but
ignores it. Here are some examples of padding:
A specification can have a @dfn{width}, which is a decimal number
between the @samp{%} and the specification character. If the printed
representation of the object contains fewer characters than this
width, @code{format} extends it with padding. The width specifier is
ignored for the @samp{%%} specification. Any padding introduced by
the width specifier normally consists of spaces inserted on the left:
@example
(format "%06d is padded on the left with zeros" 123)
@result{} "000123 is padded on the left with zeros"
(format "%-6d is padded on the right" 123)
@result{} "123 is padded on the right"
(format "%5d is padded on the left with spaces" 123)
@result{} " 123 is padded on the left with spaces"
@end example
@noindent
If the width is too small, @code{format} does not truncate the
object's printed representation. Thus, you can use a width to specify
a minimum spacing between columns with no risk of losing information.
In the following three examples, @samp{%7s} specifies a minimum width
of 7. In the first case, the string inserted in place of @samp{%7s}
has only 3 letters, and needs 4 blank spaces as padding. In the
second case, the string @code{"specification"} is 13 letters wide but
is not truncated.
In the following three examples, @samp{%7s} specifies a minimum
width of 7. In the first case, the string inserted in place of
@samp{%7s} has only 3 letters, it needs 4 blank spaces as padding. In
the second case, the string @code{"specification"} is 13 letters wide
but is not truncated. In the third case, the padding is on the right.
@smallexample
@example
@group
(format "The word `%7s' actually has %d letters in it."
"foo" (length "foo"))
@result{} "The word ` foo' actually has 3 letters in it."
@end group
@group
(format "The word `%7s' actually has %d letters in it."
"specification" (length "specification"))
@result{} "The word `specification' actually has 13 letters in it."
@end group
@end example
@cindex flags in format specifications
Immediately after the @samp{%} and before the optional width
specifier, you can also put certain @dfn{flag characters}.
The flag @samp{+} inserts a plus sign before a positive number, so
that it always has a sign. A space character as flag inserts a space
before a positive number. (Otherwise, positive numbers start with the
first digit.) These flags are useful for ensuring that positive
numbers and negative numbers use the same number of columns. They are
ignored except for @samp{%d}, @samp{%e}, @samp{%f}, @samp{%g}, and if
both flags are used, @samp{+} takes precedence.
The flag @samp{#} specifies an ``alternate form'' which depends on
the format in use. For @samp{%o}, it ensures that the result begins
with a @samp{0}. For @samp{%x} and @samp{%X}, it prefixes the result
with @samp{0x} or @samp{0X}. For @samp{%e}, @samp{%f}, and @samp{%g},
the @samp{#} flag means include a decimal point even if the precision
is zero.
The flag @samp{-} causes the padding inserted by the width
specifier, if any, to be inserted on the right rather than the left.
The flag @samp{0} ensures that the padding consists of @samp{0}
characters instead of spaces, inserted on the left. These flags are
ignored for specification characters for which they do not make sense:
@samp{%s}, @samp{%S} and @samp{%c} accept the @samp{0} flag, but still
pad with @emph{spaces} on the left. If both @samp{-} and @samp{0} are
present and valid, @samp{-} takes precedence.
@example
@group
(format "%06d is padded on the left with zeros" 123)
@result{} "000123 is padded on the left with zeros"
(format "%-6d is padded on the right" 123)
@result{} "123 is padded on the right"
(format "The word `%-7s' actually has %d letters in it."
"foo" (length "foo"))
@result{} "The word `foo ' actually has 3 letters in it."
@end group
@end smallexample
@end example
@cindex precision in format specifications
All the specification characters allow an optional @dfn{precision}
@ -887,25 +909,6 @@ shows only the first three characters of the representation for
@var{object}. Precision has no effect for other specification
characters.
@cindex flags in format specifications
Immediately after the @samp{%} and before the optional width and
precision, you can put certain ``flag'' characters.
@samp{+} as a flag inserts a plus sign before a positive number, so
that it always has a sign. A space character as flag inserts a space
before a positive number. (Otherwise, positive numbers start with the
first digit.) Either of these two flags ensures that positive numbers
and negative numbers use the same number of columns. These flags are
ignored except for @samp{%d}, @samp{%e}, @samp{%f}, @samp{%g}, and if
both flags are used, the @samp{+} takes precedence.
The flag @samp{#} specifies an ``alternate form'' which depends on
the format in use. For @samp{%o} it ensures that the result begins
with a @samp{0}. For @samp{%x} and @samp{%X}, it prefixes the result
with @samp{0x} or @samp{0X}. For @samp{%e}, @samp{%f}, and @samp{%g},
the @samp{#} flag means include a decimal point even if the precision
is zero.
@node Case Conversion
@comment node-name, next, previous, up
@section Case Conversion in Lisp