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(elisp)Formatting Strings
Formatting Strings
==================
"Formatting" means constructing a string by substitution of computed
values at various places in a constant string. This constant string
controls how the other values are printed, as well as where they appear;
it is called a "format string".
Formatting is often useful for computing messages to be displayed.
In fact, the functions `message' and `error' provide the same
formatting feature described here; they differ from `format' only in
how they use the result of formatting.
- Function: format string &rest objects
This function returns a new string that is made by copying STRING
and then replacing any format specification in the copy with
encodings of the corresponding OBJECTS. The arguments OBJECTS are
the computed values to be formatted.
The characters in STRING, other than the format specifications,
are copied directly into the output; starting in Emacs 21, if they
have text properties, these are copied into the output also.
A format specification is a sequence of characters beginning with a
`%'. Thus, if there is a `%d' in STRING, the `format' function
replaces it with the printed representation of one of the values to be
formatted (one of the arguments OBJECTS). For example:
(format "The value of fill-column is %d." fill-column)
=> "The value of fill-column is 72."
If STRING contains more than one format specification, the format
specifications correspond to successive values from OBJECTS. Thus, the
first format specification in STRING uses the first such value, the
second format specification uses the second such value, and so on. Any
extra format specifications (those for which there are no corresponding
values) cause unpredictable behavior. Any extra values to be formatted
are ignored.
Certain format specifications require values of particular types. If
you supply a value that doesn't fit the requirements, an error is
signaled.
Here is a table of valid format specifications:
`%s'
Replace the specification with the printed representation of the
object, made without quoting (that is, using `princ', not
`prin1'--Note: Output Functions). Thus, strings are represented
by their contents alone, with no `"' characters, and symbols appear
without `\' characters.
Starting in Emacs 21, if the object is a string, its text
properties are copied into the output. The text properties of the
`%s' itself are also copied, but those of the object take priority.
If there is no corresponding object, the empty string is used.
`%S'
Replace the specification with the printed representation of the
object, made with quoting (that is, using `prin1'--Note: Output
Functions). Thus, strings are enclosed in `"' characters, and
`\' characters appear where necessary before special characters.
If there is no corresponding object, the empty string is used.
`%o'
Replace the specification with the base-eight representation of an
integer.
`%d'
Replace the specification with the base-ten representation of an
integer.
`%x'
`%X'
Replace the specification with the base-sixteen representation of
an integer. `%x' uses lower case and `%X' uses upper case.
`%c'
Replace the specification with the character which is the value
given.
`%e'
Replace the specification with the exponential notation for a
floating point number.
`%f'
Replace the specification with the decimal-point notation for a
floating point number.
`%g'
Replace the specification with notation for a floating point
number, using either exponential notation or decimal-point
notation, whichever is shorter.
`%%'
Replace the specification with a single `%'. This format
specification is unusual in that it does not use a value. For
example, `(format "%% %d" 30)' returns `"% 30"'.
Any other format character results in an `Invalid format operation'
error.
Here are several examples:
(format "The name of this buffer is %s." (buffer-name))
=> "The name of this buffer is strings.texi."
(format "The buffer object prints as %s." (current-buffer))
=> "The buffer object prints as strings.texi."
(format "The octal value of %d is %o,
and the hex value is %x." 18 18 18)
=> "The octal value of 18 is 22,
and the hex value is 12."
All the specification characters allow an optional numeric prefix
between the `%' and the character. The optional numeric prefix defines
the minimum width for the object. If the printed representation of the
object contains fewer characters than this, then it is padded. The
padding is on the left if the prefix is positive (or starts with zero)
and on the right if the prefix is negative. The padding character is
normally a space, but if the numeric prefix starts with a zero, zeros
are used for padding. Here are some examples of padding:
(format "%06d is padded on the left with zeros" 123)
=> "000123 is padded on the left with zeros"
(format "%-6d is padded on the right" 123)
=> "123 is padded on the right"
`format' never truncates an object's printed representation, no
matter what width you specify. Thus, you can use a numeric prefix to
specify a minimum spacing between columns with no risk of losing
information.
In the following three examples, `%7s' specifies a minimum width of
7. In the first case, the string inserted in place of `%7s' has only 3
letters, so 4 blank spaces are inserted for padding. In the second
case, the string `"specification"' is 13 letters wide but is not
truncated. In the third case, the padding is on the right.
(format "The word `%7s' actually has %d letters in it."
"foo" (length "foo"))
=> "The word ` foo' actually has 3 letters in it."
(format "The word `%7s' actually has %d letters in it."
"specification" (length "specification"))
=> "The word `specification' actually has 13 letters in it."
(format "The word `%-7s' actually has %d letters in it."
"foo" (length "foo"))
=> "The word `foo ' actually has 3 letters in it."
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