Formatting means constructing a string by substituting 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
error provide the same
formatting feature described here; they differ from
in how they use the result of formatting.
This function returns a string equal to string, replacing any format specifications 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, including their text properties, if any. Any text properties of the format specifications are copied to the produced string representations of the argument objects.
The output string need not be newly-allocated. For example, if
xis the string
"foo", the expressions
(eq x (format x))and
(eq x (format "%s" x))might both yield
Typically grave accent and apostrophe in the format translate to matching curved quotes, e.g., "Missing `%s'" might result in "Missing ‘foo’". See Text Quoting Style, for how to influence or inhibit this translation.
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).
(format "The value of fill-column is %d." fill-column) ⇒ "The value of fill-column is 72."
format interprets ‘%’ characters as format
specifications, you should never pass an arbitrary string as
the first argument. This is particularly true when the string is
generated by some Lisp code. Unless the string is known to
never include any ‘%’ characters, pass
below, as the first argument, and the string as the second, like this:
(format "%s" arbitrary-string)
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:
prin1—see Output Functions). Thus, strings are represented by their contents alone, with no ‘"’ characters, and symbols appear without ‘\’ characters.
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.
prin1—see Output Functions). Thus, strings are enclosed in ‘"’ characters, and ‘\’ characters appear where necessary before special characters.
(format "%% %d" 30)returns
Any other format character results in an ‘Invalid format operation’ error.
Here are several examples, which assume the typical
(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." (format-message "The name of this buffer is ‘%s’." (buffer-name)) ⇒ "The name of this buffer is ‘strings.texi’." (format-message "The buffer object prints as `%s'." (current-buffer)) ⇒ "The buffer object prints as ‘strings.texi’."
By default, 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 an error. Any extra values to be formatted are ignored.
A format specification can have a field number, which is a decimal number immediately after the initial ‘%’, followed by a literal dollar sign ‘$’. It causes the format specification to convert the argument with the given number instead of the next argument. Field numbers start at 1. A format can contain either numbered or unnumbered format specifications but not both, except that ‘%%’ can be mixed with numbered specifications.
(format "%2$s, %3$s, %%, %1$s" "x" "y" "z") ⇒ "y, z, %, x"
After the ‘%’ and any field number, you can put certain flag characters.
The 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.) These flags are useful for ensuring that positive numbers and negative numbers use the same number of columns. They are ignored except for ‘%d’, ‘%e’, ‘%f’, ‘%g’, and if both flags are used, ‘+’ takes precedence.
The flag ‘#’ specifies an alternate form which depends on the format in use. For ‘%o’, it ensures that the result begins with a ‘0’. For ‘%x’ and ‘%X’, it prefixes the result with ‘0x’ or ‘0X’. For ‘%e’ and ‘%f’, the ‘#’ flag means include a decimal point even if the precision is zero. For ‘%g’, it always includes a decimal point, and also forces any trailing zeros after the decimal point to be left in place where they would otherwise be removed.
The flag ‘0’ ensures that the padding consists of ‘0’ characters instead of spaces. This flag is ignored for non-numerical specification characters like ‘%s’, ‘%S’ and ‘%c’. These specification characters accept the ‘0’ flag, but still pad with spaces.
The flag ‘-’ causes any padding inserted by the width, if specified, to be inserted on the right rather than the left. If both ‘-’ and ‘0’ are present, the ‘0’ flag is ignored.
(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 "The word '%-7s' actually has %d letters in it." "foo" (length "foo")) ⇒ "The word 'foo ' actually has 3 letters in it."
A specification can have a width, which is a decimal number
that appears after any field number and flags. If the printed
representation of the object contains fewer characters than this
format extends it with padding. Any padding introduced by
the width normally consists of spaces inserted on the left:
(format "%5d is padded on the left with spaces" 123) ⇒ " 123 is padded on the left with spaces"
If the width is too small,
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 two examples, ‘%7s’ specifies a minimum width
of 7. In the first case, the string inserted in place of ‘%7s’
has only 3 letters, and needs 4 blank spaces as padding. In the
second case, the string
"specification" is 13 letters wide but
is not truncated.
(format "The word '%7s' has %d letters in it." "foo" (length "foo")) ⇒ "The word ' foo' has 3 letters in it." (format "The word '%7s' has %d letters in it." "specification" (length "specification")) ⇒ "The word 'specification' has 13 letters in it."
All the specification characters allow an optional precision
after the field number, flags and width, if present. The precision is
a decimal-point ‘.’ followed by a digit-string. For the
floating-point specifications (‘%e’ and ‘%f’), the
precision specifies how many digits following the decimal point to
show; if zero, the decimal-point itself is also omitted. For
‘%g’, the precision specifies how many significant digits to show
(significant digits are the first digit before the decimal point and
all the digits after it). If the precision of %g is zero or
unspecified, it is treated as 1. For ‘%s’ and ‘%S’, the
precision truncates the string to the given width, so ‘%.3s’
shows only the first three characters of the representation for
object. For other specification characters, the effect of
precision is what the local library functions of the