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4.7 Formatting Strings

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 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, including their text properties, if any.

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."

Since 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 "%s", described below, as the first argument, and the string as the second, like this:

       (format "%s" arbitrary-string)

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 an error. 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—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.

%S
Replace the specification with the printed representation of the object, made with quoting (that is, using prin1—see Output Functions). Thus, strings are enclosed in ‘"’ characters, and ‘\’ characters appear where necessary before special characters.
%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."

A specification can have a width, which is a decimal number between the ‘%’ and the specification character. If the printed representation of the object contains fewer characters than this width, format extends it with padding. The width specifier is ignored for the ‘%%’ specification. Any padding introduced by the width specifier 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 three 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."

Immediately after the ‘%’ and before the optional width specifier, you can also 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’, ‘%f’, and ‘%g’, the ‘#’ flag means include a decimal point even if the precision is zero.

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 the padding inserted by the width specifier, if any, 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."

All the specification characters allow an optional precision before the character (after the width, if present). The precision is a decimal-point ‘.’ followed by a digit-string. For the floating-point specifications (‘%e’, ‘%f’, ‘%g’), the precision specifies how many decimal places to show; if zero, the decimal-point itself is also omitted. 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. Precision has no effect for other specification characters.