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The current version of SWI-Prolog provides two formatted write predicates. The‘writef' family (writef/1, writef/2, swritef/3), is compatible with Edinburgh C-Prolog and should be considered deprecated. The‘format' family (format/1, format/2, format/3), was defined by Quintus Prolog and currently available in many Prolog systems, although the details vary.
writef(Atom, ).See writef/2 for details.
Escape sequences to generate a single special character:
|Output a newline character (see also nl/[0,1])|
|Output a line separator (same as |
|Output a carriage return character (ASCII 13)|
|Output the ASCII character TAB (9)|
|The character |
|The character |
|where <nnn> is an integer (1-3 digits); the character with code <nnn> is output (NB : <nnn> is read as decimal)|
interpreted differently when character escapes are in effect. See
Escape sequences to include arguments from Arguments. Each time a % escape sequence is found in Format the next argument from Arguments is formatted according to the specification.
|print/1 the next item (mnemonic: term)|
|write/1 the next item|
|writeq/1 the next item|
|Write the term, ignoring operators. See also write_term/2. Mnemonic: old Edinburgh display/1|
the next item (identical to |
|Put the next item as a character (i.e., it is a character code)|
|Write the next item N times where N is the second item (an integer)|
|Write the next item as a String (so it must be a list of characters)|
|Perform a ttyflush/0 (no items used)|
|Write the next item Centered in N columns|
|Write the next item Left justified in N columns|
|Write the next item Right justified in N columns. N is a decimal number with at least one digit. The item must be an atom, integer, float or string.|
?- swritef(S, '%15L%w', ['Hello', 'World']). S = "Hello World"
swritef(String, Format, ).
The format family of predicates is the most versatile and portable142Unfortunately not covered by any standard. way to produce textual output.
format(Format) :- format(Format, ).’. See format/2 for details.
Special sequences start with the tilde (
followed by an optional numeric argument, optionally followed by a colon
143The colon modifiers is a
SWI-Prolog extension, proposed by Richard O'Keefe. followed
by a character describing the action to be undertaken. A numeric
argument is either a sequence of digits, representing a positive decimal
number, a sequence
representing the character code value of the character (only useful for
~t) or a asterisk (
), in which
case the numeric argument is taken from the next argument of the
argument list, which should be a positive integer. E.g., the following
three examples all pass 46 (
?- format('~w ~46t ~w~72|~n', ['Title', 'Page']). ?- format('~w ~`.t ~w~72|~n', ['Title', 'Page']). ?- format('~w ~*t ~w~72|~n', ['Title', 46, 'Page']).
Some format expressions may call back Prolog, i.e.,
~@ and user defined extensions registered
Output written to the stream
current_output is merged into
output. If there is no pending
~t) and the the position notation aligns,
only the output is switched. Otherwise the output is captured in a
temporary memory buffer and emitted after the callback finishes. The
system attempts to preserve the position and alignment promises. It sets
tty property of the temporary stream to reflect the
main stream and uses the position information of the temporary stream to
update its notion of the position. Notable ansi_format/3
cooperates properly in callbacks.144As
of version 8.3.30.
Numeric conversion (
G) accept an arithmetic expression as argument. This is
introduced to handle rational numbers transparently (see
section 22.214.171.124). The
floating point conversions allow for unlimited precision for printing
rational numbers in decimal form. E.g., the following will write as many
3's as you want by changing the‘50'.
?- format('~50f', [10 rdiv 3]). 3.33333333333333333333333333333333333333333333333333
The colon modifier (e.g.,
~:d) causes the number to be
printed according to the locale of the output stream. See section
~:d, but using the fixed English locale.
_). The argument describes the size of each digit group. The default is 3. See also section 126.96.36.199. For example:
?- A is 1<<100, format('~10I', [A]). 1_2676506002_2822940149_6703205376
~16rprints its argument hexadecimal. The argument should be in the range [2, ... , 36]. Lowercase letters are used for digits above 9. The colon modifier may be used to form locale-specific digit groups.
current_outputstream is inserted at this place. Goal is called in the module calling format/3. This option is not present in the original definition by Quintus, but supported by some other Prolog systems. The goal is executed as
\+ \+ Goal, i.e., bindings created by the goal are discarded.
~tstatements between the tab stops. This space is padded with spaces by default. If an argument is supplied, it is taken to be the character code of the character used for padding. This can be used to do left or right alignment, centering, distributing, etc. See also
~+to set tab stops. A tab stop is assumed at the start of each line.
~t’s to be distributed between the previous and this tab stop.
If the current column is at or past the requested tabstop and the
modifier (:) is used, a newline is inserted and the padding character of
~t is used to pad to the requested position.
~|) relative to the last tab stop or the beginning of the line if no tab stops are set before the
~+. This constructs can be used to fill fields. The partial format sequence below prints an integer right-aligned and padded with zeros in 6 columns. The ... sequences in the example illustrate that the integer is aligned in 6 columns regardless of the remainder of the format specification.
format('...~|~`0t~d~6+...', [..., Integer, ...])
format('~W', [Term, [numbervars(true)]]). This option is SWI-Prolog specific.
simple_statistics :- <obtain statistics> % left to the user format('~tStatistics~t~72|~n~n'), format('Runtime: ~`.t ~2f~34| Inferences: ~`.t ~D~72|~n', [RunT, Inf]), ....
Statistics Runtime: .................. 3.45 Inferences: .......... 60,345
library(backcomp). For example:
?- format(atom(A), '~D', ). A = '1,000,000'
~c(tilde, followed by some character) is found, the format/3 and friends first check whether the user has defined a predicate to handle the format. If not, the built-in formatting rules described above are used. Char is either a character code or a one-character atom, specifying the letter to be (re)defined. Head is a term, whose name and arity are used to determine the predicate to call for the redefined formatting character. The first argument to the predicate is the numeric argument of the format command, or the atom
defaultif no argument is specified. The remaining arguments are filled from the argument list. The example below defines
~Tto print a timestamp in ISO8601 format (see format_time/3). The subsequent block illustrates a possible call.
:- format_predicate('T', format_time(_Arg,_Time)). format_time(_Arg, Stamp) :- must_be(number, Stamp), format_time(current_output, '%FT%T%z', Stamp).
?- get_time(Now), format('Now, it is ~T~n', [Now]). Now, it is 2012-06-04T19:02:01+0200 Now = 1338829321.6620328.
Code is handled by the user-defined predicate specified by Head.