4.32 Formatted Write

The current version of SWI-Prolog provides two formatted write predicates. The first is writef/[1,2], which is compatible with Edinburgh C-Prolog. The second is format/[1,2], which is compatible with Quintus Prolog. We hope the Prolog community will once define a standard formatted write predicate. If you want performance use format/[1,2] as this predicate is defined in C. Otherwise compatibility reasons might tell you which predicate to use.

4.32.1 Writef

Equivalent to write(Term), nl.
Equivalent to writef(Atom, []).
writef(+Format, +Arguments)
Formatted write. Format is an atom whose characters will be printed. Format may contain certain special character sequences which specify certain formatting and substitution actions. Arguments then provides all the terms required to be output.

Escape sequences to generate a single special character:

\n Output a newline character (see also nl/[0,1])
\l Output a line separator (same as \n)
\r Output a carriage-return character (ASCII 13)
\t Output the ASCII character TAB (9)
\\ The character \ is output
\% The character % is output
\nnn where <nnn> is an integer (1-3 digits) the character with character code <nnn> is output (NB : <nnn> is read as decimal)

Note that \l, \nnn and \\ are interpreted differently when character-escapes are in effect. See section

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.

%t print/1 the next item (mnemonic: term)
%w write/1 the next item
%q writeq/1 the next item
%d Write the term, ignoring operators. See also write_term/2. Mnemonic: old Edinburgh display/1.
%p print/1 the next item (identical to %t)
%n Put the next item as a character (i.e., it is a character code)
%r Write the next item N times where N is the second item (an integer)
%s Write the next item as a String (so it must be a list of characters)
%f Perform a ttyflush/0 (no items used)
%Nc Write the next item Centered in N columns.
%Nl Write the next item Left justified in N columns.
%Nr 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(-String, +Format, +Arguments)
Equivalent to writef/2, but ``writes'' the result on String instead of the current output stream. Example:
?- swritef(S, '%15L%w', ['Hello', 'World']).

S = "Hello          World"
swritef(-String, +Format)
Equivalent to swritef(String, Format, []).

4.32.2 Format

Defined as `format(Format) :- format(Format, []).'
format(+Format, +Arguments)
Format is an atom, list of character codes, or a Prolog string. Arguments provides the arguments required by the format specification. If only one argument is required and this is not a list of character codes the argument need not be put in a list. Otherwise the arguments are put in a list.

Special sequences start with the tilde (~), followed by an optional numeric argument, 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 `<character>, representing the character code value of the character (only useful for ~t) or a asterisk (*), in when the numeric argument is taken from the next argument of the argument list, which should be a positive integer.

Numeric conversion (d, D, e, E, f, g and G) accept an arithmetic expression as argument. This is introduced to handle rational numbers transparently (see section The floating point conversions allow for unlimited precision for printing rational numbers in decimal form.


simple_statistics :-
    <obtain statistics>         % left to the user
    format('Runtime: ~`.t ~2f~34|  Inferences: ~`.t ~D~72|~n',
                                            [RunT, Inf]),

Will output


Runtime: .................. 3.45  Inferences: .......... 60,345
format(+Output, +Format, +Arguments)
As format/2, but write the output on the given Output. The de-facto standard only allows Output to be a stream. The SWI-Prolog implementation allows all valid arguments for with_output_to/2.59Earlier versions defined sformat/3 . These predicates have been moved to the library library(backcomp). For example:
?- format(atom(A), '~D', [1000000]).
A = '1,000,000'

4.32.3 Programming Format

format_predicate(+Char, +Head)
If a sequence ~c (tilde, followed by some character) is found, the format derivatives will 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 an ASCII value, 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 default if no argument is specified. The remaining arguments are filled from the argument list. The example below redefines ~n to produce Arg times return followed by linefeed (so a (Grr.) DOS machine is happy with the output).
:- format_predicate(n, dos_newline(_Arg)).

dos_newline(default) :- !,
dos_newline(N) :-
        (   N > 0
        ->  write('\r\n'),
            N2 is N - 1,
        ;   true
current_format_predicate(?Code, ?:Head)
Enumerates all user-defined format predicates. Code is the character code of the format character. Head is unified with a term with the same name and arity as the predicate. If the predicate does not reside in module user, Head is qualified with the definition module of the predicate.