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Predicate number_string/2
Availability:built-in
number_string(?Number, ?String)
Bi-directional conversion between a number and a string. At least one of the two arguments must be instantiated. Besides the type used to represent the text, this predicate differs in several ways from its ISO cousin:171Note that SWI-Prolog's syntax for numbers is not ISO compatible either.

  • If String does not represent a number, the predicate fails rather than throwing a syntax error exception.
  • Leading white space and Prolog comments are not allowed.
  • Numbers may start with + or -.
  • It is not allowed to have white space between a leading + or - and the number.
  • Floating point numbers in exponential notation do not require a dot before exponent, i.e., "1e10" is a valid number.

Unlike other predicates of this family, if instantiated, String cannot be an atom.

The corresponding‘atom-handling’predicate is atom_number/2, with reversed argument order.

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LogicalCaptain said (2020-10-20T09:47:50):0 upvotes 0 0 downvotes
Picture of user LogicalCaptain.

The corresponding "atomikky" predicate, however, is called atom_number/2

Note that unlike other predicates, the String must indeed be a string, an atom is not accepted (at least for now). However, list of characters (atoms of length 1) and list of unicode code points are allowed:

:- begin_tests(number_string).

test("usual case", true(S == "123456")) :-
   number_string(123456,S).
   
test("usual case, with leading 0s", true(S == "123456")) :-
   number_string(0000123456,S).
   
test("parsing integer", true(N == 123456)) :-
   number_string(N,"123456").
   
test("parsing integer with leading 0s", true(N == 123456)) :-
   number_string(N,"0000123456").
   
test("no leading whitespace in string", fail) :-
   number_string(_,"    123456").
   
test("no trailing whitespace in string", fail) :-
   number_string(_,"123456     ").
   
test("fails if string-to-parse is not a number (rather than throwing) #1", fail) :-
   number_string(_,"NaN").
   
test("fails if string-to-parse is not a number (rather than throwing) #2", fail) :-
   number_string(_,"Hello, World!").

test("direct verification") :-
   number_string(123456,"123456").

% ---
% UNEXPECTED: "Type error: `list' expected, found `'123456'' (an atom)"
% ---

test("weird exception 1", true(N == 123456)) :-
   number_string(N,'123456').

% ---
% UNEXPECTED: "Type error: `list' expected, found `'0000123456'' (an atom)"
% ---

test("weird exception 2", true(N == 123456)) :-
   number_string(N,'0000123456').

% ---
% The above is explained by the fact that number_string/2 can actually process
% lists of unicode code points or characters to represent valid numbers, too:
% ---

test("Bad code points", fail) :-
   number_string(_,[1,2,3]).

test("Good list of characters", true(N == 123)) :-
   number_string(N,['1','2','3']).

test("A conversion too far: list of stringy characters", [error(type_error(_,_))]) :-
   number_string(_,["1","2","3"]).

test("Good list of code points #1", true(N == 123)) :-
   number_string(N,[0'1, 0'2, 0'3]).

test("Good list of code points #2", true(N == 123)) :-
   number_string(N,[0'0, 0'0, 0'1, 0'2, 0'3]).

:- end_tests(number_string).