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read_string(+Stream, +Delimiters, ?Length, -String)

Reads a string from the stream Stream up to a delimiter or up to a specified length.
Stream
Stream handle or alias (atom)
Delimiters
String or atom.
Length
Integer or variable.
String
String or variable.

Description

A string of characters is read from the input stream Stream up to one character which occurs in the delimiter string Delimiters. This character is also consumed, but does not appear in the string which is unified with String.

Two symbolic Delimiters can be specified:

    end_of_line   a newline or carriage-return/newline sequence
    end_of_file   the end of the file/input
End of file always acts like a delimiter.

If Length is a variable, it is unified with the length of the string String. If Length is an integer, the number of characters read from the input stream Stream is limited by Length.

Modes and Determinism

Fail Conditions

There is nothing to read, i.e. the stream is at end_of_file

Exceptions

(4) instantiation fault
Delimiters is not instantiated.
(5) type error
Delimiters is not a string or atom.
(5) type error
Length is a non-integer number.
(5) type error
String is not a variable or a string.
(6) out of range
Delimiters is an atom but not a valid symbolic delimiter.
(24) number expected
Length is not a variable or number.
(190) end of file reached
End of file was encountered before reading any character.
(192) illegal stream mode
Stream is not an input stream.
(193) illegal stream specification
Stream is an illegal stream specification.
(198) reading past the file end
Trying to read even after the error 190 was raised.

Examples

Success:
      ?- read_string(input, "123", Length, String).
       abcdef2ghi
      Length = 6
      String = "abcdef"
      yes.

      ?- read_string(input, " \t", Length, String).
       one two
      Length = 3
      String = "one"
      yes.

      ?- read_string(input, end_of_line, Length, String).
      abcdefghi
      Length = 9
      String = "abcdefghi"
      yes.

      ?- read_string(input, end_of_line, 6, String).
      abcdefghi
      String = "abcdef"
      yes.

      ?- open(file1, read, s).
      yes.
      ?- system('cat file1').
      abcd
      yes.
      ?- read_string(s, "", Length, String).
      Length = 5
      String = "abcd\n"           % Read up to end of file
      yes.


% Code example: read lines from a file and return
% a list of strings, each string containing a line

    get_lines(File, Lines) :-
        open(File, read, S),
        stream_get_lines(S, Lines),
        close(S).

    stream_get_lines(S, Lines) :-
        ( read_string(S, end_of_line, _, Line) ->
            Lines = [Line|Ls],
            stream_get_lines(S, Ls)
        ;
            Lines = []
        ).


Fail:
      ?- open(string(""),read,s), read_string(s,"",L,String).

      no (more) solution.         % EOF - Error 190 - handler fails

Error:
    read_string(Stream, "", Length, String).       (Error 4).
    read_string(stream, Dels, Length, String).     (Error 4).
    read_string("stream", "", Length, String).     (Error 5).
    read_string(stream, 12, Length, String).       (Error 5).
    read_string(stream, "", "abc", String).        (Error 5).
    read_string(stream, "", Length, 12).           (Error 5).
    read_string(stream, stop, Length, String).     (Error 6).
    read_string(output, "", Length, String).       (Error 192).
    read_string(atom, "", Length, String).         (Error 193).



See Also

read_string / 5, read_token / 2, read_token / 3, open / 3