### 4.8.4 ASCII links

Via ASCII links data that can be converted to a string can be written into files for storage or communication with other programs. The data is written in plain ASCII format. The output format of polynomials is done w.r.t. the value of the global variable `short` (see short). Reading from an ASCII link returns a string -- conversion into other data is up to the user. This can be done, for example, using the command `execute` (see execute).

The ASCII link describing string has to be one of the following:

1. `"ASCII: "` + filename
   the mode (read or append) is set by the first `read` or `write` command.
2. `"ASCII:r "` + filename
   opens the file for reading.
3. `"ASCII:w "` + filename
   opens the file for overwriting.
4. `"ASCII:a "` + filename
   opens the file for appending.

There are the following default values:

- the type `ASCII` may be omitted since ASCII links are the default links.

- if non of `r`, `w`, or `a` is specified, the mode of the link is set by the first `read` or `write` command on the link. If the first command is `write`, the mode is set to `a` (append mode).

- if the filename is omitted, `read` reads from stdin and `write` writes to stdout.

Using these default rules, the string `":r temp"` describes a link which is equivalent to the link `"ASCII:r temp"`: an ASCII link to the file `temp` which is opened for reading. The string `"temp"` describes an ASCII link to the file `temp`, where the mode is set by the first `read` or `write` command. See also the example below.

Note that the filename may contain a path. On Microsoft Windows (resp. MS-DOS) platforms, names of a drive can precede the filename, but must be started with a `//` (as in `//c/temp/ex`. An ASCII link can be used either for reading or for writing, but not for both at the same time. A `close` command must be used before a change of I/O direction. Types without a conversion to `string` cannot be written.

Example:

      ring r=32003,(x,y,z),dp;
      link l=":w example.txt";     // type is ASCII, mode is overwrite
      l;
    ==> // type : ASCII
    ==> // mode : w
    ==> // name : example.txt
    ==> // open : no
    ==> // read : not ready
    ==> // write: not ready
      status(l, "open", "yes");    // link is not yet opened
    ==> 0
      ideal i=x2,y2,z2;
      write (l,1,";",2,";","ideal i=",i,";");
      status(l, "open", "yes");    // now link is open
    ==> 1
      status(l, "mode");           // for writing
    ==> w
      close(l);                    // link is closed
      write("example.txt","int j=5;");// data is appended to file
      read("example.txt");         // data is returned as string
    ==> 1
    ==> ;
    ==> 2
    ==> ;
    ==> ideal i=
    ==> x2,y2,z2
    ==> ;
    ==> int j=5;
    ==>
      execute(read(l));            // read string is executed
    ==> 1
    ==> 2
    ==> // ** redefining i **
      close(l);                    // link is closed

User manual for Singular version 4-0-3, 2016, generated by texi2html.