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5.6 Files

Asymptote can read and write text files (including comma-separated value) files and portable XDR (External Data Representation) binary files.

An input file must first be opened with input(string name, bool check=true, string commentchar="#"); reading is then done by assignment:

file fin=input("test.txt");
real a=fin;

If the optional boolean argument check is false, no check will be made that the file exists. If the file does not exist or is not readable, the function bool error(file) will return true. The first character of the string commentchar specifies a comment character. If this character is encountered in a data file, the remainder of the line is ignored. When reading strings, the comment character must be in the first column (otherwise it will be treated as an ordinary character).

One can change the current working directory to the contents of the string d with the function string cd(string d), which returns the new working directory.

When reading pairs, the enclosing parenthesis are optional. Strings are also read by assignment, by reading characters up to but not including a newline. In addition, Asymptote provides the function string getc(file) to read the next character only, returning it as a string.

A file named name can be open for output with

file output(string name, bool append=false);
data will be appended to an existing file only if the file is opened with append=true. Data of a built-in type T can be written to an output file by calling one of the functions
write(string s="", T x, suffix suffix=endl ... T[]);
write(file fout, string s="", T x, suffix suffix=none ... T[]);
write(file fout=stdout, string s="", explicit T[] x ... T[][]);
write(file fout=stdout, T[][]);
write(file fout=stdout, T[][][]);
write(suffix suffix=endl);
write(file fout, suffix suffix=none);
If fout is not specified, stdout is used and terminated by default with a newline. If specified, the optional identifying string s is written before the data x. An arbitrary number of data values may be listed when writing scalars or one-dimensional arrays. The suffix may be one of the following: none (do nothing), endl (terminate with a newline), or tab (terminate with a tab). Here are some simple examples of data output:
file fout=output("test.txt");
write(fout,1);                  // Writes "1"
write(fout);                    // Writes a new line
write(fout,"List: ",1,2,3);     // Writes "List: 1     2     3"
There are two special files: stdin, which reads from the keyboard, and stdout, which writes to the terminal.

A file may also be opened with xinput or xoutput instead of input or output, in which case it will read or write double precision values written in Sun Microsystem's XDR (External Data Representation) portable binary format (available on all UNIX platforms). The function file single(file) sets the file to read single precision XDR values; calling file single(file,false) sets it back to read doubles again. The default initializer for file is stdout.

One can test a file for end-of-file with the boolean function eof(file), end-of-line with eol(file), and for I/O errors with error(file). One can flush the output buffers with flush(file), clear a previous I/O error with clear(file), and close the file with close(file). To set the number of digits of output precision, use precision(file,int). The function int tell(file) returns the current position in an input file relative to the beginning. The function seek(file, int) can be used to change this position; for example, to rewind a file, use the command seek(file,0).

Assigning settings.scroll=n for a positive integer n requests a pause after every n output lines to stdout. One may then press Enter to continue or q followed by Enter to quit. If n is negative, the output scrolls a page at a time (i.e. by one less than the current number of display lines). The default value, settings.scroll=0, specifies continuous scrolling.

The routines

string getstring(string name="", string default="", string prompt="",
                 bool save=true);
int getint(string name="", int default=0, string prompt="",
           bool save=true);
real getreal(string name="", real default=0, string prompt="",
             bool save=true);
pair getpair(string name="", pair default=0, string prompt="",
             bool save=true)
defined in the module plain may be used to prompt for a value from stdin using the GNU readline library. If save=true, the history of values for name is saved to the file ".asy_"+name (see history). The most recent value in the history will be used to provide a default value for subsequent runs. The default value (initially default) is displayed after prompt. These routines are based on the following interface to readline, which prompts the user with the default value formatted according to prompt and saves the local history under the name ".asy_"+history, unless the string history begins with a linefeed ('\n'):
string readline(string prompt="", string history="", string initial="",
                bool tabcompletion=false);