File Operations

In this section, we will discuss general file operations including listing files, opening or closing a file, and moving cursor within a file.

List Files

To list files and sub-directories within a directory, use the files function. It returns next information of files.

1. File names.

2. A directory or not. (0:file , 1:directory)

3. Size of files.

4. Last accessed time.

5. Last modified time.

$ files("C:/DolphinDB");

filename	isDir	fileSize	lastAccessed	lastModified
LICENSE_AND_AGREEMENT.txt	0	22558	1495508675000	1483773234998
README_WIN.txt	0	5104	1495508675000	1483866232680
server	1	0	1496624932437	1496624932437
THIRD_PARTY_SOFTWARE_LICENS…	0	8435	1495508675000	1483628426506

$ files("C:/DolphinDB", "readme%");

filename	isDir	fileSize	lastAccessed	lastModified
README_WIN.txt	0	5104	1495508675000	1483866232680

Alternatively, we can use SQL expressions to manipulate the returned table.

$ select * from files("C:/DolphinDB") where filename like "DolphinDB%";

filename	isDir	fileSize	lastAccessed	lastModified
DolphinDB 1.lnk	0	824	1499933602000	1500344826000
DolphinDB 2.lnk	0	790	1499933685000	1501832205000
DolphinDB 3.lnk	0	1006	1501829666000	1501832213000
DolphinDB 4(acl).lnk	0	872	1501829402000	1502626190412

Open and Close Files

The file function can open a file with a given mode. The opening mode could be one of the 6 modes: “r”, “r+”, “w”, “w+”, “a”, and “a+”. (For details please see file ）The close function closes an opened file handle.

$ fout=file("C:/DolphinDB/test.txt","w");
$ fout.writeLine("hello world!");
1

$ fout.close();
$ fin = file("C:/DolphinDB/test.txt");
$ print fin.readLine();
hello world!

$ fin.close();

In the example below, a file is opened within a function and it will be automatically closed when the function call completes. We don’t have to explicitly close the file in many cases unless it is necessary to close the file immediately. When the system shuts down, all open files will be closed.

$ def myread(f): file(f).readLine()
$ myread("C:/DolphinDB/test.txt");
Hello World!

Move Cursor within a File

When the system reads data from a file or writes data to a file, the internal cursor moves forward. Users can manipulate the cursor manually via the seek function. In addition to the file handle, the seek function accepts another 2 arguments: offset and the starting position. The offset could be positive or negative and the starting position must be of one the 3 positions: HEAD, CURRENT, and TAIL. The seek function returns the final position of the internal cursor if no exception is raised.

// write a function to show the length of a file
$ def fileLength(f): file(f).seek(0, TAIL)
$ fileLength("C:/DolphinDB/test.txt");
14

// move the internal cursor to the beginning of the file
$ fin=file("C:/DolphinDB/test.txt")
$ fin.readLine();
Hello World!
$ fin.seek(0, HEAD);
0
$ fin.readLine();
Hello World!