Io Library Tutorial
The I/O library provides two different styles for file manipulation. The first one uses implicit file descriptors, that is, there are operations to set a default input file and a default output file, and all input/output operations are over those default files. The second style uses explicit file descriptors.
When using implicit file descriptors, all operations are supplied by table
io. When using explicit file descriptors, the operation
io.open returns a file descriptor and then all operations are supplied as methods by the file descriptor.
io also provides three predefined file descriptors with their usual meanings from C:
A file descriptor is a userdata containing the file stream
(FILE*), with a distinctive metatable created by the I/O library.
Unless otherwise stated, all I/O functions return
nil on failure (plus an error message as a second result) and some value different from
nil on success.
file = io.open (filename [, mode])
This function opens a file, in the mode specified in the string mode. It returns a new file descriptor, or, in case of errors,
nil plus an error message.
The mode string can be any of the following:
The mode string may also have a b at the end, which is needed in some systems to open the file in binary mode. This string is exactly what is used in the standard C function fopen.
file:close. Without a file, closes the default output file.
file:flush over the default output file.
When called with a file name, it opens the named file (in text mode), and uses it as the default input descriptor. When called with a file descriptor, it simply sets that file descriptor as the default input file. When called without parameters, it returns the current default input file descriptor.
In case of errors this function raises the error, instead of returning an error code.
Opens the given file name in read mode and returns an iterator function that, each time it is called, returns a new line from the file. Therefore, the construction
for line in io.lines(filename) do ... end
will iterate over all lines of the file. When the iterator function detects the end of file, it closes the file and returns
nil (to finish the loop).
io.lines() (without a file name) is equivalent to
io.input():lines(), that is, it iterates over the lines of the default input file.
io.input, but operates over the default output file.
io.read (format1, ...)
Returns a descriptor for a temporary file. This file is open in update mode and it is automatically removed when the program ends.
obj is a valid file descriptor. Returns the string
obj is an open file descriptor,
"closed file" if
obj is a closed file descriptor, and
obj is not a file descriptor.
io.write (value1, ...)
Saves any written data to file
Returns an iterator function that, each time it is called, returns a new line from file
f. Therefore, the construction
for line in f:lines() do ... end
will iterate over all lines of file
io.lines, this function does not close the file when the loop ends.)
f:read (format1, ...)
Reads the file
f, according to the given formats, which specify what to read. For each format, the function returns a string (or a number) with the characters read, or
nil if it cannot read data with the specified format. When called without formats, it uses a default format that reads the entire next line (see below).
The available formats are
nilon end of file. This is the default format.
nilon end of file. If number is zero, it reads nothing and returns an empty string, or
nilon end of file.
f:seek ([whence] [, offset])
Sets and returns the index position for file
f, measured from the beginning of the file, to the position given by offset plus a base specified by the string whence, as follows:
In case of success, function seek returns the final file position, measured in bytes from the beginning of the file. If this function fails, it returns
nil, plus a string describing the error.
The default value for whence is
"cur", and for offset is 0. Therefore, the call
file:seek() returns the current file position, without changing it; the call
file:seek("set") sets the position to the beginning of the file (and returns 0); and the call
file:seek("end") sets the position to the end of the file, and returns its size.
f:write (value1, ...)
Writes the value of each of its arguments to file
f. The arguments must be strings or numbers. To write other values, use
string.format before write.