Print data, followed by a carriage return and newline, to the File, which must have been opened for writing. Prints numbers as a sequence of digits, each an ASCII character (e.g. the number 123 is sent as the three characters ‘1’, ‘2’, ‘3’).
file.println() file.println(data) file.println(data, BASE)
file: an instance of the File class (returned by SD.open()).
data: the data to print (char, byte, int, long, or string).
BASE(optional): the base in which to print numbers; BIN for binary (base 2), DEC for decimal (base 10), OCT for octal (base 8), HEX for hexadecimal (base 16).
The number of bytes written, though reading that number is optional.