Ruby - File Pointer Seeking

Introduction

To move the pointer forward by a certain offset or move the pointer to a certain position backward from the end of the file, you need to use seek.

seek has three modes of operation:

Mode	Description
IO::SEEK_CUR	Seeks a certain number of bytes ahead of the current position.
IO::SEEK_END	Seeks to a position based on the end of the file. This means that to seek to a certain position from the end of the file.
IO::SEEK_SET	Seeks to an absolute position in the file. This is identical to pos=.

To position the file pointer 5 bytes from the end of the file and change the character to an X , you would use seek as follows:

f = File.new("test.txt", "r+") 
f.seek(-5, IO::SEEK_END) # from  w w w .  ja  va2s.com
f.putc "X" 
f.close

The following code prints every fifth character in a file:

f = File.new("test.txt", "r") 
while a = f.getc 
  puts a.chr # from  w  w w  . j a  v a 2s . c om
  f.seek(5, IO::SEEK_CUR) 
end