How do I remove lines of data in the middle of a text file with Ruby -

i know how write file, , read file, don't know how modify file besides reading entire file memory, manipulating it, , rewriting entire file. large files isn't productive.

i don't know difference between append , write.

e.g.

if have file containing:

person1,will,23 person2,richard,32 person3,mike,44 

how able delete line containing person2?

you can delete line in several ways:

• simulate deletion. is, overwrite line's content spaces. later, when read , process file, ignore such empty lines.

  pros: easy , fast. cons: it's not real deletion of data (file doesn't shrink) , need more work when reading/processing file.

  code:

  f = file.new(filename, 'r+') f.each |line|   if should_be_deleted(line)     # seek beginning of line.     f.seek(-line.length, io::seek_cur)      # overwrite line spaces , add newline char     f.write(' ' * (line.length - 1))     f.write("\n")   end end f.close  file.new(filename).each {|line| p line }  # >> "person1,will,23\n" # >> "                  \n" # >> "person3,mike,44\n" 

• do real deletion. means line no longer exist. have read next line , overwrite current line it. repeat following lines until end of file reached. seems error prone task (lines of different lengths, etc), here's error-free alternative: open temp file, write lines (but not including) line want delete, skip line want delete, write rest temp file. delete original file , rename temporary 1 use name. done.

  while technically total rewrite of file, differ asked. file doesn't need loaded memory. need 1 line @ time. ruby provides method this: io#each_line.

  pros: no assumptions. lines deleted. reading code needs not altered. cons: lots more work when deleting line (not code, io/cpu time).

  there snippet illustrates approach in @azgult's answer.