Get line count before looping over data in ruby

I need to get the total number of lines that an IO object contains before looping through each line in the IO object. How can I do this 开发者_如何学Pythonin ruby?

You can't really, unless you want to shell out to wc and parse the result of that - otherwise you'll need to do two passes - one to get the line numbers, and another to do your actual work.

(assuming we're talking about a File IO instance - neither of those approaches work for network sockets etc)

in rails (the only difference is how I generate the file object instance)

file = File.open(File.join(Rails.root, 'lib', 'assets', 'file.json'))
linecount = file.readlines.size

io.lines.count would give you the number of lines.

io.lines.each_with_index {|line, index|} would give you each line and which line number it is (starting at 0).

But I don't know if it's possible to count the number of lines without reading a file.

You may want to read a file, and then use io.rewind to read it again.

If your file is not humongous, slurp it into memory(array) and count the the number of items( ie lines).