Count the number of lines in a file without reading entire file into memory?

Question

I\'m processing huge data files (millions of lines each).

Before I start processing I\'d like to get a count of the number of lines in the file, so I can then indic

Answer 1

wc -l in Ruby with less memory, the lazy way:

(ARGV.length == 0 ?
 [["", STDIN]] :
    ARGV.lazy.map { |file_name|
        [file_name, File.open(file_name)]
})
.map { |file_name, file|
    "%8d %s\n" % [*file
                    .each_line
                    .lazy
                    .map { |line| 1 }
                    .reduce(:+), file_name]
}
.each(&:display)

as originally shown by Shugo Maeda.

Example:

$ curl -s -o wc.rb -L https://git.io/vVrQi
$ chmod u+x wc.rb
$ ./wc.rb huge_data_file.csv
  43217291 huge_data_file.csv