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Ruby Reading Files

Reading a file brings its text into a program. A program can write a small file first and then read that same file back.

Read the whole file

The program writes a short file first. File.read then brings the whole text back in one string.

File.write "notes.txt", "Apples\nPears\nPlums\n"
text = File.read "notes.txt"
puts text

Walk line by line

File.foreach hands one line at a time to a block. Each line still carries its ending, so chomp removes it.

File.write "fruits.txt", "Apples\nPears\nPlums\n"
File.foreach "fruits.txt" do |line|
  puts line.chomp.upcase
end

Check a file exists

File.exist? checks a name before a read. It answers true for a stored file and false for a missing one.

File.write "list.txt", "Milk\nEggs\n"
found = File.exist? "list.txt"
missing = File.exist? "other.txt"
puts found
puts missing

Read lines into an array

File.readlines gives one string for each line. The map call cleans each line before the array is shown.

File.write "fruits.txt", "Apples\nPears\nPlums\n"
lines = File.readlines "fruits.txt"
clean = lines.map { |line| line.chomp }
p clean
puts lines.length

Example

File.write "scores.txt", "10\n20\n30\n"
total = 0
File.foreach "scores.txt" do |line|
  total += line.to_i
end
puts "Total: #{total}"

Key points

  • File.write stores text in a file, ready for a later read.
  • File.read brings the whole file back as one string.
  • File.foreach runs a block once for each line.
  • File.exist? answers true or false for a name.
  • File.readlines gives an array with one string per line.

In this example, the program writes three scores into a file. File.foreach walks the file line by line. Each line becomes a number and joins the total.

Try it Yourself

Edit the code, then press Run.

Running is simulated

Exercise

What does File.read give back?