How to Input a File in Ruby

Reading data from and writing data to a file are common tasks in programming. The programming language Ruby has a number of methods which make it simple to input a file.

Difficulty: Challenging

Instructions

Input a File

1. Identify the filename of the file you want and open it.

2. Determine whether you want to read from the file, write to the file or both. If you want to read a file, pass "r" as the second argument to File.open. If you want to write, pass "w". To do both, pass "r+".

3. Create a new File object with the File.open method and store the result in a variable:
   f = File.open("myfile.txt", "r")

4. Use one of a number of methods for reading and writing the file. To read each line in order, you can use the each_line method, which takes a block as an argument, enclosed in do...end keywords, or {...} braces. Reading a file this way is similar to iterating over an array:
   f = File.open("myfile.txt", "r")
f.each_line do|line|
puts "I read this line: #{line}"
end

5. Read individual strings for formatted data in a number of ways. After opening the file use the gets method to read a line and store the result in a variable:
   f = File.open("myfile.txt", "r")
line = f.gets
puts "The line I read is: #{line}"

Utilize the Shortcuts

6. Use the shortcut for the each_line method, the File.foreach method. This method opens the file and uses each_line without you having to open the file yourself. It also closes the file when it's finished:
   File.foreach("myfile.txt") do|line|
puts "I read this line: #{line}"
end

7. Use the readlines method in situations where it would be easier to read all the lines of the file into an array. The readlines method returns an array of all the lines. The following example will open a file, read all the lines with readlines, then iterate over the returned array and print all the lines:
   f = File.open("myfile.txt", "r")
lines = f.readlines
lines.each do|line|
puts "I read this line: #{line}"
end

Close the File

8. Close the file with the close method. Remember to call the close method, otherwise the file might never be closed:
   f = File.open("myfile.txt", "r")
# ... Do something with the file
f.close

9. Close it automatically if you don't need to keep the file open for very long. The File.open method can take a block as an argument.

10. Pass a block, and the file will be automatically closed at the end of the block. File.open will also return the result of the block if one is passed to it as a opposed to a file handle. Blocks automatically return the result of their last expression. In this example, File.open returns the result of the readlines method despite the fact that there is no return statement:
    lines = File.open("myfile.txt", "r") do|file|
file.readlines
end

Tips & Warnings

• Be sure not to mistake any notes which come after the operator "#" as part of the code block. These notes are intended to guide programmers, and are usually used when more than one person has been assigned to a specific program.