11.1 Introduction

Until this point, all of our programs have taken a data set of values from user input. Any given program took in one data set, which consisted of one or more related values from the user, and then performed a computation based on that data set. Imagine a program that has many data sets, each consisting of multiple values. Entering all the data sets by hand could be somewhat time-consuming. Also, some data sets may come from external sources and not directly from the user.

The techniques presented in this chapter will allow you to manipulate files and utilize their data within your software. File input/output (I/O) allows software to perform computations on sets of data until a desired end result is achieved.

11.2 File Access: Reading and Writing

To access files, the built-in Ruby File class is used. This class contains multiple methods. We use the following: open, close, gets, and puts. The method File.open instantiates a new object that enables Ruby to read from or write to an existing or new file. The object returned by File.open can then be used by Ruby to access the file. The following code shows how to open a file:

myfile = File.open(file_name, access_mode)

The myfile variable is a File object that can now be used to interact with the file’s contents, depending on what access mode is used. Note that file name and access mode are strings. The variable file_name is a representation of a path to a file, such as /home/ruby/file.txt. The myfile variable in our program is local to only our program. Two of the most basic access modes and what they do are shown in Table 11-1.

To read a line of characters from a file, call the gets method on the myfile object. When the gets method is called, it reads characters until it reaches a newline character (\n), and then it returns what it read. The File object keeps track of what has been read in, so each successive call to gets will always return the next line, until it reaches the end of the file and returns nil. nil has the logical truth value of false. Every other value returned by gets has the logical value of true. Consider the code provided in Example 11-1, which reads in a file foo.txt and then prints it out.

     1 myfile = File.open("foo.txt", "r")
     2 whole_file = ""
     3 
     4 while (input_line = myfile.gets)
     5 	whole_file += input_line
     6 end
     7 
     8 puts "Contents of input file:"
     9 puts whole_file
    10 myfile.close()

• Line 1 opens foo.txt for read access.

• Line 2 instantiates an empty string variable, whole_file, which will be used to store the file to display.

• Lines 4–6 contain a loop that reads the file into input_line, one line at a time, and appends each line into whole_file.

• Line 9 prints out the file.

Sequentially writing to a file is similar to sequentially reading from a file. Instead of opening the file in read mode, the file needs to be opened with w for writing access. To write text to a file, use the local variable myfile to call the puts method:

myfile.puts("text goes here!")

The code shown in Example 11-2 opens a file for write access, accepts user input, and writes that input to the file. It will then close the file, reopen it, and print the contents to verify correctness.

     1 file_a = File.open("bar.txt", "w")
     2 
     3 puts "Please enter a line of text"
     4 line = gets()
     5 file_a.puts(line)
     6 file_a.close()
     7 
     8 file_b = File.open("bar.txt", "r")
     9 puts "Contents of file:"
    10 puts file_b.gets()
    11 file_b.close()

• Line 1 opens bar.txt as file object file_a with write access.

• Lines 3–4 take text input from the user.

• Line 5 writes the user input into "bar.txt".

• Line 6 closes the File object file_a (this saves the text inside it by closing the file and preventing further access to it).

• Line 8 instantiates a new File object file_b. This object will be used for the purpose of reading the content of bar.txt. Note the file access mode r.

• Line 10 outputs the contents of the newly created file to the console, illustrating that the code in Example 11-2 behaves in the desired fashion.

     1 class FileReader
     2 
     3 	def initialize(file_name)
     4 		@file = File.open(file_name, "r")
     5 	end
     6 
     7 	def read_file
     8 		whole_file = ""
     9 		while (input_line = @file.gets)
    10 			whole_file += input_line
    11 		end
    12 
    13 		return whole_file
    14 	end
    15 
    16 	def display
    17 		puts "Contents of input file:"
    18 		puts read_file
    19 	end
    20 
    21 	def close
    22 		@file.close()
    23 	end
    24 end

     1 class FileWriter
     2 	
     3 	def initialize(file_name)
     4 		@file = File.open(file_name, "w")
     5 	end
     6 
     7 	def write_line(output_line)
     8 		@file.puts(output_line)
     9 	end
    10 	
    11 	def close
    12 		@file.close()
    13 	end
    14 end

     1 require_relative "file_reader.rb"
     2 require_relative "file_writer.rb"
     3 
     4 fr = FileReader.new("input.txt")
     5 fw = FileWriter.new("output.txt")
     6 
     7 input = fr.read_file()
     8 fw.write_line(input)
     9 
    10 fw.close()
    11 fr.close()

    Hello world!
    A mighty fine day for ruby programming!
    Computer Science is the best!

11.3 Summary

We described basic file input and output operations.

11.4 Exercises

1. The game of Go is often played online. Many users save their game data for later analysis. The game is played by two players who take turns placing stones (one player using black stones and the other using white stones) on the intersections of a 19  × 19 board. Create a program to enter each move of a game and save that information to a file.

2. Write a program that quizzes the user on her or his vocabulary. Make the program read a set of words and definitions from a file; display definitions one at a time, in random order; and prompt for the appropriate word.

3. Write a program that will keep track of the mileage for oil changes for your car.

4. Write a program that reads the content of a file and outputs it to the screen.

5. Write a program that takes input from the user and saves it to a file.

6. Write a program that creates and stores a simple address book.