Getting Input One Line at a Time

Problem

You e writing an interactive console program, and you want to get line-based input from the user. You present the user with a prompt, and he types some data before hitting enter.

Solution

Instead of reading standard input all at once, read it a line at a time with gets or readline.

This method populates a data structure with values obtained from user input:

	def confirmation_hearings
	 questions = [[What is your name?, :name],
	 [How old are you?, :age],
	 [Why would you like to be Secretary of the Treasury?, :why]]
	 answers = questions.inject({}) do |answers, qv|
	 question, value = qv
	 print question +  
	 answers[value] = gets.chomp
	 answers
	 end
	 puts "Okay, you
e confirmed!"
	 return answers
	end

	confirmation_hearings
	# What is your name? # <= Leonard Richardson
	# How old are you? # <= 27
	# Why would you like to be Secretary of the Treasury? # <= Mainly for the money
	# Okay, you
e confirmed!
	# => {:age=>"26", :why=>"Mainly for the money", :name=>"Leonard Richardson"}

Discussion

Most console programs take their input from command-line switches or from a file passed in on standard input. This makes it easy to programatically combine console programs: you can pipe cat into grep into last without any of the programs having to know that they e connected to each other. But sometimes its more user-friendly to ask for input interactively: in text-based games, or data entry programs with workflow.

The only difference between this technique and traditional console applications is that you e writing to standard output before you e completely done reading from standard input. You can pass an input file into a program like this, and itll still work. In this example, a Ruby program containing the questionnaire code seen in the Solution is fed by an input file:

	$ ./confirmation_hearings.rb < answers
	# => What is your name? How old are you? Why would you like to be
	# Secretary of the Treasury? Okay, you
e confirmed!

The program works, but the result looks differenteven though the standard output is actually the same. When a human is running the program, the newline created when they hit enter is echoed to the screen, making the second question appear on a separate line from the first. Those newlines don get echoed when they e read from a file.

The HighLine library requires that you install a gem ( highline), but it makes sophisticated line-oriented input much easier. You can make a single method call to print a prompt, retrieve the input, and validate it. This code works the same way as the code above, but its shorter, and it makes sure you enter a reasonable age for the question "How old are you?"

	require 
ubygems
	require highline/import

	def confirmation_hearings
	 answers = {}
	 answers[:name] = ask(What is your name? )
	 answers[:age] = ask(How old are you? , Integer) { |q| q.in = 0..120 }
	 answers[:why] = ask(Why would you like to be Secretary of the Treasury? )
	 puts "Okay, you
e confirmed!"
	 return answers
	end

	confirmation_hearings
	# What is your name? # <= Leonard Richardson
	# How old are you? # <= twenty-seven
	# You must enter a valid Integer.
	# ? # <= 200
	# Your answer isn	 within the expected range (included in 0..120)
	# ? # <= 27
	# …

See Also

• Recipe 21.2, "Getting Input One Character at a Time"
• Recipe 21.9, "Reading a Password"
• The examples/basic_usage.rb script in the HighLine library has many more examples of data validation with HighLine
• If you want your program to treat its command-line arguments as filenames and read from the files one line at a time, see Recipe 21.3, "Parsing Command-Line Arguments," for a shortcut