Objectives

1. Understand the significance of capturing and operating on user input in a CLI application.
2. Create a prompt for user input in a CLI.
3. Use the #gets method to capture, store, and operate on that input.

User Input and the CLI

I think it's fair to say that personal computers have become the most empowering tool we've ever created. They're tools of communication, they're tools of creativity, and they can be shaped by their user. –– Bill Gates

It is inarguable that the advent of personal computing has changed the world and the way that individuals interact with it and with one another. The core of this revolution has been interactivity. The capability of computers to be interacted with. Who builds this interactive capability? We do! As a programmer, the underlying functionality of many of the applications you build will be interaction, the interaction between a user and your program. There are many ways in which that interaction can play out. Here, we'll discuss one of the most basic ways––the #gets method and the command line interface (CLI).

CLI Flow

The basic flow of a CLI app goes something like this:

1. Greet the user.
2. Ask the user for input.
3. Capture and store that input.
4. Do something with that input.

In this exercise, we'll be familiarizing ourselves with a CLI application that has already been built. To experience the user-flow of this application, first open this lab.

Run learn

You'll need to modify the greeting method in lib/hello_ruby_programmer.rb so that it accepts an argument called name.

HINT: Just change the line that reads def greeting to def greeting(name).

Then run ruby bin/greeting in your terminal, from within the directory of this project.

Note that you are greeted, asked to provide input and then greeted again, this time with a phrase that uses the input you provided.

Let's take a closer look at the structure of this application.

Project Structure

Check out the file structure below.

bin
   |–– greeting
lib
   |–– hello_ruby_programmer.rb
...

Let's take a moment to review:

The bin Directory

The bin directory holds our executable file. This file is responsible for running the program. It contains code that actually enacts the command line interaction––i.e. greeting the user, asking the user for input, storing that input and then acting on it.

Open up bin/greeting. Notice that we are requiring the lib/hello_ruby_programmer.rb file.

The lib Directory

The lib directory is where we place the code that our program relies on to run. It is the meat of our CLI application. Our executable file requires the files in the lib directory and uses the code (for example, calling on any methods we might define) to enact the CLI.

Open up lib/hello_ruby_programmer.rb file. Notice that it defines a #greeting method that is called in the bin/greeting file. This is the pattern you'll become familiar with for CLI applications––defining methods in a lib directory and calling those methods in bin executable files to actually run the program.

Now, let's take a closer look at our code.

The CLI Pattern

In bin/greeting you should see the following code:

puts "Hi! Welcome to the wonderful world of Ruby programming."
puts "Please enter your name so that we can greet you more personally:"
name = gets.strip
greeting(name)

Here, we have all of the CLI flow steps outlined above. Let's break it down:

1 . Greet the user:

puts "Hi! Welcome to the wonderful world of Ruby programming."

2 . Ask the user for input:

puts "Please enter your name so that we can greet you more personally:"

3 . Capture that input using #gets

name = gets.strip

4 . Use that input to do something else:

greeting(name)

In this case, we are passing the user's input into the #greeting method as an argument. The greeting method then uses string interpolation to #puts out a personalized message.

The gets Method

We've talked a lot about capturing and storing a user's input to the terminal and using it in our Ruby program. Now we'll take a closer look at exactly how that happens.

Let's take another look at our code from bin/greeting

puts "Hi! Welcome to the wonderful world of Ruby programming."
puts "Please enter your name so that we can greet you more personally:"
name = gets.strip
greeting(name)

On the third line, the gets method is being called. Calling the gets method captures the last thing the user typed into the terminal. Whenever your program makes a call to gets, it will freeze and wait for user input.

Waiting for the user Input

If the user never types anything in, your program will wait forever until it is otherwise exited. If you find your tests and your program stalling for long periods of time (anything over 5-10 seconds generally), you might be trapped in a gets.

From executing a program, a gets will look like:

From a test run, a stalled gets will look like:

Return value for gets

The return value of gets is the text typed into the terminal. So, setting a variable, name, equal to invoking the gets method sets that variable equal to the return value of gets––the last thing typed into the terminal. Then, the following line uses that name variable in string interpolation.

Once we store the return value of gets in a variable, we can treat that variable as we would any variable pointing to a string––interpolate with it, convert it to an integer, add it to an array, you name it.

Remember to run learn submit so you can move on to the next lesson.

Advanced: How gets gets input from the terminal

We already know, in general terms, how the puts method outputs text to the terminal, but here's a reminder from an earlier lesson, "Puts, Print and Return":

How do the puts and print methods actually output text to your console? They use the $stdout global variable provided to us by Ruby. You don't need to worry about global variables right now. For the purposes of understanding how puts and print work, we just need to understand the following:

Your computer has a stdout file that communicates with your operating system. So, puts and print actually send output to the $stdout variable. The $stdout variable sends that information to the stdout file on your computer which in turn communicates with your operating system which in turn outputs that information to the console.

The gets method works similarly. Just like your computer has a standard output file, it has a standard input file. When you enter text in your terminal, you are writing to that file. And, just like Ruby has a $stdout global variable, it has a $stdin global variable. The $stdin variable holds a stream from the standard input. It can be used to read input from the console.

The gets method wraps the $stdin variable, reading text from the standard input and allowing you to store that text in a variable, so that you can operate on it later.

Sanitizing User Input: The strip and chomp Methods

One thing to know about the #gets method is that it captures a new line character at the end of whatever input it is storing. We don't want extra whitespace or new lines to be appended to the user input we are trying to store. So, we can chain a call to the #stripmethod to remove any new lines or trailing whitespace.

The #chomp method works similarly, and you are likely to see #gets.chomp used in some examples online. The #chomp method removes any new lines at the end of a string while the #strip method removes both trailing whitespace and new lines.

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