Prework Videos Irb

Learning Goals

  • Describe why REPLs are useful for testing code
  • Define Evaluation in the context of a REPL
  • Use IRB to execute Ruby expressions
  • Identify return values in IRB


  • Hi guys, it's Ian from Flatiron School. In this video, we're going to learn all about IRB. By the end of this video, you should be able to:
  • Describe why REPLs are useful for testing code
  • Define Evaluation in the context of a REPL
  • Use IRB to execute Ruby expressions
  • And to identify return values in IRB
  • Let's go ahead and get started. I'm here in the Learn IDE sandbox, and I'd like to have a way to quickly test out some code.
  • Maybe I just learned about a new string method, or I'm not sure about what a line of code will return, but sometime it's nice to have a little playground to test things out.
  • A really important part of programming is being able to ask yourself - what do you expect a line of code to do? And, did it do what you were expecting? If not, why not? So you really need to be able to test things out like this.
  • One great way to do this is to use a REPL. REPL stands for Read, Evaluate, Print, and Loop
  • In this case, Evaluation means basically running one line of code and showing you what that one line would return.
  • Basically, these are quick interpreters that read the code, evaluate what it does, print you out the return value, and then start over again
  • Ruby comes with a built-in REPL called IRB - short for Interactive Ruby.
  • So, from my terminal, I can type irb and press enter
  • Now, I see this carrot character at the start of the line, and I know I'm in my Interactive Ruby shell.
  • Any code I type here will be evaluated line by line, with the result of that line printed out.
  • When I type 1 + 1 and type enter - we see 2 is printed out. And this arrow-looking thing => - that identifies that was the return value of the previous expression
  • In this case, I can see that the expression 1 + 1 did return what I was expecting. Now, let me test something else. If I do "1" + "1" - now, I might be expecting this to return "2" as a string. When I run this, I can see the return value is actually a string: "11"
  • So now I get to ask myself, why am I not seeing what I was expecting to? In this case, it's because when you use the + operator on a string, it just combines it with another string.
  • IRB is really useful if you want to test something out and just see what it returns.
  • So that's basically it - you can use IRB to test out all kinds of things, and this will be really useful going forward. Just to recap what we discussed:
  • We talked about what a REPL is and why it's useful for testing code
  • We discussed what we mean by "Evaluation" in this context - the REPL is telling us what that one line of code will return
  • We used IRB to execute, or run, a few different Ruby expressions
  • And we identified the return values by the little arrow => character
  • Thanks so much for watching, and happy coding!
Unlock your future in tech
Learn to code.

Learn about Flatiron School's Mission

With a new take on education that falls somewhere between self-taught prodigy and four-year computer science degree, the Flatiron School promises to turn students with little programming experience into developers.

In the six months since the Manhattan coding school was acquired by WeWork, it has spawned locations in Washington, D.C., Brooklyn, and London. Now, WeWork is opening a fourth Flatiron School location, this time in Houston.

Adam Enbar, Flatiron School's cofounder, believes now is the time to grow. "How the world is changing has impacted working and learning in very similar ways. We think education fundamentally is about one thing: enabling people to pursue a better life."

Learn. Love. Code.
Students come to Flatiron School to change their lives. Join our driven community of career-changers and master the skills you need to become a software engineer or a data scientist.
Find Us