Earlier this week I published a tweet that made some buzz in the community. To be honest, I didn’t expect that so many people would be interested in this topic. About 29000 views and 1600 votes is quite an impressive result for 9 lines of code.
The tweet was about the following code snippet:
class Klass
def self.square(value)
if value > 0
value * value
end
end
end
puts Klass.square(0)
The question was: what is the result of the Klass.square(0) method call?
TL;DR
The result of the Klass.square(0) method call is nil. From 1600 votes, about 1500 people gave the correct answer.
Explanation
Every expression in Ruby has a value.
A conditional statement in Ruby is an expression that returns nil if the conditional is false. Ruby methods return the last expression in the method body.
So, in the code snippet above, 0 is not greater than 0, so the if statement returns nil. The Klass.square method returns the result of the if statement (because it is the last expression), which is nil.
Elementary, right? Yes, but since there were about 100 people who gave the wrong answer (0), even such simple things can be tricky and odd.
More odd examples
In the comments to the tweet, Lucian Ghinda pointed to a similar, odd example originally found by Rob Lacey.
It turns out that local variable assignment in Ruby assigns nil even if the right part of the expression is not defined.
irb(main):001> local_var = undefined_var
(irb):1:in `<main>': undefined local variable or method `undefined_var' for main:Object (NameError)
local_var = undefined_var
^^^^^^^^^^^^^
from /Users/igor/.rbenv/versions/3.2.3/lib/ruby/gems/3.2.0/gems/irb-1.13.2/exe/irb:9:in `<top (required)>'
from /Users/igor/.rbenv/versions/3.2.3/bin/irb:25:in `load'
from /Users/igor/.rbenv/versions/3.2.3/bin/irb:25:in `<main>'
irb(main):002> local_var
=> nil
To be honest, I didn’t know about this behavior before.
Why do I ask this in interviews?
In my original post, I mentioned that I ask this question in interviews. This triggered a discussion about whether it is a good idea to ask such questions in interviews.
Yes, I believe that at the beginning of your career, it is crucial to understand how the language you are working with works. It is important to know all the nuances and dive as deep as you can. Even if the candidate doesn’t know the answer, which is usual and normal in entry-level positions, he can always demonstrate his thinking curve which, start discussion and try to explain the potential behavior. Focusing on how a candidate thinks and approaches problems can reveal a lot about their potential and willingness to learn, which is especially important early in their careers.
Conclusions
Today you learned, or, I hope, you remembered, that every expression in Ruby has a value. Conditional statements in Ruby are expressions that return nil if the conditional is false.
