21 Essential Ruby Interview Questions *

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What will val1 and val2 equal after the code below is executed? Explain your answer.

val1 = true and false  # hint: output of this statement in IRB is NOT value of val1!
val2 = true && false
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Although these two statements might appear to be equivalent, they are not, due to the order of operations. Specifically, the and and or operators have lower precedence than the = operator, whereas the && and || operators have higher precedence than the = operator, based on order of operations.

To help clarify this, here’s the same code, but employing parentheses to clarify the default order of operations:

(val1 = true) and false    # results in val1 being equal to true
val2 = (true && false)     # results in val2 being equal to false

This is, incidentally, a great example of why using parentheses to clearly specify your intent is generally a good practice, in any language. But whether or not you use parentheses, it’s important to be aware of these order of operations rules and to thereby ensure that you are properly determining when to employ and / or vs. && / ||.

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Which of the expressions listed below will result in "false"?

true    ? "true" : "false"
false   ? "true" : "false"
nil     ? "true" : "false"
1       ? "true" : "false"
0       ? "true" : "false"
"false" ? "true" : "false"
""      ? "true" : "false"
[]      ? "true" : "false"
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In Ruby, the only values that evaluate to false are false and nil. Everything else – even zero (0) and an empty array ([]) – evaluates to true.

This comes as a real surprise to programmers who have previously been working in other languages like JavaScript.

(Thanks to Ruby Gotchas for this question.)

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Write a function that sorts the keys in a hash by the length of the key as a string. For instance, the hash:

{ abc: 'hello', 'another_key' => 123, 4567 => 'third' }

should result in:

["abc", "4567", "another_key"]
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As is always true in programming, there are in fact multiple ways to accomplish this.

The most straightforward answer would be of the form:

hsh.keys.map(&:to_s).sort_by(&:length)

or:

hsh.keys.collect(&:to_s).sort_by { |key| key.length }

Alternatively, Ruby’s Enumerable mixin provides many methods to operate on collections. The key here is to turn the hash keys into a collection, convert them all to strings, then sort the array.

def key_sort hsh
	hsh.keys.collect(&:to_s).sort { |a, b| a.length <=> b.length }
end

An equivalent call of the collect method is done with the usual block syntax of:

collect { |x| x.to_s }

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Consider the following two methods:

def times_two(arg1);
  puts arg1 * 2;
end

def sum(arg1, arg2);
  puts arg1 + arg2;
end

What will be the result of each of the following lines of code:

times_two 5
times_two(5)
times_two (5)
sum 1, 2
sum(1, 2)
sum (1, 2)
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The first three lines of code will all print out 10, as expected.

The next two lines of code will both print out 3, as expected.

However, the last line of code (i.e., sum (1,2)) will result in the following:

syntax error, unexpected ',', expecting ')'
sum (1, 2)
       ^

The problem is the space between the method name and the open parenthesis. Because of the space, the Ruby parser thinks that (1, 2) is an expression that represents a single argument, but (1, 2) is not a valid Ruby expression, hence the error.

Note that the problem does not occur with single argument methods (as shown with our timesTwo method above), since the single value is a valid expression (e.g., (5) is a valid expression which simply evaluates to 5).

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Consider the following code:

VAL = 'Global'
 
module Foo
  VAL = 'Foo Local'
 
  class Bar
    def value1
      VAL
    end
  end
end
 
class Foo::Bar
  def value2
    VAL
  end
end

What will be the value of each of the following:

Foo::Bar.new.value1
Foo::Bar.new.value2

Explain your answer.

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Foo::Bar.new.value1 will be equal to 'Foo Local' and Foo::Bar.new.value2 will be equal to 'Global'.

Here’s why:

The module keyword (as well as the class and def keywords) will create a new lexical scope for all of its contents. The above module Foo therefore creates the scope Foo in which the VAL constant equal to 'Foo Local' is defined. Inside Foo, we declare class Bar, which creates another new lexical scope (named Foo::Bar) which also has access to its parent scope (i.e., Foo) and all of its constants.

However, when we then declare Foo::Bar (i.e., using ::), we are actually creating yet another lexical scope, which is also named Foo::Bar (how’s that for confusing!). However, this lexical scope has no parent (i.e., it is entirely independent of the lexcial scope Foo created earlier) and therefore does not have any access to the contents of the ‘Foo’ scope. Therefore, inside class Foo::Bar, we only have access to the VAL constant declared at the beginning of the script (i.e., outside of any module) with the value 'Global'.

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Is the line of code below valid Ruby code? If so, what does it do? Explain your answer.

-> (a) {p a}["Hello world"]
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Yes, it’s valid. Here’s how to understand what it does:

The -> operator creates a new Proc, which is one of Ruby’s function types. (The -> is often called the “stabby proc”. It’s also called the “stabby lambda”, as it creates a new Proc instance that is a lambda. All lambdas are Procs, but not all Procs are lambdas. There are some slight differences between the two.)

This particular Proc takes one parameter (namely, a). When the Proc is called, Ruby executes the block p a, which is the equivalent of puts(a.inspect) (a subtle, but useful, difference which is why p is sometimes better than puts for debugging). So this Proc simply prints out the string that is passed to it.

You can call a Proc by using either the call method on Proc, or by using the square bracket syntax, so this line of code also invokes the Proc and passes it the string “Hello World”.

So putting that all together, this line of code (a) creates a Proc that takes a single parameter a which it prints out and (b) invokes that Proc and passes it the string “Hello world”. So, in short, this line of code prints “Hello World”.

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What is the difference between calling super and calling super()?

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A call to super invokes the parent method with the same arguments that were passed to the child method. An error will therefore occur if the arguments passed to the child method don’t match what the parent is expecting.

A call to super() invokes the parent method without any arguments, as presumably expected. As always, being explicit in your code is a good thing.

(Thanks to Ruby Gotchas for this question.)

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Explain each of the following operators and how and when they should be used: ==, ===, eql?, equal?.

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== – Checks if the value of two operands are equal (often overridden to provide a class-specific definition of equality).

=== – Specifically used to test equality within the when clause of a case statement (also often overridden to provide meaningful class-specific semantics in case statements).

eql? – Checks if the value and type of two operands are the same (as opposed to the == operator which compares values but ignores types). For example, 1 == 1.0 evaluates to true, whereas 1.eql?(1.0) evaluates to false.

equal? – Compares the identity of two objects; i.e., returns true iff both operands have the same object id (i.e., if they both refer to the same object). Note that this will return false when comparing two identical copies of the same object.

(Thanks to Ruby Gotchas for this question.)

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Given:

x = "hello"

Explain the difference between:

x += " world"

and

x.concat " world"
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The += operator re-initializes the variable with a new value, so a += b is equivalent to a = a + b.

Therefore, while it may seem that += is mutating the value, it’s actually creating a new object and pointing the the old variable to that new object.

This is perhaps easier to understand if written as follows:

foo = "foo"
foo2 = foo
foo.concat "bar"

puts foo
=> "foobar"
puts foo2
=> "foobar"

foo += "baz"
puts foo
=> "foobarbaz"
puts foo2
=> "foobar"

(Examining the object_id of foo and foo2 will also demonstrate that new objects are being created.)

The difference has implications for performance and also has different mutation behavior than one might expect.

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In Ruby code, you quite often see the trick of using an expression like array.map(&:method_name) as a shorthand form of array.map { |element| element.method_name }. How exactly does it work?

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When a parameter is passed with & in front of it (indicating that is it to be used as a block), Ruby will call to_proc on it in an attempt to make it usable as a block. Symbol#to_proc quite handily returns a Proc that will invoke the method of the corresponding name on whatever is passed to it, thus enabling our little shorthand trick to work.

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Write a single line of Ruby code that prints the Fibonacci sequence of any length as an array.

(Hint: use inject/reduce)

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There are multiple ways to do this, but one possible answer is:

(1..20).inject( [0, 1] ) { | fib | fib << fib.last(2).inject(:+) }

As you go up the sequence fib, you sum, or inject(:+), the last two elements in the array and add the result to the end of fib.

Note: inject is an alias of reduce

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Consider the following code:

class A
  def self.a(b)
    if b > 0
      b * b
    end
  end
end

What will be the values of:

  • var1 = A.a(0)
  • var2 = A.a(2)
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  • var1 will be equal to nil
  • var2 will be equal to 4

Understand: * 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.

In this example:

  • A.a(0) returns nil for unsuccessful conditional
  • A.a(2) returns the square of 2, i.e 4
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Can you call a private method outside a Ruby class using its object?

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Yes, with the help of the send method.

Given the class Test:

class Test
   private
       def method
         p "I am a private method"
      end
end

We can execute the private method using send:

>> Test.new.send(:method)
"I am a private method"
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What is the differnece between extend and include in ruby?

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  • include mixes in specified module methods as instance methods in the target class
  • extend mixes in specified module methods as class methods in the target class

Given the following class definitions:

module ReusableModule
  def module_method
    puts "Module Method: Hi there! I'm a module method"
  end
end

class ClassThatIncludes
  include ReusableModule
end
class ClassThatExtends
  extend ReusableModule
end

Here’s how ClassThatIncludes behaves:

# A class method does not exist
>> ClassThatIncludes.module_method
NoMethodError: undefined method `module_method' for ClassThatIncludes:Class

# A valid instance method exists
>> ClassThatIncludes.new.module_method
Module Method: Hi there! I'm a module method
=> nil

Here’s how ClassThatExtends behaves:

# A valid class method exists
>> ClassThatExtends.module_method
Module Method: Hi there! I'm a module method
=> nil

# An instance method does not exist
ClassThatExtends.new.module_method
NoMethodError: undefined method `module_method' for #<ClassThatExtends:0x007ffa1e0317e8>

We should mention that object.extend ExampleModule makes ExampleModule methods available as singleton methods in the object.

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What is the difference between Array#map and Array#each?

Note: collect is an alias of map

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  • Array#each method iterates over the elements of the array and executes the provided block each time. However, it returns the original array unaffected.
  • Array#map will return a new array of elements containing the values returned by the block it is provided. It also does not affect the original array

Example:

# Given
>> arr = [1,2,3,4]

# This will print 2,4,6,8 but will return [1,2,3,4]
>> arr.each {|x| puts x*2; x*2 }
2
4
6
8
=> [1, 2, 3, 4]


# This will also print 2,4,6,8 but returns [2,4,6,8]
>> arr.map {|x| puts x*2; x*2 }
2
4
6
8
=> [2, 4, 6, 8]
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How do you remove nil values in array using ruby?

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Array#compact removes nil values.

Example:

>> [nil,123,nil,"test"].compact
=> [123, "test"]
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For the class ABC the given as:

class ABC
  def xyz
    puts "xyz in ABC"
  end
end

What is the return value for:

  • ABC::new::xyz
  • ABC::new.xyz
  • ABC.new::xyz
  • ABC.new.xyz
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All the statements for the invocation of the xyz method through the object are valid.

When run through IRB:

irb(main):001:0> ABC::new::xyz
xyz in ABC
=> nil
irb(main):002:0> ABC::new.xyz
xyz in ABC
=> nil
irb(main):003:0> ABC.new::xyz
xyz in ABC
=> nil
irb(main):004:0> ABC.new.xyz
xyz in ABC
=> nil
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What is the value of the variable upcased in the below piece of code?

upcased = ["one", "two", "three"].map {|n| puts n.upcase }

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upcase = [nil, nil, nil]

Let’s take a look at puts as below:

>> puts "Hi"
Hi
=> nil

Note the nil at the end: that’s the return value from puts. After all, puts is a method, so it has to return something. As it happens, it always returns nil. The printing out of the string is an action the method performs.

Similarly, evaluating the code in question we can understand that while it’s a common learner mistake to expect the result to be ["ONE", "TWO", "THREE"]. In fact, it’s [nil, nil, nil]. Each time through the block, the block evaluates to the return value of puts to be nil.

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Given that the following code has been run:

if false
  foo = 'test'
end

What will be the value of:

  • defined? foo
  • defined? bar
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defined? foo
#=> "local-variable"
defined? bar
#=> nil

foo has been at least read by the interpreter, therefore it is defined and assigned a nil value. However, since bar has never been written, it was never defined in the first place.

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What is the difference between the Object methods clone and dup?

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Object#dup creates a shallow copy of an object. For example, it will not copy any mixed-in module methods, whereas Object#clone will. This can be shown with the following code example:

class Klass
  attr_accessor :str
end

module Foo
  def foo; 'foo'; end
end

s1 = Klass.new #=> #
s1.extend(Foo) #=> #
s1.foo #=> "foo"

s2 = s1.clone #=> #
s2.foo #=> "foo"

s3 = s1.dup #=> #
s3.foo #=> NoMethodError: undefined method `foo' for #
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How many types of variables are used in Ruby and what are they?

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There are 4 types of variables used in Ruby:

  1. class variables start with @@, e.g. @@my_var
  2. instance variables start with @, e.g. @my_var
  3. global variables start with $, e.g. $my_var
  4. local variables start with _, e.g. _my_var
* There is more to interviewing than tricky technical questions, so these are intended merely as a guide. Not every “A” candidate worth hiring will be able to answer them all, nor does answering them all guarantee an “A” candidate. At the end of the day, hiring remains an art, a science — and a lot of work.
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Graham Powrie
United States
Graham is a seasoned developer focused on creating custom web applications that automate and improve business processes, while remaining flexible for iteration. He's a full-stack developer who can deliver a complete product from start to finish individually, or integrate with a team to augment output or lead and mentor. When the need arises, he is a competent project manager and enjoys assessing the long-term challenges a project will face.