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How can you construct an array in Ruby which only uses a variable if it exists, and otherwise uses nil in its place. With the conditional logic inside the array constructor.

Simplified Example:

a = 1
c = 3

arr = [a, b || nil, c]

I've tried a number of different way but they are not working so I feel like I'm missing something fundamental here.

  1. (b || nil)
  2. b ? b : nil
  3. b.nil? ? nil : b

Is this possible?

Context: This array constructor is used inside a loop used by multiple different models. Some models have the b attribute and some do not, making it difficult extrapolate the logic outside.

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  • 1
    Why would b not exist? That's really an unusual thing to see in Ruby code. Commented Sep 30, 2016 at 21:42
  • You say both "uses a variable if it exists" and "Some models have the b attribute and some do not". The first says that b is a variable, the second says that b is a method call, which is it? Commented Sep 30, 2016 at 21:49
  • The real example is more similar to @instance.attribute_b. The method with this logic iterates over query results for several different models (separately); some of the models do not have an attribute_b. Commented Sep 30, 2016 at 21:49
  • You shouldn't try to embed the conditional logic into the creation of the array; That way leads to madness at 3AM when a stressed coworker needs to fix a production bug. Instead, move the condition outside the array definition so it is clear what is happening, then define the array. Or, at a minimum, split the array into separate lines so there's some vertical space making it easier for the brain to process. Commented Sep 30, 2016 at 22:27

5 Answers 5

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3

Yea, you can use defined? method. It return "local-variable" if variable exist, if not it will return nil.

arr = [a, defined?(b) ? b : nil, c]
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4 Comments

arr = [a, defined?(b) ? b : nil, c] is a little bit cleaner
This is the correct answer, as b is not guaranteed to be defined.
@CDub I understood this question exactly in this way.
The problem here is that b may not be defined but calling the b method could still work, consider a model that uses method_missing to create accessors as needed.
2

Given that your b is actually a model attribute which may or may not be supported by the current self, then b is not a variable at all, b is a method call and you're not interested in whether or not the "variable" exists, you're interested in whether or not self responds to b. Hence you want to use respond_to?:

arr = [a, respond_to?(:b) ? b : nil, c]

or perhaps:

arr = [a, respond_to?(:b, true) ? b : nil, c]

if you want to allow b to be a non-public method.

defined?(b) should also work in this case (unless b's accessor method is automatically created via method_missing) but it will be rather puzzling to anyone looking at your code. Using respond_to? to see if an object has a method/attribute would be more idiomatic.

Using respond_to? when faced with method_missing of course assumes that both method_missing and respond_to? have been overridden but that should be a relatively safe assumption.

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0

This will do the trick:

arr = [a, b = b || nil, c]

Example 1:

a = 1
c = 3

arr = [a, b = b || nil, c]
p arr #=> [1, nil, 3]

Example 2:

a = 1
b = 2
c = 3

arr = [a, b = b || nil, c]
p arr #=> [1, 2, 3]
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4 Comments

can be shortened to arr = [a, b ||= nil, c]
@Doon yes good point, I miss these things when trying to post a speedy answer. I will however not take up your recommendation as another answer has this already, but thank you.
@sagarpandya82 - go ahead and update it. You won't offend me. :)
In a code review I'd say that arr = [a, b = b || nil, c] is not easily read. While in C or Perl that sort of construct would fly, in Ruby we try to be more obvious. Breaking it into separate lines would help or preassign b then reference it in the array.
0

You can do:

arr = [a, (b ||= nil), c]

Example:

a = 1
b = nil
c = 3

> arr = [a, (b ||= 33), c]
=> [1, 33, 3]

b = 2

> arr = [a, (b ||= 33), c]
=> [1, 2, 3]

If b may not be defined, you'll need to use defined?:

> arr = [a, (defined?(b) ? b : nil), c]
=> [1, nil, 3]
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3 Comments

It's not necessary to enclose the expression inside parenthesis, the commas will suffice, however for readability I'd put each element in the array on a separate line so it's easy to see what is happening.
While not necessary, on a single line, it's much easier to read and determine logic vs. simply a variable...
Yes, but it's questionable whether it should be a single line at all. Breaking it into multiple lines evaluates to the same thing, won't reduce speed but will improve readability.
0

Ugh. Don't write code like that. Instead break it into more understandable pieces:

a = 1
c = 3

b ||= nil

arr = [a, b, c]

Or, use vertical space to help the brain see what's going on:

arr = [
  a,
  b ||= 3,
  c
]
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