Ruby Gotcha: Operator Precedence

Posted on Mon 23 October 2023 in Technical Solutions


In one of my previous roles, the team utilized Ruby. I was not an individual contributor at that point in my career, so my exposure to it was less than much of my team. However, during my tenure, I noticed several "Gotchas" of the Ruby language. Each language has these in their own way, but in this post I'm going to cover one specific one.

What's the difference between and and &&? What about the difference between or and ||?

The Gotcha

At first pass, these two blocks of code appear to do the same thing. Let's take a look.

irb(main):003:0> true && false
=> false
irb(main):004:0> true and false
=> false

By the rules of boolean logic, both and and && statements return false.

irb(main):005:0> true || false
=> true
irb(main):006:0> true or false
=> true

Boolean logic holds true for the or and || statements and return true.

But, what happens if we assign the result of this to a variable?

irb(main):007:0> result = true and false
=> false
irb(main):008:0> result1 = true && false
=> false
irb(main):009:0> result
=> true
irb(main):010:0> result1
=> false

Now we see the problem. With the and operator, result = true. With the && operator, result1 = false.

Looking at the or/|| operators quickly,

irb(main):011:0> result2 = true or false
=> true
irb(main):012:0> result3 = true || false
=> true
irb(main):013:0> result2
=> true
irb(main):014:0> result3
=> true

This all looks reasonable. But, let's swap the order of our true/false in the statement.

irb(main):015:0> result2 = false or true
=> true
irb(main):016:0> result3 = false || true
=> true
irb(main):017:0> result2
=> false
irb(main):018:0> result3
=> true

Here we see the differences again, with the or operator setting result2 = false and the || operator setting result3 = true

What is going on?

and and && / or and || are the same thing, but they have different orders of precedence. The (not well formatted in my opinion) documentation shows the precedence table for Ruby.

The key things to point out here:

  • && and || are above and and or, meaning they will be evaluated first.
  • = is also above and and or (but below && and ||)

Let's look at one of the examples again:

irb(main):015:0> result2 = false or true
=> true
irb(main):017:0> result2
=> false

With the order of precedence in mind, this is what happens:

  • result2 is assigned the value of false because = occurs before or
  • The remainder of the statement is evaluated as true

This was hidden when true and false was swapped because result2 was assigned a value of true and then the remainder of the statement was also evaluated to true.

How do you prevent this?

There is very few reasons to utilize the English words and and or in Ruby's logical evaluations. Instead, these should be utilized as flow control modifiers (think if and unless) and not for boolean logic.

Here's an example of how it should be utilized for flow control

irb(main):019:0> numeral = 10 && numeral / 5
(irb):19:in `<main>': undefined method `/' for nil:NilClass (NoMethodError)
        from C:/Ruby32-x64/lib/ruby/gems/3.2.0/gems/irb-1.6.2/exe/irb:11:in `<top (required)>'
        from C:/Ruby32-x64/bin/irb:33:in `load'
        from C:/Ruby32-x64/bin/irb:33:in `<main>'
irb(main):020:0> numeral = 10 and numeral / 5
=> 2

In our first example, with &&, the statement is broken up like this

numeral = (10 && numeral) / 5

This won't work because numeral doesn't have a value when && is evaluated. Contrast this with the second version where the code:

  • Assigns a value of 10 to numeral
  • Divides numeral by 5

While basic, it should get the point across. Another example could be something like this:

user = User.find_by_email(email) and user.send_email!

Here, we are only sending an email to the user if we find their information. No information, no email is sent.

- is a father, an engineer and a computer scientist. He is interested in online community building, tinkering with new code and building new applications. He writes about his experiences with each of these.