Difference between "and" and && in Ruby?

Question

What is the difference between the && and and operators in Ruby?

Solution 1

and is the same as && but with lower precedence. They both use short-circuit evaluation.

WARNING: and even has lower precedence than = so you'll usually want to avoid and. An example when and should be used can be found in the Rails Guide under "Avoiding Double Render Errors".

Solution 2

The practical difference is binding strength, which can lead to peculiar behavior if you're not prepared for it:

foo = :foo
bar = nil

a = foo and bar
# => nil
a
# => :foo

a = foo && bar
# => nil
a
# => nil

a = (foo and bar)
# => nil
a
# => nil

(a = foo) && bar
# => nil
a
# => :foo

The same thing works for || and or.

Solution 3

The Ruby Style Guide says it better than I could:

Use &&/|| for boolean expressions, and/or for control flow. (Rule of thumb: If you have to use outer parentheses, you are using the wrong operators.)

# boolean expression
if some_condition && some_other_condition
  do_something
end

# control flow
document.saved? or document.save!

Solution 4

|| and && bind with the precedence that you expect from boolean operators in programming languages (&& is very strong, || is slightly less strong).

and and or have lower precedence.

For example, unlike ||, or has lower precedence than =:

> a = false || true
 => true 
> a
 => true 
> a = false or true
 => true 
> a
 => false

Likewise, unlike &&, and also has lower precedence than =:

> a = true && false
 => false 
> a
 => false 
> a = true and false
 => false 
> a
 => true 

What's more, unlike && and ||, and and or bind with equal precedence:

> !puts(1) || !puts(2) && !puts(3)
1
 => true
> !puts(1) or !puts(2) and !puts(3)
1
3
 => true 
> !puts(1) or (!puts(2) and !puts(3))
1
 => true

The weakly-binding and and or may be useful for control-flow purposes: see http://devblog.avdi.org/2010/08/02/using-and-and-or-in-ruby/ .

Solution 5

and has lower precedence than &&.

But for an unassuming user, problems might occur if it is used along with other operators whose precedence are in between, for example, the assignment operator:

def happy?() true; end
def know_it?() true; end

todo = happy? && know_it? ? "Clap your hands" : "Do Nothing"

todo
# => "Clap your hands"

todo = happy? and know_it? ? "Clap your hands" : "Do Nothing"

todo
# => true

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