Difference between "and" and && in Ruby?

Question

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

See also [Difference between `or` and `||`](http://stackoverflow.com/q/2083112/211563). — Andrew Marshall, May 18 '12 at 06:10

score 396 · Accepted Answer · edited Nov 22 '19 at 18:09

396

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".

edited Nov 22 '19 at 18:09

the Tin Man

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answered Sep 15 '09 at 12:18

Dominic Rodger

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It would be a good idea to specify that one should usually use `&&`, while `and` should be used for very specific cases only. – Marc-André Lafortune Apr 09 '12 at 03:05
13

Another good explanation here: http://devblog.avdi.org/2010/08/02/using-and-and-or-in-ruby/. – Andrew Marshall May 18 '12 at 05:58
21

From Andrew Marshall's link: "Another way of thinking about `and` is as a reversed `if` statement modifier: `next if widget = widgets.pop` becomes `widget = widgets.pop and next`. That's a great way of putting it, really made it "click" in my head. (And `or` is like a reversed `unless` modifier.) – GMA Oct 29 '13 at 07:31
1

Combine this answer with the details of tadman's answer and you get the whole picture. – sargas Apr 30 '14 at 22:14
5

Avdi updated his take on when to use and vs. &&. Basically use 'and' and 'or' for control flow because of their lower precedence. http://devblog.avdi.org/2014/08/26/how-to-use-rubys-english-andor-operators-without-going-nuts/ – EricC Nov 06 '14 at 21:53
1

because of the lower precedence, evaluations such as `x = true and false` makes x equal to true and `x = false or true` makes x equal to false – Eiiki Jul 08 '15 at 21:56
1

Updated link from Avdi: https://avdi.codes/how-to-use-rubys-english-andor-operators-without-going-nuts/ – Mark Robinson Jul 21 '20 at 11:44

score 259 · Answer 2 · edited May 18 '12 at 05:59

259

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.

edited May 18 '12 at 05:59

Andrew Marshall

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answered Sep 15 '09 at 20:09

tadman

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`a = foo and bar` _and_ `(a = foo ) && bar` proves that `and` has lower precedence than `&&`. – sargas Apr 30 '14 at 22:15
i don't get it: what is "foo and bar" meant to return? – BenKoshy Apr 26 '16 at 06:08
`a = foo and bar` is equivalent to `(a = :foo) and nil`. Since the assignment returns a logically true value (`:foo`) then the second part evaluates, which fails, returning `nil`. – tadman Apr 26 '16 at 07:31

score 65 · Answer 3 · answered May 18 '12 at 07:53

65

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!

answered May 18 '12 at 07:53

Andrew Grimm

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Actually the [**guide now says**](https://github.com/bbatsov/ruby-style-guide/commit/5920497452c1f6f604742a735f5684e86d4c0003) to avoid `and`/`or` completely, and they might have a point. Often their usage in control flow [could be more obviously written with `if`/`unless` operators](http://devblog.avdi.org/2010/08/02/using-and-and-or-in-ruby/) anyway (e.g. `document.save! unless document.saved?`) – Yarin Sep 14 '13 at 12:08
1

@akostadinov in case you weren't trolling: the Ruby Style guide isn't written by the creators of Ruby. Ruby was created by Yukihiro Matsumoto and others, while the Ruby Style Guide was [mainly](https://github.com/bbatsov/ruby-style-guide/graphs/contributors) by Bozhidar Batsov. – Andrew Grimm Jul 03 '14 at 23:03
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@AndrewGrimm, thanks, good to know. Sorry for trolling but I'm sincerely confused with some aspects of ruby reality. One thing is sure - every ruby project needs strict style policies to keep the codebase maintainable. – akostadinov Jul 04 '14 at 07:59

score 41 · Answer 4 · answered Oct 19 '12 at 23:12

|| 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/ .

"unlike `||`, `or` has lower precedence than `=`" ...now it makes more sense, thanks! — Steph Sharp, Feb 20 '14 at 00:16

score 22 · Answer 5 · edited Nov 22 '19 at 18:10

22

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

edited Nov 22 '19 at 18:10

the Tin Man

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answered Jun 12 '14 at 07:43

Santhosh

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Thank you, but how is the precedence of "and" different from "&&"? – BenKoshy May 14 '16 at 09:10
3

@BKSpurgeon See [here](https://ruby-doc.org/core-2.4.1/doc/syntax/precedence_rdoc.html) for a ordered list of operator precedence in Ruby. – thutt Jul 27 '17 at 08:25

score 5 · Answer 6 · edited Nov 22 '19 at 18:12

and has lower precedence, mostly we use it as a control-flow modifier such as if:

next if widget = widgets.pop

becomes

widget = widgets.pop and next

For or:

raise "Not ready!" unless ready_to_rock?

becomes

ready_to_rock? or raise "Not ready!"

I prefer to use if but not and, because if is more intelligible, so I just ignore and and or.

Refer to "Using “and” and “or” in Ruby" for more information.

score 1 · Answer 7 · answered Mar 09 '22 at 06:57

1

and checks only first condition and gives result on other hand && strongly checks both conditions and gives logical result.

answered Mar 09 '22 at 06:57

Dheeraj Maheshwari

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score 0 · Answer 8 · answered Jul 27 '18 at 17:36

0

I don't know if this is Ruby intention or if this is a bug but try this code below. This code was run on Ruby version 2.5.1 and was on a Linux system.

puts 1 > -1 and 257 < 256
# => false

puts 1 > -1 && 257 < 256
# => true

answered Jul 27 '18 at 17:36

Kevin Ng

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@JakubArnold Sarcasm is never helpful. Examples sometimes are. – BobRodes Feb 14 '19 at 04:38
@BobRodes It wasn't a sarcasm. There are 7 answers, 6 of which already have examples. – Jakub Arnold Feb 14 '19 at 09:42
1

@JakubArnold I still found this example helpful. – BobRodes Feb 15 '19 at 06:03
I'm getting odd results as well. v1 = true and false p v1 # => app.rb: true, IRB: false v2 = true && false p v2 # => app.rb: false, IRB: false puts 1 > -1 && 257 < 256 # => app.rb: false, IRB: false – Rich_F Apr 15 '19 at 09:31

Difference between "and" and && in Ruby?

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