How can I get the opposite of a boolean in Ruby (I know that it is converted to 0/1) using a method inline?
say I have the given instance:
class Vote
def return_opposite
self.value
end
end
Which obviously doesn't do anything, but I can't seem to find a method that is simple and short something like opposite() or the like. Does something like this exist and I'm just not looking at the right place in the docs? If one doesn't exist is there a really short ternary that would toggle it from 1 => 0 or 0 => 1?
I like to use this
#object.boolean = !#object.boolean
Boolean expressions are not 0 or 1 in Ruby, actually, 0 is not false
If n is numeric we are swapping 0 and 1...
n == 0 ? 1 : 0 # or...
1 - n # or...
[1, 0][n] # or maybe [1, 0][n & 1] # or...
class Integer; def oh_1; self==0 ? 1:0; end; end; p [12.oh_1, 0.oh_1, 1.oh_1] # or...
n.succ % 2 # or...
n ^= 1
If b already makes sense as a Ruby true or false conditional, it's going to be hard to beat:
!b
These examples differ in how they treat out-of-range input...
You can use XOR operator (exclusive or)
a = true
a # => true
a ^= true
a # => false
a ^= true
a # => true
Edit: See comment by #philomory below.
I believe this is basically an oversight in the boolean classes (TrueClass and FalseClass) in Ruby.
You can negate any object:
nil.! => true
false.! => true
true.! => false
0.! => false
1.! => false
a.! => false (all other objects)
But you cannot in-place negate the boolean objects:
a.!! => does not compile
I guess this would call for trouble with the compiler's grammar.
The best you can do, is thus:
a = a.!
If you just want to access the opposite of the value, use ! as some people have said in the comments. If you want to actually change the value, that would be bool_value = !bool_value. Unless I've misunderstood your question. Which is quite possible.
In order to toggle a boolean data in rails you can do this vote.toggle!(:boolean)
If you want to toggle boolean (that is false and true) , that would be as simple as just use ! as others has stated.
If you want to toggle between 0 and 1 , I can only think of something naive as below :)
def opposite
op = {0=>1,1=>0}
op[self.value]
end
In a Rails app, I use this method to toggle a post between paused and active, so ! (not) is my preferred method.
def toggle
#post = Post.find(params[:id])
#post.update_attributes paused: !#post.paused?
msg = #post.paused? ? 'The post is now paused.' : 'The post is now active.'
redirect_to :back, notice: msg
end
I think that using a ternary can be interesting :
check = false
check == true ? check = false : check = true
Related
I'm trying to express a condition like this:
if 33.75 < degree <= 56.25
# some code
end
But Ruby gives this error:
undefined method `<=' for true:TrueClass
I'm guessing that one way to do it is something like:
if 33.75 < degree and degree <= 56.25
# code
end
But there is no another, easier way?
Ruby also has between?:
if value.between?(lower, higher)
There are many ways of doing the same things in Ruby.
You can check if value is in the range by use of following methods,
14.between?(10,20) # true
(10..20).member?(14) # true
(10..20).include?(14) # true
But, I would suggest using between than member? or include?. You can find more about it here.
You can express a <= x <= b as (a..b).include? x and a <= x < b as (a...b).include? x.
>> (33.75..56.25).include? 33.9
=> true
>> (33.75..56.25).include? 56.25
=> true
>>
>> (33.75..56.25).include? 56.55
=> false
Unfortunately, there seems no such thing for a < x <= b, a < x < b, ..
UPDATE
You can accomplish using (-56.25...-33.75).include? -degree. But it's hard to read. So I recommend you to use 33.75 < degree and degree <= 56.25.
use between? is the easiest way, I found most answers here didn't mention (ruby doc explanation is hard to understand too), using between? does INCLUDE the min and max value.
for example:
irb(main):001:0> 2.between?(1, 3)
=> true
irb(main):002:0> 3.between?(1, 3)
=> true
irb(main):003:0> 1.between?(1, 3)
=> true
irb(main):004:0> 0.between?(1, 3)
=> false
by the way, ruby doc quote:
between?(min, max) → true or false Returns false if obj <=> min is
less than zero or if anObject <=> max is greater than zero, true
otherwise.
undefined method `<=' for true:TrueClass
means that Ruby is not parsing your if-condition the way you expect it.
Using && and adding parentheses helps!
if (33.75<degree) && (degree<=56.25)
...
end
It's a bad habit to leave out parentheses -- as soon as the expression gets more difficult, you could get a surprising outcome. I've seen this many times in other people's code.
Using and instead of && in Ruby is a very bad idea, see:
https://www.tinfoilsecurity.com/blog/ruby-demystified-and-vs
http://rubyinrails.com/2014/01/30/difference-between-and-and-in-ruby/
You can also use this notation:
(1..5) === 3 # => true
(1..5) === 6 # => false
Adjust value to range?
value = 99
[[value,0].max, 5].min # 5
value = -99
[[value,0].max, 5].min # 0
value = 3
[[value,0].max, 5].min # 3
I have a exercise of the school and i can't resolve it. Can you help me?
The problem is this:
Try using a method that takes two arguments - use the between? method
to determine if the number 2 lies between the numbers 1 and 3.
I tried to find what is the .between? method but í couldn't find it.
I just know that is a method
The method is Comparable#between?, and you can use it like this:
2.between?(1, 3)
# => true
use between? is the easiest way, I found most answers here didn't mention (ruby doc explanation is hard to understand too), using between? does INCLUDE the min and max value.
for example:
irb(main):001:0> 2.between?(1, 3)
=> true
irb(main):002:0> 3.between?(1, 3)
=> true
irb(main):003:0> 1.between?(1, 3)
=> true
irb(main):004:0> 0.between?(1, 3)
=> false
by the way, ruby doc quote (too hard to understand for newbie):
between?(min, max) → true or false Returns false if obj <=> min is
less than zero or if anObject <=> max is greater than zero, true
otherwise.
From "between" ruby documentation:
between?(min, max) → true or false
Returns false if obj <=> min is less than zero or if anObject <=> max is greater than zero, true otherwise.
Uh oh, and of course, it's #=== method for ranges:
( 1..3 ) === 2 #=> true
( 1..3 ) === 4 #=> false
You can use Range#cover? as a solution :
(1..3).cover? 2 #=> true
Is there a simple way in Ruby to get a true/false value from something without explicitly evaluating it to true or false
e.g. how would one more succinctly express
class User
def completed_initialization?
initialization_completed == 1 ? true : false
end
end
is there some way to do something along the lines of
initialization_completed.true?
There's obviously not much in it but since I'm in the zen garden of Ruby I might as well embrace it
EDIT (I've updated the example)
This question was extremely badly phrased as was very gently pointed out by #Sergio Tulentsev. The original example (below) does of course evaluate directly to a boolean. I'm still struggling to find an example of what I mean however Sergio's double-negative was in fact exactly what I was looking for.
Original example
class User
def top_responder
responses.count > 10 ? true : false
end
end
> operator already returns boolean value. So it can be just
def top_responder
responses.count > 10
end
To convert arbitrary values to booleans, I offer you this little double-negation trick.
t = 'foo'
!!t # => true
t = 1
!!t # => true
t = 0
!!t # => true
t = nil
!!t # => false
The first negation "casts" value to boolean and inverts it. That is, it will return true for nil / false and false for everything else. We need another negation to make it produce "normal" values.
I can't seem to check if an object is a boolean easily. Is there something like this in Ruby?
true.is_a?(Boolean)
false.is_a?(Boolean)
Right now I'm doing this and would like to shorten it:
some_var = rand(1) == 1 ? true : false
(some_var.is_a?(TrueClass) || some_var.is_a?(FalseClass))
Simplest way I can think of:
# checking whether foo is a boolean
!!foo == foo
I find this to be concise and self-documenting:
[true, false].include? foo
If using Rails or ActiveSupport, you can even do a direct query using in?
foo.in? [true, false]
Checking against all possible values isn't something I'd recommend for floats, but feasible when there are only two possible values!
There is no Boolean class in Ruby, the only way to check is to do what you're doing (comparing the object against true and false or the class of the object against TrueClass and FalseClass). Can't think of why you would need this functionality though, can you explain? :)
If you really need this functionality however, you can hack it in:
module Boolean; end
class TrueClass; include Boolean; end
class FalseClass; include Boolean; end
true.is_a?(Boolean) #=> true
false.is_a?(Boolean) #=> true
As stated above there is no boolean class just TrueClass and FalseClass however you can use any object as the subject of if/unless and everything is true except instances of FalseClass and nil
Boolean tests return an instance of the FalseClass or TrueClass
(1 > 0).class #TrueClass
The following monkeypatch to Object will tell you whether something is an instance of TrueClass or FalseClass
class Object
def boolean?
self.is_a?(TrueClass) || self.is_a?(FalseClass)
end
end
Running some tests with irb gives the following results
?> "String".boolean?
=> false
>> 1.boolean?
=> false
>> Time.now.boolean?
=> false
>> nil.boolean?
=> false
>> true.boolean?
=> true
>> false.boolean?
=> true
>> (1 ==1).boolean?
=> true
>> (1 ==2).boolean?
=> true
If your code can sensibly be written as a case statement, this is pretty decent:
case mybool
when TrueClass, FalseClass
puts "It's a bool!"
else
puts "It's something else!"
end
An object that is a boolean will either have a class of TrueClass or FalseClass so the following one-liner should do the trick
mybool = true
mybool.class == TrueClass || mybool.class == FalseClass
=> true
The following would also give you true/false boolean type check result
mybool = true
[TrueClass, FalseClass].include?(mybool.class)
=> true
So try this out (x == true) ^ (x == false) note you need the parenthesis but this is more beautiful and compact.
It even passes the suggested like "cuak" but not a "cuak"... class X; def !; self end end ; x = X.new; (x == true) ^ (x == false)
Note: See that this is so basic that you can use it in other languages too, that doesn't provide a "thing is boolean".
Note 2: Also you can use this to say thing is one of??: "red", "green", "blue" if you add more XORS... or say this thing is one of??: 4, 5, 8, 35.
This gem adds a Boolean class to Ruby with useful methods.
https://github.com/RISCfuture/boolean
Use:
require 'boolean'
Then your
true.is_a?(Boolean)
false.is_a?(Boolean)
will work exactly as you expect.
No. Not like you have your code. There isn't any class named Boolean. Now with all the answers you have you should be able to create one and use it. You do know how to create classes don't you? I only came here because I was just wondering this idea myself. Many people might say "Why? You have to just know how Ruby uses Boolean". Which is why you got the answers you did. So thanks for the question. Food for thought. Why doesn't Ruby have a Boolean class?
NameError: uninitialized constant Boolean
Keep in mind that Objects do not have types. They are classes. Objects have data. So that's why when you say data types it's a bit of a misnomer.
Also try rand 2 because rand 1 seems to always give 0. rand 2 will give 1 or 0
click run a few times here.
https://repl.it/IOPx/7
Although I wouldn't know how to go about making a Boolean class myself. I've experimented with it but...
class Boolean < TrueClass
self
end
true.is_a?(Boolean) # => false
false.is_a?(Boolean) # => false
At least we have that class now but who knows how to get the right values?
Just wondering what !! is in Ruby.
Not not.
It's used to convert a value to a boolean:
!!nil #=> false
!!"abc" #=> true
!!false #=> false
It's usually not necessary to use though since the only false values to Ruby are nil and false, so it's usually best to let that convention stand.
Think of it as
!(!some_val)
One thing that is it used for legitimately is preventing a huge chunk of data from being returned. For example you probably don't want to return 3MB of image data in your has_image? method, or you may not want to return your entire user object in the logged_in? method. Using !! converts these objects to a simple true/false.
It returns true if the object on the right is not nil and not false, false if it is nil or false
def logged_in?
!!#current_user
end
! means negate boolean state, two !s is nothing special, other than a double negation.
!true == false
# => true
It is commonly used to force a method to return a boolean. It will detect any kind of truthiness, such as string, integers and what not, and turn it into a boolean.
!"wtf"
# => false
!!"wtf"
# => true
A more real use case:
def title
"I return a string."
end
def title_exists?
!!title
end
This is useful when you want to make sure that a boolean is returned. IMHO it's kind of pointless, though, seeing that both if 'some string' and if true is the exact same flow, but some people find it useful to explicitly return a boolean.
Note that this idiom exists in other programming languages as well. C didn't have an intrinsic bool type, so all booleans were typed as int instead, with canonical values of 0 or 1. Takes this example (parentheses added for clarity):
!(1234) == 0
!(0) == 1
!(!(1234)) == 1
The "not-not" syntax converts any non-zero integer to 1, the canonical boolean true value.
In general, though, I find it much better to put in a reasonable comparison than to use this uncommon idiom:
int x = 1234;
if (!!x); // wtf mate
if (x != 0); // obvious
It's useful if you need to do an exclusive or. Copying from Matt Van Horn's answer with slight modifications:
1 ^ true
TypeError: can't convert true into Integer
!!1 ^ !!true
=> false
I used it to ensure two variables were either both nil, or both not nil.
raise "Inconsistency" if !!a ^ !!b
It is "double-negative", but the practice is being discouraged. If you're using rubocop, you'll see it complain on such code with a Style/DoubleNegation violation.
The rationale states:
As this is both cryptic and usually redundant, it should be avoided
[then paraphrasing:] Change !!something to !something.nil?
Understanding how it works can be useful if you need to convert, say, an enumeration into a boolean. I have code that does exactly that, using the classy_enum gem:
class LinkStatus < ClassyEnum::Base
def !
return true
end
end
class LinkStatus::No < LinkStatus
end
class LinkStatus::Claimed < LinkStatus
def !
return false
end
end
class LinkStatus::Confirmed < LinkStatus
def !
return false
end
end
class LinkStatus::Denied < LinkStatus
end
Then in service code I have, for example:
raise Application::Error unless !!object.link_status # => raises exception for "No" and "Denied" states.
Effectively the bangbang operator has become what I might otherwise have written as a method called to_bool.
Other answers have discussed what !! does and whether it is good practice or not.
However, none of the answers give the "standard Ruby way" of casting a value into a boolean.
true & variable
TrueClass, the class of the Ruby value true, implements a method &, which is documented as follows:
Returns false if obj is nil or false, true otherwise.
Why use a dirty double-negation when the standard library has you covered?