I encountered some code that looks like:
(input_array || []).each do |a|
some stuff
end
What is the purpose of input_array || []? I would naively think that this would evaluate to a boolean value which would cause each to throw an error, but that's clearly not whats happening.
The semantics of || are:
if the first expression is not nil or false, return it
if the first expression is nil or false, return the second expression
This is used to provide a default value if the first is nil.
if firstcheck? || secondcheck?
nil
else
true
end
How can i reduce this block of code to one single line?
If the condition is correct i need to return nil, else true
Please help
This line is equivalent to the snippet of code you wrote:
true unless firstcheck? || secondcheck?
I have to wonder if you really have a requirement to return nil instead of false. If you are OK with returning false instead of nil, you could write it as:
!firstcheck? && !secondcheck?
I have a function similar to the following:
def check
return 2 == 2 || 3 != 2 || 4 != 5
end
My question is, will Ruby perform all the comparisons even though the first is true, and thus the function return true. My checks are much more intensive, so I'd like to know if I should break this out in a different way to avoid making all the checks every time.
irb(main):004:0> 2 == 2 || 3 != 2 || 4 != 5
=> true
Thank you.
Ruby uses short-circuit evaluation.
This applies to both || and &&.
With || the right operand is not evaluated if the left operand is truthy.
With && the right operand is not evaluated if the left operand is falsy.
|| short-circuits as soon as the first condition is true. So yes, it will help if you put the most expensive conditions at the end.
|| will by default short-circuit evaluate, meaning that once the first "true" expression is encountered it will stop evaluation (unless you explicitly state you want all expressions to evaluate with the 'or' operator).
reference:
http://en.wikipedia.org/wiki/Short-circuit_evaluation
As soon as one of the condition is true, the function will return.
You can test it yourself in irb, like this:
irb> p('Hello') || p('World')
As we know the function p prints its parameters(in an inspect manner) then returns them, so if the || short circuits, only "Hello" is printed, otherwise both "Hello" and "World" are printed.
You can also test the logical && operator, by using puts instead of p, as puts always returns nil.
BTW, irb is a perfect place to play around ruby. You can test everything there, except a small portion of concurrency.
I've noticed this little oddity(I think) in If Statements in Ruby. Here's an example:
my_number = nil
if my_number < 3
'number is less than 3'
end
Obviously, when you run this code you'll get a "comparison of Fixnum with nil failed" error. Now here's something strange. If I make a little change in the If Statement to check for nil, it works fine. Example:
my_number = nil
if my_number && my_number < 3
'number is less than 3'
end
Adding the check for nil makes it not crash. This may all sound stupid but I can't figure out why that works. Shouldn't it still throw an error?
Thanks to anyone who can explain this. :) Thanks!
Boolean expressions are evaluated in what is known as "short circuit" fashion. That is, as soon as it know the result, it doesn't keep trying to evaluate expressions.
So it does the if my_number and since that's false, there is no need to continue, because false && <anything> is always false.
This is a useful language feature, and many languages work like this (the original Visual Basic is one exception that I can think of) because it lets you do exactly this sort of test without requiring cumbersome nested 'if's.
This is no way specific to Ruby: I've noticed this behaviour in all languages I've ever used, from Pascal to Java.
If you have boolean expression b1 && b2 && b3 ... && bn, most languages guarantee that bi will be evaluated from left to right and if some bi turns out to be false, evaluation will be stopped. (because whole expression is false then). Same for boolean || operator.
It has to be OK to test for nil, so if nil or something like it is OK in almost every language.
But for an arithmetic comparison, if it didn't throw an exception it would have to return true or false. Either way is problematic.
True kind of doesn't make sense, as nil would be both < and > than 3.
False is almost as bad, as now nil < 3 and nil >= 3 are both false, and that's not ideal.
So, the comparison method throws an exception. Problem solved.
nil is "falsy". Try this in irb:
irb(main):001:0> puts "Nil is true " if nil
=> nil
irb(main):002:0> puts "Nil is not true " if !nil
Nil is not true
=> nil
nil isn't the same as false, but making it act as such help a lot in loops and other tests:
false == nil
=> false
Since the first part of your and is false ruby does the lazy thing and doesn't even bother to evaluate the next bit, if it's > 3.
What's happening is called short-circuit evaluation. You're statement can be thought of like this:
if( my_number )
if( my_number < 3)
'number is less than 3'
end
end
Since the first condition is false there's no reason to evaluate the second condition of the statement -- the my_number < 3 part.
I am looking for a concise way to check a value to see if it is nil or zero. Currently I am doing something like:
if (!val || val == 0)
# Is nil or zero
end
But this seems very clumsy.
Objects have a nil? method.
if val.nil? || val == 0
[do something]
end
Or, for just one instruction:
[do something] if val.nil? || val == 0
From Ruby 2.3.0 onward, you can combine the safe navigation operator (&.) with Numeric#nonzero?. &. returns nil if the instance was nil and nonzero? - if the number was 0:
unless val&.nonzero?
# Is nil or zero
end
Or postfix:
do_something unless val&.nonzero?
If you really like method names with question marks at the end:
if val.nil? || val.zero?
# do stuff
end
Your solution is fine, as are a few of the other solutions.
Ruby can make you search for a pretty way to do everything, if you're not careful.
First off I think that's about the most concise way you can check for that particular condition.
Second, to me this is a code smell that indicates a potential flaw in your design. Generally nil and zero shouldn't mean the same thing. If possible you should try to eliminate the possibility of val being nil before you hit this code, either by checking that at the beginning of the method or some other mechanism.
You might have a perfectly legitimate reason to do this in which case I think your code is good, but I'd at least consider trying to get rid of the nil check if possible.
You can use the Object.nil? to test for nil specifically (and not get caught up between false and nil). You can monkey-patch a method into Object as well.
class Object
def nil_or_zero?
return (self.nil? or self == 0)
end
end
my_object = MyClass.new
my_object.nil_or_zero?
==> false
This is not recommended as changes to Object are difficult for coworkers to trace, and may make your code unpredictable to others.
nil.to_i returns zero, so I often do this:
val.to_i.zero?
However, you will get an exception if val is ever an object that does not respond_to #to_i.
I believe your code is incorrect; it will in fact test for three values: nil, false, and zero. This is because the !val expression is true for all values that are false, which in Ruby is nil and false.
The best I can come up with right now is
if val == nil || val == 0
# do stuff
end
Which of course is not very clever, but (very) clear.
My solution also use Refinements, minus the conditionals.
module Nothingness
refine Numeric do
alias_method :nothing?, :zero?
end
refine NilClass do
alias_method :nothing?, :nil?
end
end
using Nothingness
if val.nothing?
# Do something
end
Short and clear
[0, nil].include?(val)
Shortest and best way should be
if val&.>(0)
# do something
end
For val&.>(0)
it returns nil when val is nil since > basically is also a method, nil equal to false in ruby. It return false when val == 0.
Rails does this via attribute query methods, where in addition to false and nil, 0 and "" also evaluate to false.
if (model.attribute?) # => false if attribute is 0 and model is an ActiveRecord::Base derivation
However it has its share of detractors. http://www.joegrossberg.com/archives/002995.html
To be as idiomatic as possible, I'd suggest this.
if val.nil? or val == 0
# Do something
end
Because:
It uses the nil? method.
It uses the "or" operator, which is preferable to ||.
It doesn't use parentheses, which are not necessary in this case. Parentheses should only be used when they serve some purpose, such as overriding the precedence of certain operators.
I deal with this by defining an "is?" method, which I can then implement differently on various classes. So for Array, "is?" means "size>0"; for Fixnum it means "self != 0"; for String it means "self != ''". NilClass, of course, defines "is?" as just returning nil.
You can use case if you like:
case val with nil, 0
# do stuff
end
Then you can use anything that works with ===, which is nice sometimes. Or do something like this:
not_valid = nil, 0
case val1 with *not_valid
# do stuff
end
#do other stuff
case val2 with *not_valid, false #Test for values that is nil, 0 or false
# do other other stuff
end
It's not exactly good OOP, but it's very flexible and it works. My ifs usually end up as cases anyway.
Of course Enum.any?/Enum.include? kind of works too ... if you like to get really cryptic:
if [0, nil].include? val
#do stuff
end
The right thing to do is of course to define a method or function. Or, if you have to do the same thing with many values, use a combination of those nice iterators.
I really like Rails blank? method for that kind of things, but it won't return true for 0. So you can add your method:
def nil_zero?
if respond_to?(:zero?)
zero?
else
!self
end
end
And it will check if some value is nil or 0:
nil.nil_zero?
=> true
0.nil_zero?
=> true
10.nil_zero?
=> false
if val.nil_zero?
#...
end
Instead of monkey patching a class, you could use refinements starting in Ruby 2.1. Refinements are similar to monkey patching; in that, they allow you to modify the class, but the modification is limited to the scope you wish to use it in.
This is overkill if you want to do this check once, but if you are repeating yourself it's a great alternative to monkey patching.
module NilOrZero
refine Object do
def nil_or_zero?
nil? or zero?
end
end
end
using NilOrZero
class Car
def initialize(speed: 100)
puts speed.nil_or_zero?
end
end
car = Car.new # false
car = Car.new(speed: nil) # true
car = Car.new(speed: 0) # true
Refinements were changed in the last minute to be scoped to the file. So earlier examples may have shown this, which will not work.
class Car
using NilOrZero
end
This is very concise:
if (val || 0) == 0
# Is nil, false, or zero.
end
It works as long as you don't mind treating false the same as nil. In the projects I've worked on, that distinction only matters once in a while. The rest of the time I personally prefer to skip .nil? and have slightly shorter code.
[Update: I don't write this sort of thing any more. It works but is too cryptic. I have tried to set right my misdeeds by changing the few places where I did it.]
By the way, I didn't use .zero? since this raises an exception if val is, say, a string. But .zero? would be fine if you know that's not the case.
This evaluates to true for nil and zero: nil.to_s.to_d == 0
unless (val || 0).zero?
# do stufff
end
In a single stretch you can do this:
[do_something] if val.to_i == 0
nil.to_i will return 0
Another solution:
if val.to_i == 0
# do stuff
end
val ||= 0
if val == 0
# do something here
end