What does or-equals mean in Ruby

Question

What does the following code mean in Ruby         Does it have any meaning or reason for the syntax

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is a conditional assignment operator    x     y   is equivalent to    x   x    y   or alternatively  if defined  x  and x     x   x else      x   y end

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It means or-equals to  It checks to see if the value on the left is defined  then use that  If it s not  use the value on the right  You can use it in Rails to cache instance variables in models   A quick Rails-based example  where we create a function to fetch the currently logged in user   class User  gt  ActiveRecord  Base    def current user      current user     User find by id session  user id     end  end   It checks to see if the  current user instance variable is set  If it is  it will return it  thereby saving a database call  If it s not set however  we make the call and then set the  current user variable to that  It s a really simple caching technique but is great for when you re fetching the same instance variable across the application multiple times

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This ruby-lang syntax  The correct answer is to check the ruby-lang documentation  All other explanations obfuscate   Google   ruby-lang docs Abbreviated Assignment     Ruby-lang docs  https   docs ruby-lang org en 2 4 0 syntax assignment rdoc html label-Abbreviated Assignment

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a     b is a conditional assignment operator  It means if a is undefined or falsey  then evaluate b and set a to the result  Equivalently  if a is defined and evaluates to truthy  then b is not evaluated  and no assignment takes place  For example   a     nil     gt  nil a     0     gt  0 a     2     gt  0  foo   false     gt  false foo     true     gt  true foo     false     gt  true   Confusingly  it looks similar to other assignment operators  such as      but behaves differently    a    b translates to a   a   b a     b roughly translates to a    a   b   It is a near-shorthand for a    a   b  The difference is that  when a is undefined  a    a   b would raise NameError  whereas a     b sets a to b  This distinction is unimportant if a and b are both local variables  but is significant if either is a getter setter method of a class   Further reading    http   www rubyinside com what-rubys-double-pipe-or-equals-really-does-5488 html

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b   5 a     b   This translates to   a   a    b   which will be  a   nil    5   so finally  a   5   Now if you call this again   a     b a   a    b a   5    5 a   5  b   6   Now if you call this again   a     b a   a    b a   5    6 a   5    If you observe  b value will not be assigned to a  a will still have 5   Its a Memoization Pattern that is being used in Ruby to speed up accessors   def users    users     User all end   This basically translates to    users    users    User all   So you will make a call to database for the first time you call this method   Future calls to this method will just return the value of  users instance variable

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is called a conditional assignment operator    It basically works as   but with the exception that if a variable has already been assigned it will do nothing    First example   x     10   Second example   x   20 x     10   In the first example x is now equal to 10  However  in the second example x is already defined as 20  So the conditional operator has no effect  x is still 20 after running x     10

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Suppose  a   2 and b   3  THEN  a     b  will be resulted to a s value i e  2   As when a evaluates to some value not resulted to false or nil   That s why it ll not evaluate b s value   Now Suppose  a   nil and b   3   Then a     b will be resulted to  3 i e  b s value   As it first try to evaluates a s value which resulted to nil   so it evaluated b s value   The best example used in ror app is     To get currently logged in iser def current user    current user     User find by id session  user id   end    Make current user available in templates as a helper helper method  current user   Where  User find by id session  user id   is fired if and only if  current user is not initialized before

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As a common misconception  a     b is not equivalent to a   a    b  but it behaves like a    a   b   But here comes a tricky case  If a is not defined  a    a   42 raises NameError  while a     42 returns 42  So  they don t seem to be equivalent expressions

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This is the default assignment notation  for example  x     1  this will check to see if x is nil or not  If x is indeed nil it will then assign it that new value  1 in our example   more explicit   if x    nil  x   1 end

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In short  a   b means  If a is undefined  nil or false  assign b to a  Otherwise  keep a intact

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This question has been discussed so often on the Ruby mailing-lists and Ruby blogs that there are now even threads on the Ruby mailing-list whose only purpose is to collect links to all the other threads on the Ruby mailing-list that discuss this issue  Here s one  The definitive list of      OR Equal  threads and pages If you really want to know what is going on  take a look at Section 11 4 2 3  quot Abbreviated assignments quot  of the Ruby Language Draft Specification  As a first approximation  a     b  is equivalent to a    a   b  and not equivalent to a   a    b  However  that is only a first approximation  especially if a is undefined  The semantics also differ depending on whether it is a simple variable assignment  a method assignment or an indexing assignment  a        b a c      b a c      b  are all treated differently

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a     b   is equivalent to  a    a   b   and not  a   a    b   because of the situation where you define a hash with a default  the hash will return the default for any undefined keys   a   Hash new true   Which is       if you use   a 10      10  same as a 10     a 10    10   a is still        but when you write it like so   a 10    a 10     10   a becomes    10   gt  true    because you ve assigned the value of itself at key 10  which defaults to true  so now the hash is defined for the key 10  rather than never performing the assignment in the first place

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If X does NOT have a value  it will be assigned the value of Y  Else  it will preserve it s original value  5 in this example    irb main  020 0 gt  x   5   gt  5 irb main  021 0 gt  y   10   gt  10 irb main  022 0 gt  x     y   gt  5    Now set x to nil    irb main  025 0 gt  x   nil   gt  nil irb main  026 0 gt  x     y   gt  10

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a     b is the same as saying a   b if a nil  or a   b unless a  But do all 3 options show the same performance  With Ruby 2 5 1 this  1000000 times do   a     1   a     1   a     1   a     1   a     1   a     1   a     1   a     1   a     1   a     1 end   takes 0 099 Seconds on my PC  while  1000000 times do   a   1 unless a   a   1 unless a   a   1 unless a   a   1 unless a   a   1 unless a   a   1 unless a   a   1 unless a   a   1 unless a   a   1 unless a   a   1 unless a end   takes 0 062 Seconds  That s almost 40  faster   and then we also have   1000000 times do   a   1 if a nil    a   1 if a nil    a   1 if a nil    a   1 if a nil    a   1 if a nil    a   1 if a nil    a   1 if a nil    a   1 if a nil    a   1 if a nil    a   1 if a nil  end   which takes 0 166 Seconds   Not that this will make a significant performance impact in general  but if you do need that last bit of optimization  then consider this result  By the way  a   1 unless a is easier to read for the novice  it is self-explanatory   Note 1  reason for repeating the assignment line multiple times is to reduce the overhead of the loop on the time measured   Note 2  The results are similar if I do a nil nil before each assignment

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To be precise  a     b means  quot if a is undefined or falsy  false or nil   set a to b and evaluate to  i e  return  b  otherwise evaluate to a quot   Others often try to illustrate this by saying that a     b is equivalent to a    a   b or a   a    b  These equivalencies can be helpful for understanding the concept  but be aware that they are not accurate under all conditions  Allow me to explain   a     b   a    a   b  The behavior of these statements differs when a is an undefined local variable  In that case  a     b will set a to b  and evaluate to b   whereas a    a   b will raise NameError  undefined local variable or method  a  for main Object   a     b   a   a    b  The equivalency of these statements are often assumed  since a similar equivalence is true for other abbreviated assignment operators  i e     -                amp          lt  lt    and  gt  gt     However  for     the behavior of these statements may differ when a  is a method on an object and a is truthy  In that case  a     b will do nothing  other than evaluate to a   whereas a   a    b will call a  a  on a s receiver  As others have pointed out  this can make a difference when calling a a has side effects  such as adding keys to a hash   a     b   a   b unless a   The behavior of these statements differs only in what they evaluate to when a is truthy  In that case  a   b unless a will evaluate to nil  though a will still not be set  as expected   whereas a     b will evaluate to a   a     b   defined  a     a    a   b     a   b      Still no  These statements can differ when a method missing method exists which returns a truthy value for a  In this case  a     b will evaluate to whatever method missing returns  and not attempt to set a  whereas defined  a     a    a   b     a   b  will set a to b and evaluate to b    Okay  okay  so what is a     b equivalent to  Is there a way to express this in Ruby  Well  assuming that I m not overlooking anything  I believe a     b is functionally equivalent to     drumroll  begin   a   nil if false   a    a   b end  Hold on  Isn t that just the first example with a noop before it  Well  not quite  Remember how I said before that a     b is only not equivalent to a    a   b when a is an undefined local variable  Well  a   nil if false ensures that a is never undefined  even though that line is never executed  Local variables in Ruby are lexically scoped

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unless x   x   y end  unless x has a value  it s not nil or false   set it equal to y  is equivalent to   x     y

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It s like lazy instantiation  If the variable is already defined it will take that value instead of creating the value again

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Basically    x     y means   if x has any value leave it alone and do not change the value  otherwise set x to y

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Please also remember that     isn t an atomic operation and so  it isn t thread safe  As rule of thumb  don t use it for class methods

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irb main  001 0 gt  a   1   gt  1 irb main  002 0 gt  a     2   gt  1   Because a was already set to 1  irb main  003 0 gt  a   nil   gt  nil irb main  004 0 gt  a     2   gt  2   Because a was nil

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Concise and complete answer  a     b   evaluates the same way as each of the following lines  a    a   b a   a   a   b if a then a else a   b end   -  On the other hand   a   a    b   evaluates the same way as each of the following lines  a   a   a   b if a then a   a else a   b end   -  Edit  As AJedi32 pointed out in the comments  this only holds true if  1  a is a defined variable  2  Evaluating a one time and two times does not result in a difference in program or system state

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a     b  Signifies if any value is present in  a  and you dont want to alter it the keep using that value  else if  a  doesnt have any value  use value of  b    Simple words  if left hand side if not null  point to existing value  else point to value at right side

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x     y   is  x    x   y    if x is false or undefined  then x point to y

[ruby] What does ||= (or-equals) mean in Ruby?

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