What does ||= mean?
Basically, a ||= b
means assign b to a if a is null or undefined or false (i.e. false-ish value in ruby)
, it is similar to a = b unless a
, except it will always evaluate to the final value of a
(whereas a = b unless a
would result in nil
if a
was true-ish).
What does ||= (or-equals) mean in Ruby?
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 "Abbreviated assignments" 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.
What does ||= mean?
It is an assignment operator which means: or assign this value to a variable.
So if you did something like x ||= y
this meansx || x = y
so if x is nil or false set x to be the value of y.
What does |= mean? (pipe equal operator)
|=
reads the same way as +=
.
notification.defaults |= Notification.DEFAULT_SOUND;
is the same as
notification.defaults = notification.defaults | Notification.DEFAULT_SOUND;
where |
is the bit-wise OR operator.
All operators are referenced here.
A bit-wise operator is used because, as is frequent, those constants enable an int to carry flags.
If you look at those constants, you'll see that they're in powers of two :
public static final int DEFAULT_SOUND = 1;
public static final int DEFAULT_VIBRATE = 2; // is the same than 1<<1 or 10 in binary
public static final int DEFAULT_LIGHTS = 4; // is the same than 1<<2 or 100 in binary
So you can use bit-wise OR to add flags
int myFlags = DEFAULT_SOUND | DEFAULT_VIBRATE; // same as 001 | 010, producing 011
so
myFlags |= DEFAULT_LIGHTS;
simply means we add a flag.
And symmetrically, we test a flag is set using &
:
boolean hasVibrate = (DEFAULT_VIBRATE & myFlags) != 0;
What does the |= operator mean in C++?
Assuming you are using built-in operators on integers, or sanely overloaded operators for user-defined classes, these are the same:
a = a | b;
a |= b;
The '|=
' symbol is the bitwise OR assignment operator. It computes the value of OR'ing the RHS ('b') with the LHS ('a') and assigns the result to 'a', but it only evaluates 'a' once while doing so.
The big advantage of the '|=' operator is when 'a' is itself a complex expression:
something[i].array[j]->bitfield |= 23;
vs:
something[i].array[i]->bitfield = something[i].array[j]->bitfield | 23;
Was that difference intentional or accidental?
...
Answer: deliberate - to show the advantage of the shorthand expression...the first of the complex expressions is actually equivalent to:
something[i].array[j]->bitfield = something[i].array[j]->bitfield | 23;
Similar comments apply to all of the compound assignment operators:
+= -= *= /= %=
&= |= ^=
<<= >>=
Any compound operator expression:
a XX= b
is equivalent to:
a = (a) XX (b);
except that a
is evaluated just once. Note the parentheses here - it shows how the grouping works.
What does &.!= mean in Ruby?
What looks like one operator in the middle (&.!=
) is actually 1 operator and 1 method call : &.
followed by !=
with :click
as argument:
action &. != :click
It checks if action
is not nil
but distinct from :click
:
action = nil
action&.!=:click
# => nil
action = :not_click
action&.!=:click
# => true
action = :click
action&.!=:click
# => false
action = false
action&.!=:click
# => true
It's an abusive way to use &.
in my humble opinion. &.
is called a "safe navigation operator", because it prevents you from calling undefined methods on nil
objects.
!=
is defined for any object though, so there's nothing safe about using &.
before !=
.
You could write :
!(action.nil? || action == :click)
or
!action.nil? && action != :click
or even:
![nil, :click].include?(action)
What does ||= mean?
Basically, a ||= b
means assign b to a if a is null or undefined or false (i.e. false-ish value in ruby)
, it is similar to a = b unless a
, except it will always evaluate to the final value of a
(whereas a = b unless a
would result in nil
if a
was true-ish).
In Ruby what does = mean and how does it work?
=>
separates the keys from the values in a hashmap literal. It is not overloadable and not specifically connected to symbols.
A hashmap literal has the form {key1 => value1, key2 => value2, ...}
, but when used as the last parameter of a function, you can leave off the curly braces. So when you see a function call like f(:a => 1, :b => 2)
, f
is called with one argument, which is a hashmap that has the keys :a
and :b
and the values 1
and 2
.
What does ||= mean in Ruby?
It's mainly used as a shortform for initializing a variable to a certain value, if it is not yet set.
Think about the statement as returning x || (x = y)
. If x
has a value (other than false
), only the left side of the ||
will be evalutated (since ||
short-circuts), and x
will be not be reassigned. However, if x
is false
or nil
, the right side will be evaluated, which will set x
to y
, and y
will be returned (the result of an assignment statement is the right-hand side).
See http://dablog.rubypal.com/2008/3/25/a-short-circuit-edge-case for more discussion.
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