A statement like w = x = y
= z is called a chained assignment in which the value of z is assigned
to multiple variables w, x, and y. Chained assignments are often used to
initialize multiple variables, as in
a = b = c = d = f = 0
Not all programming languages support chained assignment.
In
some programming languages (C for example), chained assignments are
supported because assignments return values. In C++ they are also
available for values of class types by making appropriate return type
for the assignment operator. This is however strongly discouraged
because it also allows values of class types to be assigned to temporary
objects, which results in a statement with no effect (assignment to
temporary objects is detected as error only for non-class types).
In
Python, assignment statements are not expressions and thus do not
return a value, but chained assignment is recognized and supported as a
special case of the assignment statement.
Parallel assignment
Some
programming languages, such as JavaScript (since 1.7), occam 2,[9]
Perl,[10] Python,[11] REBOL, Ruby,[12], C++ (since C++11 using tuples),
and Windows PowerShell allow several variables to be assigned in
parallel, with syntax like:
a,b := 0,1
which
simultaneously assigns 0 to a and 1 to b. If the right-hand side of the
assignment is an array variable, this feature is sometimes called
sequence unpacking:
var list := {0, 1}
a,b := list
The list will be unpacked so that 0 is assigned to a and 1 to b. More interestingly,
a,b := b,a
Swaps the values of a and b. In languages without parallel assignment, this would have to be written to use a temporary variable
var t := a
a := b
b := t
since a:=b ; b:=a leaves both a and b with the original value of b.
Parallel assignment was introduced in CPL in 1963, under the name simultaneous assignment.[13]
Assignment versus equality
A notorious example for a bad idea was the choice of the equal sign to
denote assignment. It goes back to Fortran in 1957 and has blindly been
copied by armies of language designers. Why is it a bad idea? Because it
overthrows a century old tradition to let “=” denote a comparison for
equality, a predicate which is either true or false. But Fortran made it
to mean assignment, the enforcing of equality. In this case, the
operands are on unequal footing: The left operand (a variable) is to be
made equal to the right operand (an expression). x = y does not mean the
same thing as y = x.[14]
—Niklaus Wirth, Good Ideas, Through the Looking Glass
Beginning
programmers sometimes confuse assignment with the relational operator
for equality, as "=" means equality in mathematics, and is used for
assignment in many languages. But assignment alters the value of a
variable, while equality testing tests whether two expressions have the
same value.
In some languages, such as BASIC, a single
equals sign ("=") is used for both the assignment operator and the
equality relational operator, with context determining which is meant.
Other languages use different symbols for the two operators. For
example:
In Pascal, the assignment operator is a
colon and an equals sign (":=") while the equality operator is a single
equals ("=").
In C, the assignment operator is a single equals sign ("=") while the equality operator is a pair of equals signs ("==").
The
similarity in the two symbols can lead to errors if the programmer
forgets which form ("=", "==", ":=") is appropriate, or mistypes "="
when "==" was intended. This is a common programming problem with
languages such as C, where the assignment operator also returns the
value assigned, and can be validly nested inside expressions (in the
same way that a function returns a value). If the intention was to
compare two values in an if statement, for instance, an assignment is
quite likely to return a value interpretable as Boolean true, in which
case the then clause will be executed, leading the program to behave
unexpectedly. Some language processors (such as gcc) can detect such
situations, and warn the programmer of the potential error.