Editing Assignment (computer science) (section)

===Parallel assignment===
Some programming languages, such as [[APL (programming language)|APL]], [[Common Lisp]],<ref>{{cite web |title=CLHS: Macro SETF, PSETF |url=http://www.lispworks.com/documentation/HyperSpec/Body/m_setf_.htm#psetf |website=Common Lisp Hyperspec |publisher=LispWorks |access-date=23 April 2019}}</ref> [[Go (programming language)|Go]],<ref>The Go Programming Language Specification: [http://golang.org/ref/spec#Assignments Assignments]</ref> [[JavaScript]] (since 1.7), [[Julia (programming language)|Julia]], [[PHP]], [[Maple (software)|Maple]], [[Lua (programming language)|Lua]], [[occam (programming language)#occam 2|occam 2]],<ref name="occam">{{cite book |editor=INMOS Limited |year=1988 |title=Occam 2 Reference Manual |location=New Jersey |publisher=Prentice Hall |isbn=0-13-629312-3}}</ref> [[Perl]],<ref name="perl">{{cite book |last=Wall |first=Larry |author-link=Larry Wall |author2=Christiansen, Tom |author3=Schwartz, Randal C. |year=1996 |edition=2 |title=Perl Programming Language |location=Cambridge |publisher=O´Reilly |isbn=1-56592-149-6 |url=https://archive.org/details/programmingperl00wall }}</ref> [[Python (programming language)|Python]],<ref name="python">{{cite book |last=Lutz |first=Mark |year=2001 |edition=2 |title=Python Programming Language |location=Sebastopol |publisher=O´Reilly |isbn=0-596-00085-5 |url=https://archive.org/details/programmingpytho00lutz }}</ref> [[REBOL]], [[Ruby (programming language)|Ruby]],<ref name="ruby">{{cite book |last=Thomas |first=David |author2=Hunt, Andrew |year=2001 |title=Programming Ruby: The Pragmatic Programmer's Guide |location=Upper Saddle River |publisher=Addison Wesley |isbn=0-201-71089-7 |url=https://archive.org/details/programmingruby000thom }}</ref> and [[PowerShell]] allow several variables to be assigned in parallel, with syntax like:

 a, b := 0, 1

which simultaneously assigns 0 to <code>a</code> and 1 to <code>b</code>. This is most often known as '''parallel assignment'''; it was introduced in [[CPL (programming language)|CPL]] in 1963, under the name '''simultaneous assignment''',<ref>D.W. Barron ''et al.'', "The main features of CPL", ''Computer Journal'' '''6''':2:140 (1963). [https://archive.today/20120707200431/http://comjnl.oxfordjournals.org/cgi/reprint/6/2/134 full text (subscription)]</ref> and is sometimes called '''multiple assignment''', though this is confusing when used with "single assignment", as these are not opposites. If the right-hand side of the assignment is a single variable (e.g. an array or structure), the feature is called '''unpacking'''<ref>{{cite web|url=http://legacy.python.org/dev/peps/pep-3132/|title=PEP 3132 -- Extended Iterable Unpacking|website=legacy.python.org|access-date=20 April 2018}}</ref> or '''destructuring assignment''':<ref>{{cite web|url=https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Destructuring_assignment|title=Destructuring assignment|website=MDN Web Docs|access-date=20 April 2018}}</ref>

 '''var''' list := {0, 1}
 a, b := list

The list will be unpacked so that 0 is assigned to <code>a</code> and 1 to <code>b</code>. Furthermore,

 a, b := b, a

swaps the values of <code>a</code> and <code>b</code>. In languages without parallel assignment, this would have to be written to use a temporary variable

 '''var''' t := a
 a := b
 b := t

since <code>a := b; b := a</code> leaves both <code>a</code> and <code>b</code> with the original value of <code>b</code>.

Some languages, such as [[Go (programming language)|Go]], [[F Sharp (programming language)|F#]] and [[Python (programming language)|Python]], combine parallel assignment, tuples, and automatic [[b:Python Programming/Tuples#Packing and Unpacking|tuple unpacking]] to allow multiple return values from a single function, as in this Python example,

<syntaxhighlight lang="python">
def f():
    return 1, 2
a, b = f()
</syntaxhighlight>

while other languages, such as [[C sharp (programming language)|C#]] and [[Rust (programming language)|Rust]], shown here, require explicit tuple construction and deconstruction with parentheses:

<syntaxhighlight lang="csharp">
// Valid C# or Rust syntax
(a, b) = (b, a);
</syntaxhighlight>

<syntaxhighlight lang="csharp">
// C# tuple return
(string, int) f() => ("foo", 1);
var (a, b) = f();
</syntaxhighlight>

<syntaxhighlight lang="rust">
// Rust tuple return
let f = || ("foo", 1);
let (a, b) = f();
</syntaxhighlight>

This provides an alternative to the use of [[output parameter]]s for returning multiple values from a function. This dates to [[CLU (programming language)|CLU]] (1974), and CLU helped popularize parallel assignment generally.

C# additionally allows generalized ''deconstruction assignment'' with implementation defined by the expression on the right-hand side, as the compiler searches for an appropriate [[instance method|instance]] or [[extension method|extension]] <code>Deconstruct</code> method on the expression, which must have output parameters for the variables being assigned to.<ref>{{cite web |title=Deconstructing tuples and other types |url=https://docs.microsoft.com/en-us/dotnet/csharp/deconstruct |website=Microsoft Docs |publisher=Microsoft |access-date=29 August 2019}}</ref> For example, one such method that would give the [[class (computer programming)|class]] it appears in the same behavior as the return value of <code>f()</code> above would be
<syntaxhighlight lang="csharp">
void Deconstruct(out string a, out int b) { a = "foo"; b = 1; }
</syntaxhighlight>

In C and C++, the [[comma operator]] is similar to parallel assignment in allowing multiple assignments to occur within a single statement, writing <code>a = 1, b = 2</code> instead of <code>a, b = 1, 2</code>. 
This is primarily used in [[Comma operator#For loops|for loops]], and is replaced by parallel assignment in other languages such as Go.<ref>[http://golang.org/doc/effective_go.html Effective Go]: [http://golang.org/doc/effective_go.html#for for],
"Finally, Go has no comma operator and ++ and -- are statements not expressions. Thus if you want to run multiple variables in a for you should use parallel assignment (although that precludes ++ and --)."</ref>
However, the above C++ code does not ensure perfect simultaneity, since the right side of the following code <code>a = b, b = a+1</code> is evaluated after the left side. In languages such as Python, <code>a, b = b, a+1</code> will assign the two variables concurrently, using the initial value of a to compute the new b.