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e is automatically declared the first time it is assigned to with the scope it is declared in varying by language single assignment edit see also static single assignment form any assignment that changes an existing value e g x x 1 is disallowed in purely functional languages 4 in functional programming assignment is discouraged in favor of single assignment more commonly known as initialization single assignment is an example of name binding and differs from assignment as described in this article in that it can only be done once usually when the variable is created no subsequent reassignment is allowed an evaluation of an expression does not have a side effect if it does not change an observable state of the machine 5 other than producing the result and always produces same value for the same input 4 imperative assignment can introduce side effects while destroying and making the old value unavailable while substituting it with a new one 6 and is referred to as destructive assignment for that reason in lisp and functional programming similar to destructive updating single assignment is the only form of assignment available in purely functional languages such as haskell which do not have variables in the sense of imperative programming languages 4 but rather named constant values possibly of compound nature with their elements progressively defined on demand for the lazy languages purely functional languages can provide an opportunity for computation to be performed in parallel avoiding the von neumann bottleneck of sequential one step at a time execution since values are independent of each other 7 impure functional languages provide both single assignment as well as true assignment though true assignment is typically used with less frequency than in imperative programming languages for example in scheme both single assignment with let and true assignment with set can be used on all variables and specialized primitives are provided for destructive update inside lists vectors strings etc in ocaml only single assignment is allowed for variables via the let name value syntax however destructive update can be used on elements of arrays and strings with separate operator as well as on fields of records and objects that have been explicitly declared mutable meaning capable of being changed after their initial declaration by the programmer functional programming languages that use single assignment include clojure for data structures not vars erlang it accepts multiple assignment if the values are equal in contrast to haskell f haskell javascript for constants lava ocaml oz for dataflow variables not cells racket for some data structures like lists not symbols sasl scala for vals sisal standard ml non backtracking prolog code can be considered explicit single assignment explicit in a sense that its named variables can be in explicitly unassigned state or be set exactly once in haskell by contrast there can be no unassigned variables and every variable can be thought of as being implicitly set when it is created to its value or rather to a computational object that will produce its value on demand value of an assignment edit in some programming languages an assignment statement returns a value while in others it does not in most expression oriented programming languages for example c the assignment statement returns the assigned value allowing such idioms as x y a in which the assignment statement y a returns the value of a which is then assigned to x in a statement such as while ch getchar eof the return value of a function is used to control a loop while assigning that same value to a variable in other programming languages scheme for example the return value of an assignment is undefined and such idioms are invalid in haskell 8 there is no variable assignment but operations similar to assignment like assigning to a field of an array or a field of a mutable data structure usually evaluate to the unit type which is represented as this type has only one possible value therefore containing no information it is typically the type of an expression that is evaluated purely for its side effects variant forms of assignment edit certain use patterns are very common and thus often have special syntax to support them these are primarily syntactic sugar to reduce redundancy in the source code but also assists readers of the code in understanding the programmer s intent and provides the compiler with a clue to possible optimization augmented assignment edit main article augmented assignment the case where the assigned value depends on a previous one is so common that many imperative languages most notably c and the majority of its descendants provide special operators called augmented assignment like so a 2 a can instead be written as a 2 3 beyond syntactic sugar this assists the task of the compiler by making clear that in place modification of the variable a is possible chained assignment edit 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 chained assignments are equivalent to a sequence of assignments but the evaluation strategy differs between languages for simple chained assignments like initializing multiple variables the evaluation strategy does not matter but if the targets l values in the assignment are connected in some way the evaluation strategy affects the result in some programming languages c for example chained assignments are supported because assignments are expressions and have values in this case chain assignment can be implemented by having a right associative assignment and assignments happen right to left for example i arr i f is equivalent to arr i f i arr i in c they are also available for values of class types by declaring the appropriate return type for the assignment operator in python assignment statements are not expressions and thus do not have a value instead chained assignments are a series of statements with multiple targets for a single expression the assignments are executed left to right so that i arr i f evaluates the expression f then assigns the result to the leftmost target i and then assigns the same result to the next target arr i using the new value of i 9 this is essentially equivalent to tmp f i tmp arr i tmp though no actual variable is produced for the temporary value parallel assignment edit some programming languages such as apl common lisp 10 go 11 javascript since 1 7 12 julia php maple lua occam 2 13 perl 14 python 15 rebol ruby 16 and 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 this is most often known as parallel assignment it was introduced in cpl in 1963 under the name simultaneous assignment 17 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 18 or destructuring assignment 19 var list 0 1 a b list the list will be unpacked so that 0 is assigned to a and 1 to b furthermore 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 some languages such as go f and python combine parallel assignment tuples and automatic tuple unpacking to allow multiple return values from a single function as in this python example def f tuple int int return 1 2 a int b int a b f while other languages such as c and rust shown here require explicit tuple construction and deconstruction with parentheses valid c or rust syntax a b b a c tuple return string int f foo 1 var a b f rust tuple return let f fn str i32 foo 1 let a b str i32 f this provides an alternative to the use of output parameters for returning multiple values from a function this dates to 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 or extension deconstruct method on the expression which must have output parameters for the variables being assigned to 20 for example one such method that would give the class it appears in the same behavior as the return value of f above would be void deconstruct out string a out int b a foo b 1 in c and c the comma operator is similar to parallel assignment in allowing multiple assignments to occur within a single statement writing a 1 b 2 instead of a b 1 2 this is primarily used in for loops and is replaced by parallel assignment in other languages such as go 21 however the above c code does not ensure perfect simultaneity since the right side of the following code a b b a 1 is evaluated after the left side in languages such as python a b b a 1 will assign the two variables concurrently using the initial value of a to compute the new b assignment versus equality edit see also relational operator confusion with assignment operators the use of the equals sign as an assignment operator has been frequently criticized due to the conflict with equals as comparison for equality this results both in confusion by novices in writing code and confusion even by experienced programmers in reading code the use of equals for assignment dates back to heinz rutishauser s language superplan designed from 1949 to 1951 and was particularly popularized by fortran a notorious example for a bad idea was the choice of the equal sign to denote assignment it goes back to fortran in 1957 a 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 22 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 23 for example in algol and 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 in r the assignment operator is generally as in x value but a single equals sign can also be used in certain contexts 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 including one famous attempt to backdoor the linux kernel 24 where the assignment operator also returns the value assigned in the same way that a function returns a value and can be validly nested inside expressions 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 25 26 notation edit see also comparison of programming languages variable and constant declarations the two most common representations for the copying assignment are equals sign and colon equals both forms may semantically denote either an assignment statement or an assignment operator which also has a value depending on language and or usage variable expression fortran pl i c and descendants such as c java etc bourne shell python go assignment to pre declared variables r powershell nim etc variable expression algol and derivatives simula cpl bcpl pascal 27 and descendants such as modula mary pl m ada smalltalk eiffel 28 29 oberon dylan 30 python an assignment expression 31 go shorthand for declaring and defining a variable 32 io ampl ml assigning to a reference value 33 autohotkey etc other possibilities include a left arrow or a keyword though there are other rarer variants variable expression magik variable expression f ocaml r s variable expression r assign variable expression r variable expression apl 34 smalltalk atari 2600 basic programming variable expression j let variable expression basic let variable expression xquery set variable to expression applescript set variable expression c shell set variable variable expression powershell variable expression macsyma maxima k variable expression rebol var variable expression mirc scripting language reference variable reference expression simula mathematical pseudo code assignments are generally depicted with a left arrow some platforms put the expression on the left and the variable on the right move expression to variable cobol expression variable ti basic casio basic expression variable pop 2 beta r put expression into variable hypertalk livecode put expression in variable abc some expression oriented languages such as lisp 35 36 and tcl uniformly use prefix or postfix syntax for all statements including assignment setf variable expression common lisp set variable expression scheme 37 38 39 set variable expression tcl expression variable forth see also edit assignment operator c static single assignment form unification computer science immutable object assignment problem notes edit use of predates fortran though it was popularized by fortran references edit 1 2 2cs24 declarative www csc liv ac uk archived from the original on 24 april 2006 retrieved 20 april 2018 1 2 3 imperative programming uah edu archived from the original on 4 march 2016 retrieved 20 april 2018 1 2 3 ruediger marcus flaig 2008 bioinformatics programming in python a practical course for beginners wiley vch pp 98 99 isbn 978 3 527 32094 3 retrieved 25 december 2010 1 2 3 4 5 crossing borders explore functional programming with haskell archived november 19 2010 at the wayback machine by bruce tate mitchell john c 2003 concepts in programming languages cambridge university press p 23 isbn 978 0 521 78098 8 retrieved 3 january 2011 imperative programming lan...