{"history":[],"article":{"id":"8458ed52-eba3-4bcd-a8d3-da2a0db60516","title":"Relational operator","slug":"relational-operator","content":":::summary\n:::\n\n# \nIn [computer science](https://bliptext.com/articles/computer-science), a **[relation](https://bliptext.com/articles/relation-mathematics)al [operator](https://bliptext.com/articles/operator-programming)** is a [programming language](https://bliptext.com/articles/programming-language) construct or [operator](https://bliptext.com/articles/operator-programming) that tests or defines some kind of [relation](https://bliptext.com/articles/relation-mathematics) between [two entities](https://bliptext.com/articles/binary-function).\nThese include numerical [equality](https://bliptext.com/articles/equality-mathematics) (e.g., ) and [inequalities](https://bliptext.com/articles/inequality-mathematics) (e.g., 4 ≥ 3).\nIn programming languages that include a distinct [boolean data type](https://bliptext.com/articles/boolean-data-type) in their [type system](https://bliptext.com/articles/type-system), like [Pascal](https://bliptext.com/articles/pascal-programming-language), [Ada](https://bliptext.com/articles/ada-programming-language), [Python](https://bliptext.com/articles/python-programming-language) or [Java](https://bliptext.com/articles/java-programming-language), these operators usually evaluate to true or false, depending on if the conditional relationship between the two [operands](https://bliptext.com/articles/operand) holds or not.\nIn languages such as [C](https://bliptext.com/articles/c-programming-language), relational operators return the [integers](https://bliptext.com/articles/integer) 0 or 1, where 0 stands for false and any non-zero value stands for true.\nAn [expression](https://bliptext.com/articles/expression-programming) created using a relational operator forms what is termed a relational [expression](https://bliptext.com/articles/expression-programming) or a condition.\nRelational operators can be seen as special cases of logical [predicates](https://bliptext.com/articles/predicate-mathematical-logic).\n\n# Equality\n\n## Usage\nEquality is used in many programming language constructs and data types.\nIt is used to test if an element already exists in a [set](https://bliptext.com/articles/set-computer-science), or to access to a value through a key.\nIt is used in [switch statements](https://bliptext.com/articles/switch-statement) to dispatch the control flow to the correct branch, and during the unification process in logic programming.\nThere can be multiple valid definitions of equality, and any particular language might adopt one or more of them, depending on various design aspects.\nOne possible meaning of equality is that \"if a equals b, then either a or b can be used interchangeably in any context without noticing any difference\".\nBut this statement does not necessarily hold, particularly when taking into account mutability together with content equality.\n\n## Location equality vs. content equality\nSometimes, particularly in [object-oriented programming](https://bliptext.com/articles/object-oriented-programming), the comparison raises questions of [data types](https://bliptext.com/articles/data-type) and [inheritance](https://bliptext.com/articles/inheritance-object-oriented-programming), [equality](https://bliptext.com/articles/equality-mathematics), and [identity](https://bliptext.com/articles/identity-mathematics).\nIt is often necessary to distinguish between:\nIn many modern programming languages, objects and data structures are accessed through [references](https://bliptext.com/articles/reference-computer-science).\nIn such languages, there becomes a need to test for two different types of equality:\nThe first type of equality usually implies the second (except for things like not a number ([NaN](https://bliptext.com/articles/nan)) which are unequal to themselves), but the converse is not necessarily true.\nFor example, two [string](https://bliptext.com/articles/string-computer-science) objects may be distinct objects (unequal in the first sense) but contain the same sequence of characters (equal in the second sense).\nSee [identity](https://bliptext.com/articles/identity-object-oriented-programming) for more of this issue.\nReal numbers, including many simple [fractions](https://bliptext.com/articles/fraction-mathematics), cannot be represented exactly in [floating-point arithmetic](https://bliptext.com/articles/floating-point-arithmetic), and it may be necessary to test for equality within a given tolerance.\nSuch tolerance, however, can easily break desired properties such as transitivity, whereas reflexivity breaks too: the [IEEE floating-point](https://bliptext.com/articles/ieee-754) standard requires that NaN ≠ NaN holds.\nIn contrast, the (2022) [private standard](https://bliptext.com/articles/technical-standard) for [posit](https://bliptext.com/articles/posit-number-format) arithmetic ([posit](https://bliptext.com/articles/posit-number-format) proponents mean to replace IEEE floats) has a similar concept, NaR (Not a Real), where NaR = NaR holds.\nOther programming elements such as computable functions, may either have no sense of equality, or an equality that is uncomputable.\nFor these reasons, some languages define an explicit notion of \"comparable\", in the form of a base class, an interface, a trait or a protocol, which is used either explicitly, by declaration in source code, or implicitly, via the structure of the type involved.\n\n## Comparing values of different types\nIn [JavaScript](https://bliptext.com/articles/javascript), [PHP](https://bliptext.com/articles/php), [VBScript](https://bliptext.com/articles/vbscript) and a few other [dynamically typed](https://bliptext.com/articles/type-system) languages, the standard equality operator follows so-called loose typing, that is it evaluates to true even if two values are not equal and are of incompatible types, but can be coerced to each other by some set of language-specific rules, making the number 4 compare equal to the text string \"4\", for instance.\nAlthough such behaviour is typically meant to make the language easier, it can lead to surprising and difficult to predict consequences that many programmers are unaware of.\nFor example, Javascript's loose equality rules can cause equality to be intransitive (ie. and , but ), or make certain values be equal to their own negation.\nA strict equality operator is also often available in those languages, returning true only for values with identical or equivalent types (in PHP, is false although is true).\nFor languages where the number 0 may be interpreted as false, this operator may simplify things such as checking for zero (as would be true for x being either 0 or \"0\" using the type agnostic equality operator).\n\n# Ordering\nGreater than and less than comparison of non-numeric data is performed according to a sort convention (such as, for text strings, [lexicographical order](https://bliptext.com/articles/lexicographical-order)) which may be built into the programming language and/or configurable by a programmer.\nWhen it is desired to associate a numeric value with the result of a comparison between two data items, say a and b, the usual convention is to assign −1 if a < b, 0 if a = b and 1 if a > b. For example, the C function performs a [three-way comparison](https://bliptext.com/articles/three-way-comparison) and returns −1, 0, or 1 according to this convention, and [qsort](https://bliptext.com/articles/qsort) expects the comparison function to return values according to this convention.\nIn [sorting algorithms](https://bliptext.com/articles/sorting-algorithm), the efficiency of comparison code is critical since it is one of the major factors contributing to sorting performance.\nComparison of programmer-defined [data types](https://bliptext.com/articles/data-type) ([data types](https://bliptext.com/articles/data-type) for which the programming language has no in-built understanding) may be carried out by custom-written or library functions (such as mentioned above), or, in some languages, by [overloading](https://bliptext.com/articles/operator-overloading) a comparison operator – that is, assigning a programmer-defined meaning that depends on the [data types](https://bliptext.com/articles/data-type) being compared.\nAnother alternative is using some convention such as member-wise comparison.\n\n# Logical equivalence\nThough perhaps unobvious at first, like the [boolean](https://bliptext.com/articles/boolean-logic) [logical operators](https://bliptext.com/articles/logical-operator) XOR, AND, OR, and NOT, relational operators can be designed to have [logical equivalence](https://bliptext.com/articles/logical-equivalence), such that they can all be defined in terms of one another.\nThe following four conditional statements all have the same logical equivalence E (either all true or all false) for any given x and y values:\nThis relies on the domain being [well ordered](https://bliptext.com/articles/well-ordered).\n\n# Standard relational operators\nThe most common numerical relational operators used in programming languages are shown below.\nStandard [SQL](https://bliptext.com/articles/sql) uses the same operators as BASIC, while many databases allow in addition to from the standard.\nSQL follows strict [boolean algebra](https://bliptext.com/articles/boolean-algebra), i.e. doesn't use [short-circuit evaluation](https://bliptext.com/articles/short-circuit-evaluation), which is common to most languages below.\nE.g. [PHP](https://bliptext.com/articles/php) has it, but otherwise it has these same two operators defined as aliases, like many SQL databases.\nOther conventions are less common: [Common Lisp](https://bliptext.com/articles/common-lisp) and [Macsyma](https://bliptext.com/articles/macsyma)/[Maxima](https://bliptext.com/articles/maxima-software) use Basic-like operators for numerical values, except for inequality, which is in [Common Lisp](https://bliptext.com/articles/common-lisp) and in [Macsyma](https://bliptext.com/articles/macsyma)/[Maxima](https://bliptext.com/articles/maxima-software).\nCommon Lisp has multiple other sets of equality and relational operators serving different purposes, including, , , , and.\nOlder [Lisps](https://bliptext.com/articles/lisp-programming-language) used, , and ; and negated them using for the remaining operators.\n\n# Syntax\nRelational operators are also used in technical literature instead of words.\nRelational operators are usually written in [infix notation](https://bliptext.com/articles/infix-notation), if supported by the programming language, which means that they appear between their operands (the two expressions being related).\nFor example, an expression in Python will print the message if the x is less than y:\nOther programming languages, such as [Lisp](https://bliptext.com/articles/lisp-programming-language), use [prefix notation](https://bliptext.com/articles/prefix-notation), as follows:\n\n## Operator chaining\nIn mathematics, it is common practice to chain relational operators, such as in 3 < x < y < 20 (meaning 3 < x and x < y and y < 20).\nThe syntax is clear since these relational operators in mathematics are transitive.\nHowever, many recent programming languages would see an expression like 3 < x < y as consisting of two left (or right-) associative operators, interpreting it as something like.\nIf we say that x=4, we then get, and evaluation will give which generally does not make sense.\nHowever, it does compile in C/C++ and some other languages, yielding surprising result (as true would be represented by the number 1 here).\nIt is possible to give the expression its familiar mathematical meaning, and some programming languages such as Python and [Raku](https://bliptext.com/articles/raku-programming-language) do that.\nOthers, such as C# and Java, do not, partly because it would differ from the way most other infix operators work in C-like languages.\nThe [D programming language](https://bliptext.com/articles/d-programming-language) does not do that since it maintains some compatibility with C, and \"Allowing C expressions but with subtly different semantics (albeit arguably in the right direction) would add more confusion than convenience\".\nSome languages, like [Common Lisp](https://bliptext.com/articles/common-lisp), use multiple argument predicates for this.\nIn Lisp is true when x is between 1 and 10.\n\n## Confusion with assignment operators\nEarly FORTRAN (1956–57) was bounded by heavily restricted character sets where was the only relational operator available.\nThere were no or (and certainly no or ).\nThis forced the designers to define symbols such as, , , etc. and subsequently made it tempting to use the remaining character for copying, despite the obvious incoherence with mathematical usage ( should be impossible).\nInternational Algebraic Language (IAL, [ALGOL 58](https://bliptext.com/articles/algol-58)) and [ALGOL](https://bliptext.com/articles/algol) (1958 and 1960) thus introduced for assignment, leaving the standard available for equality, a convention followed by [CPL](https://bliptext.com/articles/cpl-programming-language), [ALGOL](https://bliptext.com/articles/algol) W, [ALGOL](https://bliptext.com/articles/algol) 68, Basic Combined Programming Language (B[CPL](https://bliptext.com/articles/cpl-programming-language)), [Simula](https://bliptext.com/articles/simula), SET Language ([SETL](https://bliptext.com/articles/setl)), [Pascal](https://bliptext.com/articles/pascal-programming-language), [Smalltalk](https://bliptext.com/articles/smalltalk), [Modula-2](https://bliptext.com/articles/modula-2), [Ada](https://bliptext.com/articles/ada-programming-language), [Standard ML](https://bliptext.com/articles/standard-ml), [OCaml](https://bliptext.com/articles/ocaml), [Eiffel](https://bliptext.com/articles/eiffel-programming-language), Object [Pascal](https://bliptext.com/articles/pascal-programming-language) ([Delphi](https://bliptext.com/articles/delphi-programming-language)), [Oberon](https://bliptext.com/articles/oberon-programming-language), [Dylan](https://bliptext.com/articles/dylan-programming-language), VHSIC Hardware Description Language ([VHDL](https://bliptext.com/articles/vhdl)), and several other languages.\n\n### B and C\nThis uniform de facto standard among most programming languages was eventually changed, indirectly, by a minimalist compiled language named [B](https://bliptext.com/articles/b-programming-language).\nIts sole intended application was as a vehicle for a first port of (a then very primitive) [Unix](https://bliptext.com/articles/unix), but it also evolved into the very influential [C](https://bliptext.com/articles/c-programming-language) language.\nB started off as a syntactically changed variant of the systems programming language [BCPL](https://bliptext.com/articles/bcpl), a simplified (and typeless) version of [CPL](https://bliptext.com/articles/cpl-programming-language).\nIn what has been described as a \"strip-down\" process, the and operators of BCPL were replaced with and (which would later become and , respectively. ). In the same process, the ALGOL style of BCPL was replaced by in B. The reason for all this being unknown.\nAs variable updates had no special syntax in B (such as or similar) and were allowed in expressions, this non standard meaning of the equal sign meant that the traditional semantics of the equal sign now had to be associated with another symbol.\n[Ken Thompson](https://bliptext.com/articles/ken-thompson) used the ad hoc combination for this.\nAs a small type system was later introduced, B then became C. The popularity of this language along with its association with Unix, led to Java, C#, and many other languages following suit, syntactically, despite this needless conflict with the mathematical meaning of the equal sign.\n\n### Languages\nAssignments in C have a [value](https://bliptext.com/articles/value-programming) and since any non-zero scalar [value](https://bliptext.com/articles/value-programming) is interpreted as true in [conditional expressions](https://bliptext.com/articles/conditional-programming), the code is legal, but has a very different meaning from.\nThe former code fragment means \"assign y to x, and if the new value of x is not zero, execute the following statement\".\nThe latter fragment means \"[if and only if](https://bliptext.com/articles/if-and-only-if) x is equal to y, execute the following statement\".\nThough [Java](https://bliptext.com/articles/java-programming-language) and [C#](https://bliptext.com/articles/c-sharp-programming-language) have the same operators as C, this mistake usually causes a compile error in these languages instead, because the if-condition must be of type, and there is no implicit way to convert from other types (e.g., numbers) into s. So unless the variable that is assigned to has type (or wrapper type ), there will be a compile error.\nIn ALGOL-like languages such as Pascal, Delphi, and Ada (in the sense that they allow [nested function definitions](https://bliptext.com/articles/nested-function-definition)), and in [Python](https://bliptext.com/articles/python-programming-language), and many functional languages, among others, assignment operators cannot appear in an [expression](https://bliptext.com/articles/expression-programming) (including clauses), thus precluding this class of error.\nSome compilers, such as [GNU Compiler Collection](https://bliptext.com/articles/gnu-compiler-collection) (GCC), provide a warning when compiling code containing an assignment operator inside an if statement, though there are some legitimate uses of an assignment inside an if-condition.\nIn such cases, the assignment must be wrapped in an extra pair of parentheses explicitly, to avoid the warning.\nSimilarly, some languages, such as [BASIC](https://bliptext.com/articles/basic) use just the symbol for both assignment and equality, as they are syntactically separate (as with Pascal, Ada, Python, etc., assignment operators cannot appear in expressions).\nSome programmers get in the habit of writing comparisons against a constant in the reverse of the usual order:\nIf is used accidentally, the resulting code is invalid because 2 is not a variable.\nThe compiler will generate an error message, on which the proper operator can be substituted.\nThis coding style is termed left-hand comparison, or [Yoda conditions](https://bliptext.com/articles/yoda-conditions).\nThis table lists the different mechanisms to test for these two types of equality in various languages:\nRuby uses to mean \"b is a member of the set a\", though the details of what it means to be a member vary considerably depending on the data types involved.\nis here known as the \"case equality\" or \"case subsumption\" operator.\n\n# Notes and references","current_revision":null,"created_by":"kACnuQmH5k9Nr5VNVEpVeg7XpvUmMctD","created_at":"2025-01-25T21:13:43.464Z","updated_at":"2025-01-25T21:13:43.464Z","search_vector":"'oper':2 'relat':1","search_content":" \n \n\n# \nIn [computer science](https://bliptext.com/articles/computer-science), a **[relation](https://bliptext.com/articles/relation-mathematics)al [operator](https://bliptext.com/articles/operator-programming)** is a [programming language](https://bliptext.com/articles/programming-language) construct or [operator](https://bliptext.com/articles/operator-programming) that tests or defines some kind of [relation](https://bliptext.com/articles/relation-mathematics) between [two entities](https://bliptext.com/articles/binary-function).\nThese include numerical [equality](https://bliptext.com/articles/equality-mathematics) (e.g., ) and [inequalities](https://bliptext.com/articles/inequality-mathematics) (e.g., 4 ≥ 3).\nIn programming languages that include a distinct [boolean data type](https://bliptext.com/articles/boolean-data-type) in their [type system](https://bliptext.com/articles/type-system), like [Pascal](https://bliptext.com/articles/pascal-programming-language), [Ada](https://bliptext.com/articles/ada-programming-language), [Python](https://bliptext.com/articles/python-programming-language) or [Java](https://bliptext.com/articles/java-programming-language), these operators usually evaluate to true or false, depending on if the conditional relationship between the two [operands](https://bliptext.com/articles/operand) holds or not.\nIn languages such as [C](https://bliptext.com/articles/c-programming-language), relational operators return the [integers](https://bliptext.com/articles/integer) 0 or 1, where 0 stands for false and any non-zero value stands for true.\nAn [expression](https://bliptext.com/articles/expression-programming) created using a relational operator forms what is termed a relational [expression](https://bliptext.com/articles/expression-programming) or a condition.\nRelational operators can be seen as special cases of logical [predicates](https://bliptext.com/articles/predicate-mathematical-logic).\n\n# Equality\n\n## Usage\nEquality is used in many programming language constructs and data types.\nIt is used to test if an element already exists in a [set](https://bliptext.com/articles/set-computer-science), or to access to a value through a key.\nIt is used in [switch statements](https://bliptext.com/articles/switch-statement) to dispatch the control flow to the correct branch, and during the unification process in logic programming.\nThere can be multiple valid definitions of equality, and any particular language might adopt one or more of them, depending on various design aspects.\nOne possible meaning of equality is that \"if a equals b, then either a or b can be used interchangeably in any context without noticing any difference\".\nBut this statement does not necessarily hold, particularly when taking into account mutability together with content equality.\n\n## Location equality vs. content equality\nSometimes, particularly in [object-oriented programming](https://bliptext.com/articles/object-oriented-programming), the comparison raises questions of [data types](https://bliptext.com/articles/data-type) and [inheritance](https://bliptext.com/articles/inheritance-object-oriented-programming), [equality](https://bliptext.com/articles/equality-mathematics), and [identity](https://bliptext.com/articles/identity-mathematics).\nIt is often necessary to distinguish between:\nIn many modern programming languages, objects and data structures are accessed through [references](https://bliptext.com/articles/reference-computer-science).\nIn such languages, there becomes a need to test for two different types of equality:\nThe first type of equality usually implies the second (except for things like not a number ([NaN](https://bliptext.com/articles/nan)) which are unequal to themselves), but the converse is not necessarily true.\nFor example, two [string](https://bliptext.com/articles/string-computer-science) objects may be distinct objects (unequal in the first sense) but contain the same sequence of characters (equal in the second sense).\nSee [identity](https://bliptext.com/articles/identity-object-oriented-programming) for more of this issue.\nReal numbers, including many simple [fractions](https://bliptext.com/articles/fraction-mathematics), cannot be represented exactly in [floating-point arithmetic](https://bliptext.com/articles/floating-point-arithmetic), and it may be necessary to test for equality within a given tolerance.\nSuch tolerance, however, can easily break desired properties such as transitivity, whereas reflexivity breaks too: the [IEEE floating-point](https://bliptext.com/articles/ieee-754) standard requires that NaN ≠ NaN holds.\nIn contrast, the (2022) [private standard](https://bliptext.com/articles/technical-standard) for [posit](https://bliptext.com/articles/posit-number-format) arithmetic ([posit](https://bliptext.com/articles/posit-number-format) proponents mean to replace IEEE floats) has a similar concept, NaR (Not a Real), where NaR = NaR holds.\nOther programming elements such as computable functions, may either have no sense of equality, or an equality that is uncomputable.\nFor these reasons, some languages define an explicit notion of \"comparable\", in the form of a base class, an interface, a trait or a protocol, which is used either explicitly, by declaration in source code, or implicitly, via the structure of the type involved.\n\n## Comparing values of different types\nIn [JavaScript](https://bliptext.com/articles/javascript), [PHP](https://bliptext.com/articles/php), [VBScript](https://bliptext.com/articles/vbscript) and a few other [dynamically typed](https://bliptext.com/articles/type-system) languages, the standard equality operator follows so-called loose typing, that is it evaluates to true even if two values are not equal and are of incompatible types, but can be coerced to each other by some set of language-specific rules, making the number 4 compare equal to the text string \"4\", for instance.\nAlthough such behaviour is typically meant to make the language easier, it can lead to surprising and difficult to predict consequences that many programmers are unaware of.\nFor example, Javascript's loose equality rules can cause equality to be intransitive (ie. and , but ), or make certain values be equal to their own negation.\nA strict equality operator is also often available in those languages, returning true only for values with identical or equivalent types (in PHP, is false although is true).\nFor languages where the number 0 may be interpreted as false, this operator may simplify things such as checking for zero (as would be true for x being either 0 or \"0\" using the type agnostic equality operator).\n\n# Ordering\nGreater than and less than comparison of non-numeric data is performed according to a sort convention (such as, for text strings, [lexicographical order](https://bliptext.com/articles/lexicographical-order)) which may be built into the programming language and/or configurable by a programmer.\nWhen it is desired to associate a numeric value with the result of a comparison between two data items, say a and b, the usual convention is to assign −1 if a b. For example, the C function performs a [three-way comparison](https://bliptext.com/articles/three-way-comparison) and returns −1, 0, or 1 according to this convention, and [qsort](https://bliptext.com/articles/qsort) expects the comparison function to return values according to this convention.\nIn [sorting algorithms](https://bliptext.com/articles/sorting-algorithm), the efficiency of comparison code is critical since it is one of the major factors contributing to sorting performance.\nComparison of programmer-defined [data types](https://bliptext.com/articles/data-type) ([data types](https://bliptext.com/articles/data-type) for which the programming language has no in-built understanding) may be carried out by custom-written or library functions (such as mentioned above), or, in some languages, by [overloading](https://bliptext.com/articles/operator-overloading) a comparison operator – that is, assigning a programmer-defined meaning that depends on the [data types](https://bliptext.com/articles/data-type) being compared.\nAnother alternative is using some convention such as member-wise comparison.\n\n# Logical equivalence\nThough perhaps unobvious at first, like the [boolean](https://bliptext.com/articles/boolean-logic) [logical operators](https://bliptext.com/articles/logical-operator) XOR, AND, OR, and NOT, relational operators can be designed to have [logical equivalence](https://bliptext.com/articles/logical-equivalence), such that they can all be defined in terms of one another.\nThe following four conditional statements all have the same logical equivalence E (either all true or all false) for any given x and y values:\nThis relies on the domain being [well ordered](https://bliptext.com/articles/well-ordered).\n\n# Standard relational operators\nThe most common numerical relational operators used in programming languages are shown below.\nStandard [SQL](https://bliptext.com/articles/sql) uses the same operators as BASIC, while many databases allow in addition to from the standard.\nSQL follows strict [boolean algebra](https://bliptext.com/articles/boolean-algebra), i.e. doesn't use [short-circuit evaluation](https://bliptext.com/articles/short-circuit-evaluation), which is common to most languages below.\nE.g. [PHP](https://bliptext.com/articles/php) has it, but otherwise it has these same two operators defined as aliases, like many SQL databases.\nOther conventions are less common: [Common Lisp](https://bliptext.com/articles/common-lisp) and [Macsyma](https://bliptext.com/articles/macsyma)/[Maxima](https://bliptext.com/articles/maxima-software) use Basic-like operators for numerical values, except for inequality, which is in [Common Lisp](https://bliptext.com/articles/common-lisp) and in [Macsyma](https://bliptext.com/articles/macsyma)/[Maxima](https://bliptext.com/articles/maxima-software).\nCommon Lisp has multiple other sets of equality and relational operators serving different purposes, including, , , , and.\nOlder [Lisps](https://bliptext.com/articles/lisp-programming-language) used, , and ; and negated them using for the remaining operators.\n\n# Syntax\nRelational operators are also used in technical literature instead of words.\nRelational operators are usually written in [infix notation](https://bliptext.com/articles/infix-notation), if supported by the programming language, which means that they appear between their operands (the two expressions being related).\nFor example, an expression in Python will print the message if the x is less than y:\nOther programming languages, such as [Lisp](https://bliptext.com/articles/lisp-programming-language), use [prefix notation](https://bliptext.com/articles/prefix-notation), as follows:\n\n## Operator chaining\nIn mathematics, it is common practice to chain relational operators, such as in 3 < x < y < 20 (meaning 3 < x and x < y and y < 20).\nThe syntax is clear since these relational operators in mathematics are transitive.\nHowever, many recent programming languages would see an expression like 3 < x < y as consisting of two left (or right-) associative operators, interpreting it as something like.\nIf we say that x=4, we then get, and evaluation will give which generally does not make sense.\nHowever, it does compile in C/C++ and some other languages, yielding surprising result (as true would be represented by the number 1 here).\nIt is possible to give the expression its familiar mathematical meaning, and some programming languages such as Python and [Raku](https://bliptext.com/articles/raku-programming-language) do that.\nOthers, such as C# and Java, do not, partly because it would differ from the way most other infix operators work in C-like languages.\nThe [D programming language](https://bliptext.com/articles/d-programming-language) does not do that since it maintains some compatibility with C, and \"Allowing C expressions but with subtly different semantics (albeit arguably in the right direction) would add more confusion than convenience\".\nSome languages, like [Common Lisp](https://bliptext.com/articles/common-lisp), use multiple argument predicates for this.\nIn Lisp is true when x is between 1 and 10.\n\n## Confusion with assignment operators\nEarly FORTRAN (1956–57) was bounded by heavily restricted character sets where was the only relational operator available.\nThere were no or (and certainly no or ).\nThis forced the designers to define symbols such as, , , etc. and subsequently made it tempting to use the remaining character for copying, despite the obvious incoherence with mathematical usage ( should be impossible).\nInternational Algebraic Language (IAL, [[ALGOL](https://bliptext.com/articles/algol) 58](https://bliptext.com/articles/algol-58)) and [ALGOL](https://bliptext.com/articles/algol) (1958 and 1960) thus introduced for assignment, leaving the standard available for equality, a convention followed by [CPL](https://bliptext.com/articles/cpl-programming-language), [ALGOL](https://bliptext.com/articles/algol) W, [ALGOL](https://bliptext.com/articles/algol) 68, Basic Combined Programming Language (B[CPL](https://bliptext.com/articles/cpl-programming-language)), [Simula](https://bliptext.com/articles/simula), SET Language ([SETL](https://bliptext.com/articles/setl)), [Pascal](https://bliptext.com/articles/pascal-programming-language), [Smalltalk](https://bliptext.com/articles/smalltalk), [Modula-2](https://bliptext.com/articles/modula-2), [Ada](https://bliptext.com/articles/ada-programming-language), [Standard ML](https://bliptext.com/articles/standard-ml), [OCaml](https://bliptext.com/articles/ocaml), [Eiffel](https://bliptext.com/articles/eiffel-programming-language), Object [Pascal](https://bliptext.com/articles/pascal-programming-language) ([Delphi](https://bliptext.com/articles/delphi-programming-language)), [Oberon](https://bliptext.com/articles/oberon-programming-language), [Dylan](https://bliptext.com/articles/dylan-programming-language), VHSIC Hardware Description Language ([VHDL](https://bliptext.com/articles/vhdl)), and several other languages.\n\n### B and C\nThis uniform de facto standard among most programming languages was eventually changed, indirectly, by a minimalist compiled language named [B](https://bliptext.com/articles/b-programming-language).\nIts sole intended application was as a vehicle for a first port of (a then very primitive) [Unix](https://bliptext.com/articles/unix), but it also evolved into the very influential [C](https://bliptext.com/articles/c-programming-language) language.\nB started off as a syntactically changed variant of the systems programming language [B[CPL](https://bliptext.com/articles/cpl-programming-language)](https://bliptext.com/articles/bcpl), a simplified (and typeless) version of [CPL](https://bliptext.com/articles/cpl-programming-language).\nIn what has been described as a \"strip-down\" process, the and operators of BCPL were replaced with and (which would later become and , respectively. ). In the same process, the ALGOL style of BCPL was replaced by in B. The reason for all this being unknown.\nAs variable updates had no special syntax in B (such as or similar) and were allowed in expressions, this non standard meaning of the equal sign meant that the traditional semantics of the equal sign now had to be associated with another symbol.\n[Ken Thompson](https://bliptext.com/articles/ken-thompson) used the ad hoc combination for this.\nAs a small type system was later introduced, B then became C. The popularity of this language along with its association with Unix, led to Java, C#, and many other languages following suit, syntactically, despite this needless conflict with the mathematical meaning of the equal sign.\n\n### Languages\nAssignments in C have a [value](https://bliptext.com/articles/value-programming) and since any non-zero scalar [value](https://bliptext.com/articles/value-programming) is interpreted as true in [conditional expressions](https://bliptext.com/articles/conditional-programming), the code is legal, but has a very different meaning from.\nThe former code fragment means \"assign y to x, and if the new value of x is not zero, execute the following statement\".\nThe latter fragment means \"[if and only if](https://bliptext.com/articles/if-and-only-if) x is equal to y, execute the following statement\".\nThough [Java](https://bliptext.com/articles/java-programming-language) and [C#](https://bliptext.com/articles/c-sharp-programming-language) have the same operators as C, this mistake usually causes a compile error in these languages instead, because the if-condition must be of type, and there is no implicit way to convert from other types (e.g., numbers) into s. So unless the variable that is assigned to has type (or wrapper type ), there will be a compile error.\nIn ALGOL-like languages such as Pascal, Delphi, and Ada (in the sense that they allow [nested function definitions](https://bliptext.com/articles/nested-function-definition)), and in [Python](https://bliptext.com/articles/python-programming-language), and many functional languages, among others, assignment operators cannot appear in an [expression](https://bliptext.com/articles/expression-programming) (including clauses), thus precluding this class of error.\nSome compilers, such as [GNU Compiler Collection](https://bliptext.com/articles/gnu-compiler-collection) (GCC), provide a warning when compiling code containing an assignment operator inside an if statement, though there are some legitimate uses of an assignment inside an if-condition.\nIn such cases, the assignment must be wrapped in an extra pair of parentheses explicitly, to avoid the warning.\nSimilarly, some languages, such as [BASIC](https://bliptext.com/articles/basic) use just the symbol for both assignment and equality, as they are syntactically separate (as with Pascal, Ada, Python, etc., assignment operators cannot appear in expressions).\nSome programmers get in the habit of writing comparisons against a constant in the reverse of the usual order:\nIf is used accidentally, the resulting code is invalid because 2 is not a variable.\nThe compiler will generate an error message, on which the proper operator can be substituted.\nThis coding style is termed left-hand comparison, or [Yoda conditions](https://bliptext.com/articles/yoda-conditions).\nThis table lists the different mechanisms to test for these two types of equality in various languages:\nRuby uses to mean \"b is a member of the set a\", though the details of what it means to be a member vary considerably depending on the data types involved.\nis here known as the \"case equality\" or \"case subsumption\" operator.\n\n# Notes and references","revisionCount":0},"hasMore":false,"nextCursor":null}

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