What is the difference between declarative and imperative paradigm in programming

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I have been searching the web looking for a definition for declarative and imperative programming that would shed some light for me  However  the language used at some of the resources that I have found is daunting - for instance at Wikipedia  Does anyone have a real-world example that they could show me that might bring some perspective to this subject  perhaps in C

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I ll add another example that rarely pops up in declarative imperative programming discussion  the User Interface   In C   you can build an UI using various technologies   On the imperative end  you could use DirectX or OpenGL to very imperatively draw your buttons  checkboxes  etc    line-by-line  or really  triangle by triangle    It is up to you to say how to draw the user interface   At the declarative end  you have WPF   You basically write some XML  yeah  yeah   XAML  technically  and the framework does the work for you   You say what the user interface looks like   It is up to the system to figure out how to do it   Anyway  just another thing to think about   Just because one language is declarative or imperative does not mean that it doesn t have certain features of the other   Also  one benefit of declarative programming is that purpose is usually more easily understood from reading the code whereas imperative gives you finer control over execution   The gist of it all   Declarative -  what you want done  Imperative -  how you want it done

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Imperative programming is telling the computer explicitly what to do  and how to do it  like specifying order and such  C    for  int i   0  i  lt  10  i          System Console WriteLine  Hello World         Declarative is when you tell the computer what to do  but not really how to do it  Datalog   Prolog is the first language that comes to mind in this regard  Basically everything is declarative  You can t really guarantee order   C  is a much more imperative programming language  but certain C  features are more declarative  like Linq  dynamic foo   from c in someCollection            let x   someValue   2            where c SomeProperty  lt  x            select new  c SomeProperty  c OtherProperty     The same thing could be written imperatively   dynamic foo   SomeCollection Where                  c   gt  c SomeProperty  lt   SomeValue   2               Select                  c   gt  new  c SomeProperty  c OtherProperty            example from wikipedia Linq

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From my understanding  both terms have roots in philosophy  there are declarative and imperative kinds of knowledge  Declarative knowledge are assertions of truth  statements of fact like math axioms  It tells you something  Imperative  or procedural knowledge  tells you step by step how to arrive at something  That s what the definition of an algorithm essentially is  If you would  compare a computer programming language with the English language  Declarative sentences state something  A boring example  but here s a declarative way of displaying whether two numbers are equal to each other  in Java   public static void main String   args        System out print  4   4         Imperative sentences in English  on the other hand  give a command or make some sort of request  Imperative programming  then  is just a list of commands  do this  do that   Here s an imperative way of displaying whether two numbers are equal to each other or not while accepting user input  in Java   private static Scanner input       public static void main String   args         input   new Scanner System in       System out println        System out print  Enter an integer value for x          int x   input nextInt        System out print  Enter an integer value for y                  int y   input nextInt         System out println        System out printf   d     d   s n   x  y  x    y       Essentially  declarative knowledge skips over certain elements to form a layer of abstraction over those elements  Declarative programming does the same

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Stealing from Philip Roberts here    Imperative programming tells the machine how to do something  resulting in what you want to happen  Declarative programming tells the machine what you would like to happen  and the computer figures out how to do it    Two examples   1  Doubling all numbers in an array  Imperatively   var numbers    1 2 3 4 5  var doubled       for var i   0  i  lt  numbers length  i        var newNumber   numbers i    2   doubled push newNumber    console log doubled      gt   2 4 6 8 10    Declaratively   var numbers    1 2 3 4 5   var doubled   numbers map function n      return n   2    console log doubled      gt   2 4 6 8 10    2  Summing all items in a list  Imperatively  var numbers    1 2 3 4 5  var total   0  for var i   0  i  lt  numbers length  i        total    numbers i    console log total      gt  15   Declaratively  var numbers    1 2 3 4 5   var total   numbers reduce function sum  n      return sum   n     console log total      gt  15   Note how the imperative examples involve creating a new variable  mutating it  and returning that new value  i e   how to make something happen   whereas the declarative examples execute on a given input and return the new value based on the initial input  i e   what we want to happen

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Declarative programming is when you say what you want  and imperative language is when you say how to get what you want   A simple example in Python     Declarative small nums    x for x in range 20  if x  lt  5     Imperative small nums      for i in range 20       if i  lt  5          small nums append i    The first example is declarative because we do not specify any  implementation details  of building the list   To tie in a C  example  generally  using LINQ results in a declarative style  because you aren t saying how to obtain what you want  you are only saying what you want   You could say the same about SQL   One benefit of declarative programming is that it allows the compiler to make decisions that might result in better code than what you might make by hand   Running with the SQL example  if you had a query like   SELECT score FROM games WHERE id  lt  100    the SQL  compiler  can  optimize  this query because it knows that id is an indexed field -- or maybe it isn t indexed  in which case it will have to iterate over the entire data set anyway   Or maybe the SQL engine knows that this is the perfect time to utilize all 8 cores for a speedy parallel search    You  as a programmer  aren t concerned with any of those conditions  and you don t have to write your code to handle any special case in that way

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Just to add another example in terms of mobile app development  In iOS and Android  we have Interface Builders  where we can define UI of the apps   The UI drawn using these Builders is declarative in nature  where we drag and drop the components  The actual drawing happens underneath and performed by the framework and system   But we can also draw the whole components in code  and that is imperative in nature   Also  some new languages like Angular JS are focussing on designing UIs declaratively and we may see a lot of other languages offering the same support  Like Java doesn t have any good declarative way to draw native desktop apps in Java swing or Java FX but in the near future  they just might

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A great C  example of declarative vs  imperative programming is LINQ   With imperative programming  you tell the compiler what you want to happen  step by step   For example  let s start with this collection  and choose the odd numbers   List lt int gt  collection   new List lt int gt    1  2  3  4  5      With imperative programming  we d step through this  and decide what we want   List lt int gt  results   new List lt int gt     foreach var num in collection        if  num   2    0            results Add num       Here  we re saying    Create a result collection Step through each number in the collection Check the number  if it s odd  add it to the results   With declarative programming  on the other hand  you write code that describes what you want  but not necessarily how to get it  declare your desired results  but not the step-by-step    var results   collection Where  num   gt  num   2    0     Here  we re saying  Give us everything where it s odd   not  Step through the collection   Check this item  if it s odd  add it to a result collection    In many cases  code will be a mixture of both designs  too  so it s not always black-and-white

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declarative program is just a data for its some more-or-less  universal  imperative implementation vm   pluses  specifying just a data  in some hardcoded  and checked  format  is simpler and less error-prone than specifying variant of some imperative algorithm directly  some complex specifications just cant be written directly  only in some DSL form  best and freq used in DSLs data structures is sets and tables  because you not have dependencies between elements rows  and when you havent dependencies you have freedom to modify and ease of support   compare for example modules with classes - with modules you happy and with classes you have fragile base class problem  all goods of declarativeness and DSL follows immediately from benefits of that data structures  tables and sets   another plus - you can change implementation of declarative language vm  if DSL is more-or-less abstract  well designed   make parallel implementation  for example  or port it to other os etc  all good specifed modular isolating interfaces or protocols gives you such freedom and easyness of support   minuses  you guess right  generic  and parameterized by DSL  imperative algorithm vm implementation may be slower and or memory hungry than specific one  in some cases  if that cases is rare - just forget about it  let it be slow  if it s frequient - you always can extend your DSL vm for that case  somewhere slowing down all other cases  sure     P S  Frameworks is half-way between DSL and imperative  and as all halfway solutions     they combines deficiences  not benefits  they not so safe AND not so fast    look at jack-of-all-trades haskell - it s halfway between strong simple ML and flexible metaprog Prolog and    what a monster it is  you can look at Prolog as a Haskell with boolean-only functions predicates  and how simple its flexibility is against Haskell

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Declarative vs  Imperative A programming paradigm is a fundamental style of computer programming  There are four main paradigms  imperative  declarative  functional  which is considered a subset of the declarative paradigm  and object-oriented  Declarative programming   is a programming paradigm that expresses the logic of a computation What do  without describing its control flow How do   Some well-known examples of declarative domain specific languages  DSLs  include CSS  regular expressions  and a subset of SQL  SELECT queries  for example  Many markup languages such as HTML  MXML  XAML  XSLT    are often declarative  The declarative programming try to blur the distinction between a program as a set of instructions and a program as an assertion about the desired answer  Imperative programming   is a programming paradigm that describes computation in terms of statements that change a program state  The declarative programs can be dually viewed as programming commands or mathematical assertions  Functional programming   is a programming paradigm that treats computation as the evaluation of mathematical functions and avoids state and mutable data  It emphasizes the application of functions  in contrast to the imperative programming style  which emphasizes changes in state  In a pure functional language  such as Haskell  all functions are without side effects  and state changes are only represented as functions that transform the state  The following example of imperative programming in MSDN  loops through the numbers 1 through 10  and finds the even numbers  var numbersOneThroughTen   new List lt int gt    1  2  3  4  5  6  7  8  9  10      With imperative programming  we d step through this  and decide what we want  var evenNumbers   new List lt int gt     foreach  var number in numbersOneThroughTen       if  number   2    0                evenNumbers Add number             The following code uses declarative programming to accomplish the same thing     Here  we re saying  quot Give us everything where it s even quot  var evenNumbers   numbersOneThroughTen Select number   gt  number   2    0    Both examples yield the same result  and one is neither better nor worse than the other  The first example requires more code  but the code is testable  and the imperative approach gives you full control over the implementation details  In the second example  the code is arguably more readable  however  LINQ does not give you control over what happens behind the scenes  You must trust that LINQ will provide the requested result

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I liked an explanation from a Cambridge course   their examples    Declarative - specify what to do  not how to do it  E g   HTML describes what should appear on a web page  not how it should be drawn on the screen  Imperative - specify both what and how  int x   - what  declarative  x x 1   - how

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All above answers and other online posts mention following   With declarative programming  you write code that describes what you want  but not necessarily how to get it You should prefer declarative programming over the imperative programming  What they have not told us is how to achieve it  For part of the program to be more declarative  other parts must provide the abstraction to hide the implementation details  which are the imperative codes    E g   LINQ is more declarative than loops  for  while  etc    e g   you can use list Where   to get a new filtered list  For this to work  Microsoft has done all the heavy lifting behind the LINQ abstraction   In fact  one of the reason functional programming and functional libraries are more declarative is because they have abstracted away loops and list creations  hiding all the implementation details  most likely imperative codes with loops  behind the scene  In any program  you will always have both imperative and declarative codes  what you should aim for is to hide all imperative codes behind the domain-specific abstractions  so that other parts of the program can use them declaratively  Finally  although functional programming and LINQ can make your program more declarative  you can always make it even more declarative by providing more abstractions  For example     JavaScript example     Least declarative const bestProducts       for let i   0  i  lt  products length  i          let product   products i       if  product rating  gt   5  amp  amp  product price  lt  100            bestProducts push product                More declarative const bestProducts   products filter function product        return product rating  gt   5  amp  amp  product price  lt  100          Most declarative  implementation details are hidden in a function const bestProducts   getBestProducts       P S  the extreme of declarative programming is to invent new domain specific languages  DSL    String Search  Regular Expression instead of custom imperative code React js  JSX instead of direct DOM manipulation AWS CloudFormation  YAML instead of CLI Relational Database  SQL instead of older read   write APIs such as ISAM or VSAM

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In computer science  declarative programming is a programming paradigm that expresses the logic of a computation without describing its control flow    From http   en wikipedia org wiki Declarative programming  in a nutshell the declarative language is simpler because it lacks the complexity of control flow   loops  if statements  etc     A good comparison is the ASP Net  code-behind  model   You have declarative   ASPX  files and then the imperative  ASPX CS  code files   I often find that if I can do all I need in the declarative half of the script a lot more people can follow what s being done

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Imperative programming A programming language that requires programming discipline such as C C    Java  COBOL  FORTRAN  Perl and JavaScript  Programmers writing in such languages must develop a proper order of actions in order to solve the problem  based on a knowledge of data processing and programming   Declarative programming A computer language that does not require writing traditional programming logic  Users concentrate on defining the input and output rather than the program steps required in a procedural programming language such as C   or Java     Declarative programming examples are CSS  HTML  XML  XSLT  RegX

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Imperative programming requires developers to define step by step how   code should be executed  To give directions in an imperative fashion    you say     Go to 1st Street  turn left onto Main  drive two blocks    turn right onto Maple  and stop at the third house on the left     The   declarative version might sound something like this     Drive to Sue   s   house     One says how to do something  the other says what needs to be   done       The declarative style has two advantages over the imperative style          It does not force the traveler to memorize a long set of    instructions    It allows the traveler to optimize the route when possible       Calvert C Kulkarni D  2009   Essential LINQ  Addison Wesley  48

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The difference has mostly to do with the overall level of abstraction   With declarative  at some point  you re so far away from the individual steps that the program has a lot of latitude regarding how to get your result     You could look at every piece of instruction as falling somewhere on a continuum    Degree of abstraction    Declarative  lt  lt                          gt  gt  Imperative     Declarative Real World Example    Librarian  please check me out a copy of Moby Dick   Librarian  at their discretion chooses the best method for performing the request    Imperative Real World Example    Go into Library Find Book Organization System  Card Catalog - Old school  Research how to use Card Catalogs  You forgot too  right  Figure out how shelves are labeled and organized  Figure out how books are organized on a shelf  Cross-reference book location from card catalog with organization system to find said book  Take book to check-out system  Check out book

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I just wonder why no one has mentioned Attribute classes as a declarative programming tool in C   The popular answer of this page has just talked about LINQ as a declarative programming tool   According to Wikipedia      Common declarative languages include those of database query languages    e g   SQL  XQuery   regular expressions  logic programming    functional programming  and configuration management systems    So LINQ  as a functional syntax  is definitely a declarative method  but Attribute classes in C   as a configuration tool  are declarative too  Here is a good starting point to read more about it  Quick Overview of C  Attribute Programming

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