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Top Answer
Answered Aug 29, 2015 · 1075 votes
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<int> collection = new List<int> { 1, 2, 3, 4, 5 };-
With imperative programming, we'd step through this, and decide what we want:
List<int> results = new List<int>();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 => 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.
Other Answers
Answered Nov 23, 2009 · 201 votes
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:
# Declarativesmall_nums = [x for x in range(20) if x < 5]-# Imperativesmall_nums = []for i in range(20): if i < 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 < 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.
Other Answers
Answered Sep 13, 2021 · 175 votes
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 imperative 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<int> { 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<int>();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 "Give us everything where it's even"var evenNumbers = numbersOneThroughTen.Where(number => 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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Top Answer
Answered Jan 26, 2021 · 130 votes
Your example of declarative programming above is not an actual program, so it's not a good example.
The main difference is between imperative and declarative. Functional is a particular kind of declarative.
C, C++, Java, Javascript, BASIC, Python, Ruby, and most other programming languages are imperative. As a rule, if it has explicit loops (for, while, repeat) that change variables with explicit assignment operations at each loop, then it's imperative.
SQL and XSLT are two well-known examples of declarative programming. Markup languages such as HTML and CSS are declarative too, although they are usually not powerful enough to describe arbitrary algorithms.
Here is an example computation (summing the income by gender, from a suitable data source) first written in an imperative language (Javascript) and then in a declarative language (SQL).
Imperative programming
var income_m = 0, income_f = 0;for (var i = 0; i < income_list.length; i++) { if (income_list[i].gender == 'M') income_m += income_list[i].income; else income_f += income_list[i].income;}
Notice:
- explicit initialization of variables that will contain the running totals;
- explicit loop over the data, modifying the control variable ( i) and the running totals at each iteration;
- conditionals ( ifs) are only used to choose the code path at each iteration.
Declarative programming
select gender, sum(income)from income_listgroup by gender;
Notice:
- memory cells to contain running totals are implied by the output you declare you want;
- any loop the CPU will need to perform (eg. over the income_list table) is implied by the output you declare you want and by the structure of the source data;
- conditionals (eg. case in SQL) are used in a functional way to specify the output value you want based on the input values, not to choose a code path.
Functional programming
As I mentioned above, SQL's case is a great example of the functional way of programming, which is a restricted subset of Declarative programming in which the desired computation is specified by composing functions.
Functions are things that accept inputs and return outputs (eg. case, sum()…)
Composition means chaining two or more together by specifying how the output of one is fed as the input to the next (typically by writing one inside the other.) Finally the whole composition, which is still itself a big function, is applied to the available inputs to get the desired output.
In this snippet I am declaring the output I want by composing the functions sum() and case. This is called functional programming:
select sum(case when some_flag = 'X' then some_column else some_other_column end)from ...
If the composition of two or more functions and their application to the input data are the only constructs available in a given language, that language is said to be purely functional. In those languages you will notice the complete absence of loops, variable assignment and other typically imperative statements.
Edit: I recommend watching some of Anjana Vakil's talks on functional programming in Javascript, to get a better idea of what it's about.
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Top Answer
Answered Mar 30, 2022 · 2 votes
This question seems to originate from the misunderstanding that there's no state in Functional Programming (FP). While this notion is understandable, it's not true.
In short, FP is an approach to programming that makes an explicit distinction between pure functions and everything else (often called impure actions). Simon Peyton-Jones (SPJ, one of the core Haskell developers) once gave a lecture where he said something to the effect that if you couldn't have any side effects, the only thing you could do with a pure function would be to heat the CPU, whereafter one student remarked that that would also be a side effect. (It's difficult to find the exact source of this story. I recall having seen an interview with SPJ where he related the story, but searching the web for a quote in a video is still hard in 2022.)
- Changing a pixel on the screen is a side effect.
- Sending an email is a side effect.
- Deleting a file is a side effect.
- Creating a row in a database is a side effect.
- Changing an internal variable that, via cascading consequences, causes anything like the above to happen, is a side effect.
It is impossible to write (useful) software that has no side effects.
Furthermore, pure functions also don't allow non-deterministic behaviour. That excludes even more necessary actions:
- Getting a (truly) random number is non-deterministic.
- Getting the time or date is non-deterministic.
- Reading a file is non-deterministic.
- Querying a database is non-deterministic.
- Etc.
FP acknowledges that all such impure actions need to take place. The difference in philosophy is the emphasis on pure functions. A pure function has many desirable traits (predictability, referential transparency, possible memoization, testability) that makes it worthwhile to pursue a programming philosophy that favours such functions.
A functional architecture is one that minimises the impure actions to their essentials. One label for that is Functional Core, Imperative Shell, where you push all impure actions to the edge of your system. That would, for example, include an HTML counter. Actually changing an HTML element stays imperative, while the calculation required to produce the new value can be implemented as a pure function.
Different languages have varying approaches to how explicitly they model the distinction between pure functions and impure actions. Most languages (even 'functional' languages like F# and Clojure) don't explicitly make that distinction, so it's up to the programmer to keep that separation in mind.
Haskell is one language that famously does make that distinction. It uses the IO monad to explicitly model impure actions. I've attempted to explain IO for object-oriented programmers using C# as an example language in an article: The IO Container.
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Top Answer
Answered Aug 19, 2010 · 8 votes
Declarative is the way forward.
Imperative describes the actual UI steps you have to take as a user, rather than the results you're trying to achieve. If you write your scenarios this way they'll become really brittle and impossible to maintain. Imagine if someone put a confirmation box in that imperative scenario, and there were 80 similar scenarios which also required changing.
With declarative steps you only need to change it in the one place where the step is defined; that same declarative step is then reused for all the scenarios which need it.
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People also ask
What is imperative programming?
- Imperative programming is the paradigm which is based on providing the result by focusing on the step-by-step command with manipulated state. There are many programming languages which are based on this approach of programming such as C++, python, R and Java. Control flow which exists as the major feature, facilitates the imperative programming.
What is Imperative Programming? - GeeksforGeeks
www.geeksforgeeks.org/what-is-imperative-programming/Which programming language provides the paradigm of imperative programming?
- There are multiple languages which can provide the paradigm of imperative programming. Programming languages which majorly able to implement imperative programming are Java, C, C++, C#, Ruby, Matlab, Python and R.
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www.geeksforgeeks.org/what-is-imperative-programming/What is a computer programming paradigm?
- The computer programming paradigm defines the style of programming, approach to solve problem and method of computer systems towards providing solutions use programming. There is a classification of programming paradigms into two broad paradigms i.e., imperative and declarative.
What is Imperative Programming? - GeeksforGeeks
www.geeksforgeeks.org/what-is-imperative-programming/Is functional programming the only programming style in practice?
- Of course, functional programming is not the only programming style in practice. Broadly speaking, programming styles can be categorized into imperative and declarative programming paradigms. The imperative approach defines a program as a sequence of statements that change the program’s state until it reaches the final state.
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www.baeldung.com › java-functional-programmingMay 11, 2024 · Broadly speaking, programming styles can be categorized into imperative and declarative programming paradigms. The imperative approach defines a program as a sequence of statements that change the program’s state until it reaches the final state.
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www.techgeekbuzz.com › blog › programming-paradigm5 days ago · Imperative Programming. If you have coded in C++, Python, Java, or any other high-level language, that means you have used imperative programming. In this programming approach, you may have noticed that the outcome of the program depends upon the steps. In simple words, the problem solving step by step.
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www.techgeekbuzz.com › blog › what-is-programming4 days ago · Imperative Programming Paradigm This programming paradigm has a close relation to machine architecture and is based on Von Neumann architecture. It works by leveraging the assignment statements to change a program's state.
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