Is C an imperative or declarative programming language

Question

It is quite confusing to know difference between Imperative and Declarative programming can any one explain difference between both in real world terms?

Kindly clarify whether C is an Imperative or Declarative Language?

Answer 1

Caveat

I am writing with a lot of generalities, so please bear with me.

In Theory

C is imperative, because code reads like a recipe for how to do something. However, if you use a lot of well-named functions and function pointers for polymorphism, it's possible to make C code look like a declarative language.

In imperative languages, you are focused on the algorithm/implementation. Engineering is inherently imperative, because you are focused on efficiency of a process: the cost of doing something in terms of time or money (or memory in CS) required.

In contrast, Mathematics is generally declarative (but writing a proof tends to be more imperative). In math, you care more about correctness and defining invariant relationships/operations, as opposed to how quickly you can get the answer.

Note that many functional languages tend to be declarative in nature (eg R, Lisp).

What does z = x + y mean? (Semantics)

In an imperative language, it means read from memory locations x and y, add those values together, and put the result into memory location z, and do it right now. If you assign a different value to x, you will have to use the z = x + y statement again to recalculate z.

In a declarative (lazy) language, it means z is a variable whose value is the sum of the values of two other variables x and y. The addition operation isn't executed until you try to read the value of z. What's the implication? If you read from z, the value will always be the sum of x and y at that moment in time; you do not need to reissue the statement. In pure declarative languages where there are no variables, a reissue can actually be caught as an error!!!

Keep this example in mind and you will see why mathematicians tend to prefer declarative languages. For example, I can define hypotenuse = sqrt( height^2 + length^2 ) and never worry about having to reissue that statement. The relationship is an invariant that will always hold, just like a mathematical truth always holds.

In Real Life (and why should I care?)

Proponents of declarative languages claim: an efficient solution that is wrong (buggy) is useless. They want bug-free, state-less functions without side-effects that can be reused without modification.

Proponents of imperative languages claim: a correct solution that takes forever to run is also useless. They want control over the memory/speed tradeoff. They want to be able to optimised based on physical and time constraints.

Of course, nothing is 100% imperative or declarative. Imperative code that is correct and well-written implies certain relationships. OTOH, declarative code, in sufficient depth and in conjunction with the language specifications, describes those relationships well enough for the compiler/interpreter to turn your code into a series of CPU instructions.

Because we are dealing with computers, a declarative compiler/interpreter must be smart enough to make time vs memory tradeoffs, whereas in an imperative language, it is up to the programmer to make those decisions more explicitly.

So a declarative language requires that the programmer focus on defining relationships between variables and other invariants. It is up to the compiler/interpreter to turn those relationships into a series of instructions/operations for the CPU. Most declarative compilers/interpreters are smart enough to handle most real-world cases, but may have trouble with edge cases. Unfortunately, in those situations you will have to coax the compiler/interpreter.

Which one is better?

Proponents of declarative languages claim that such languages allow the programmers to focus on the domain and to write code that reads easier for non-programmers. It is easier to write correct code, claim the advocates. However, the trade-off is, coaxing the compiler/interpreter to make the correct memory vs speed tradeoff can require some intricate knowledge of the language. You will understand this problem if you use a declarative language like R or SQL or LISP. It is certainly possible to define a new declarative language which has nothing to do with computers (but doing so may make it harder for the writer of the interpreter/compiler). Many mathematicians and pure CS researchers like declarative languages.

Imperative languages tend to give you finer grained control over the machine. There is no question that you are programming a computer. The trap is, we can end up pre-maturely focusing on unnecessary speed optimisations that hurt code maintenance and readability. In the early days of computing where speed or memory were severely limited, you needed to have imperative languages to get useful work done, optimised correctly for your situation. Engineers and tinkerers tend to gravitate towards imperative languages.

Answer 2

C is an imperative programming language.

A one line difference between the two would be Declarative programming is when you say what you want, and imperative language is when you say how to get what you want. In Declarative programming the focus is on what the computer should do rather than how it should do it (ex. SQL) whereas in the Imperative programming the focus is on what steps the computer should take rather than what the computer will do (ex. C, C++, Java).

Imperative programming is a programming paradigm that describes computation in terms of statements that change a program state

Declarative programming is a programming paradigm, a style of building the structure and elements of computer programs, that expresses the logic of a computation without describing its control flow

Many imperative programming languages (such as Fortran, BASIC and C) are abstractions of assembly language.

The wiki says:-

As an imperative language, C uses statements to specify actions. The most common statement is an expression statement, consisting of an expression to be evaluated, followed by a semicolon; as a side effect of the evaluation, functions may be called and variables may be assigned new values. To modify the normal sequential execution of statements, C provides several control-flow statements identified by reserved keywords. Structured programming is supported by if(-else) conditional execution and by do-while, while, and for iterative execution (looping). The for statement has separate initialization, testing, and reinitialization expressions, any or all of which can be omitted. break and continue can be used to leave the innermost enclosing loop statement or skip to its reinitialization. There is also a non-structured goto statement which branches directly to the designated label within the function. switch selects a case to be executed based on the value of an integer expression.