Difference between a function and procedure?

I had a doubt I know that main difference between a function and procedure is The function compulsory returns a value where as a procedure may or may not returns value. But when we use a function of type void it returns nothing. Can u people please clarify my dou开发者_如何学JAVAbt.

Traditionally, a procedure returning a value has been called a function (see below), however, many modern languages dispense with the term procedure altogether, preferring to use the term function for all named code blocks.

Read more at Suite101: Procedure, subroutine or function?: Programming terminology 101 - a look at the differences in approach and definition of procedures, subroutines and functions. http://www.suite101.com/content/procedure--subroutine-or-function--a8208#ixzz1GqkE7HjE

In C and its derivatives, the term "procedure" is rarely used. C has functions some of which return a value and some of which don't. I think this is an artefact of C's heritage where before the introduction of void in ANSI C, there was no way to not return a value. By default functions returned an int which you could ignore (can still) and might be some random number if no explicit return value was specified.

In the Pascal language family, the difference is explicit, functions return a value and procedures don't. A different keyword is used in each case for the definition. Visual Basic also differentiates with functions and subroutines(?).

Since we are talking about Objective-C, there are some further issues to confuse you. Functions associated with a class or object are known as "methods" (class methods and instance methods respectively).

Also, if we are being pedantic, you don't call Objective-C methods, you invoke them by sending a message to the object. The distinction is actually quite important because the message name (aka "selector") does not necessarily always refer to the same method, it can be changed at run time. This is fundamentally different to languages like Java and C++ where a particular method name for a particular class is really just a symbolic name for the address of the block of code constituting the body of the method.

Depending on the programming language, the distinction may be not so clear. Let's take a conservative language, Pascal:

• procedure indeed has no return value. It is used for operations which do not have a return value, or have multiple return values. In the latter case, multiple arguments (the return-arguments or output-arguments) are passed by reference (using the var keyword) and their values are directly modified from inside the procedure. (Note that this latter case may not be considered good practice, depending on the circumstances).
• function has a single return value, and usually we do not expect it to change the value of any of its arguments (which arguments may then be passed by value, or via the const keyword). Multiple return values may be returned by bundling them into a record.

C or Java does not distinguish syntactically, so a function of return type void can be thought of as a procedure. Scala distinguished between them by the presence of an equals sign between the method head and method body.

Generally, no matter how an actual language calls its construct, we would ideally expect that

• A function takes arguments, doesn't modify any state (like mutating arguments, global variables, or printing info for the user to the console), and returns the result of computation.
• A procedure takes arguments, performs operations which can have side-effects (writing to a database, printing to the console, maybe mutating variables), but hopefully doesn't mutate any arguments.

In practice however, depending on the situation, blends of these expectations can be observed. Sticking to these guidelines helps I think.