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Simple question, but I often hear these three terms defined with such ferocity, but which have been known to me to mean different things over the years.

What are the "correct" definitions of "Procedures", "Methods", "Function", "Subroutines", etc?

serv-inc
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Django Reinhardt
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6 Answers6

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I'm going with a different answer here: practically speaking, there's really no difference, with the slight exception that "method" usually refers to a subroutine associated with an object in OO languages.

The terms "procedure, function, subroutine, subprogram, and method" all really mean the same thing: a callable sub-program within a larger program. But it's difficult to come up with a definition that captures all variant usages of these terms, because they are not used consistently across programming languages or paradigms.

You might say a function returns a value. Well, the following C function doesn't return a value:

void f() { return; }

...but I doubt you'd find anyone who would call it a procedure.

Sure, in Pascal, procedures don't return values and functions return values, but that's merely a reflection of how Pascal was designed. In Fortran, a function returns a value, and a subroutine returns multiple values. Yet none of this really allows us to come up with a "universal" definition for these terms.

In fact, the term "procedural programming" refers to a whole class of languages, including C, Fortran and Pascal, only one of which actually uses the term "procedure" to mean anything.

So none of this is really consistent. The only exception is probably "method", which seems to be used almost entirely with OO languages, referring to a function that is associated with an object. Although, even this is not always consistent. C++, for example, usually uses the term "member function" rather than method, (even though the term "method" has crept into the C++ vernacular among programmers.)

The point is, none of this is really consistent. It simply reflects the terminology employed by whatever languages are en vogue at the time.

Charles Salvia
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  • That's precisely what I thought the answer was. (I should have added "subroutine" as another variant in hindsight.) Can I ask: Why wouldn't you find anyone who call that C function a "Procedure"? Because it's technically incorrect, or because the term "procedure" is currently out of vogue? – Django Reinhardt Nov 23 '10 at 19:42
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    C programmers use the term "function" simply because the designers of C used that term. – Charles Salvia Nov 23 '10 at 19:54
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    Let's not throw the baby out with the bathwater. Just because the terminology is not used with full consistency, doesn't mean that the different terms don't have different meanings. @Bruce's and @Frank's definitions are widely recognized, not idiosyncratic. The fact that the meanings are not universal is important, but it doesn't justify the leap to "practically speaking, there's really no difference". (@Django) – LarsH Nov 23 '10 at 23:33
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    Kind of dual to C++, which calls methods "member functions", Java and C# call functions "static methods". – Jörg W Mittag Nov 24 '10 at 01:28
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    Bruce's answer is definitely the one you should go for if you're new to programming. His definitions will be absolutely correct, 99% of the time. But I was looking for more of a technical/theoretical answer. Sometimes newer programmers know only their own domain, and insist that is all there is. In reality there are programmers working today who still use older languages, and who are not "wrong" for using different definitions. That was what I was most interested in. – Django Reinhardt Apr 12 '11 at 18:37
  • Actually the term `procedure` has a strict defined meaning in Fortran. It means a `function` or a `subroutine` (for example, there are `procedure pointers`). Also subroutine does not have to return anything, or more accurately it does not return anything. It may, but doesn't have to, alter its arguments. – Vladimir F Героям слава Dec 03 '12 at 15:13
  • You don't know _me_. I might just put that into production ;) – Cole Tobin May 08 '14 at 23:54
  • I would call it a procedure. – Don Larynx Aug 21 '15 at 20:46
  • And the slight exception that (pure) functions don't use external state as input. – AturSams May 07 '17 at 13:01
  • A method has an implicit `this` parameter that holds a reference to the object instance the method is associated with. That's the key distinguishing factor between a method and the function/procedure. – Berin Loritsch Feb 06 '18 at 17:37
  • nice answer, what is missing imho is that within each language those terms usually do have a quite clear definition (just across languages those are not necesarily consistent) and sometimes there really is a difference made between the terms – 463035818_is_not_a_number Oct 19 '18 at 12:09
  • In Ruby, Smalltalk etc. other Pure OO languages, there's no function except module functions in Ruby. They are not the same thing. – 15 Volts Sep 07 '19 at 16:56
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A function returns a value, but a procedure does not.

A method is similar to a function, but is internal to part of a class. The term method is used almost exclusively in object-oriented programming.

Bruce Alderman
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    Not exactly internal. A method is any function or procedure that is part of a class. – Scott Whitlock Nov 23 '10 at 17:48
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    So a "Stored Procedure" in SQL doesn't return any values? What about a "Procedure" in something like Pascal? Are your definitions based on current trends or should they be considered universal definitions? Thanks! – Django Reinhardt Nov 23 '10 at 19:10
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    @Django: In Pascal a procedure *cannot* have a return value, and a function *must* have a return value. In some other languages, the terminology may be used more loosely. – Bruce Alderman Nov 23 '10 at 20:04
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    FWIW, FORTRAN had SUBROUTINEs and FUNCTIONs from the early days, the difference being that a SUBROUTINE didn't return a value. I don't remember about ALGOL, which Pascal is descended from. – David Thornley Nov 23 '10 at 22:42
  • This is definitely the correct answer, in the current, general usage of those terms. Thanks. – Django Reinhardt Apr 12 '11 at 18:31
  • Procedures in PureBasic do return a value. http://www.purebasic.com/documentation/reference/procedures.html – Gary Willoughby Apr 12 '11 at 20:39
  • Procedures in Fortran are both functions and subroutines, they may return a value. – Vladimir F Героям слава Dec 03 '12 at 15:15
  • @DjangoReinhardt `So a "Stored Procedure" in SQL doesn't return any values?` - Depends on how technical you want to get with "return". You typically either read from a cursor to get data, use an "out" parameter (note Frank's answer), or store the result in another table. Not exactly "returning" a value, but a result is retrieved in other ways... – Izkata Jul 06 '14 at 22:07
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    Ada uses the same terminology as Pascal - therefore any argument that trumps this answer would require at least three languages using the same terminology. Given how unlikely this is, I think we can call it a win. – mattnz Jul 07 '14 at 01:38
  • Functions need not return values in JavaScript. – Kelmikra Oct 20 '15 at 04:05
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    @3p1c_d3m0n It's certainly true that `function` fulfills both roles in JS, but JS functions do all return. When a return statement has no value, the value is implicitly `undefined`. When a return statement is absent, the interpreter adds an implicit return statement. Esoteric, maybe, but it is consistent with the definition given here. This is why `var x = function() {}();` is legal in JS; if not for implicit returns, this would need to be an error, as it would be in Pascal. – Semicolon Nov 08 '15 at 05:25
  • Spoken like someone who speaks SQL – cwallenpoole Jun 14 '17 at 16:18
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A function is something that takes a bunch of inputs and returns one or more values. If the returned values are entirely determined by the inputs, and the function doesn't have any side effects (logging, perhaps, or causing state changes outside itself), then it's called a pure function.

A procedure is a function that doesn't return a value. In particular, this means that a procedure can only cause side effects. (That might include mutating an input parameter!)

A method is a function that closes over a set of variables, that is, a closure. It takes zero or more input parameters, has access to this set of variables, and returns zero or more values. In OO languages these methods are attached to objects or classes.

In most mainstream OO languages, those closed-over variables are called the member fields, or instance variables, of an object. A method can be a pure function, an impure function or a procedure.

The latter definition leads to the object = struct + closures correspondence.

Frank Shearar
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    So an object is a collection of variables and a collection of closures over these common variables. Basically, object-oriented languages have always had closures and nobody knew? Interesting view! +1 – Giorgio Jan 04 '13 at 23:45
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    I do not believe most methods close over anything. `foo.doSomething()` is not parameterless. It has one parameter (the object `foo`) with given with some syntactic sugar. A closure would be able to reference its object without needing such a parameter. That's not to say methods _can't_ be closures, just that most are not, and that being OO is not sufficient for a language to support closures. – 8bittree Aug 14 '14 at 20:43
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    `foo.doSomething()` closes over the `foo` variable. Any statement in `doSomething` can access `foo` through `this` or `self`, depending on your language. This is the very definition of "close over". Classes close over their member variables, therefore (ignoring "what is OO"), OO is sufficient. This is pretty well-known in the literature... – Frank Shearar Aug 15 '14 at 08:47
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    Er, no. See that little `foo.` at the front of `foo.doSomething()`? That's you passing `doSomething()` a parameter. Just because it's not in between the parenthesis does not mean it's not a parameter. The `this` or `self` inside the method is simply syntactic sugar for referencing that parameter. – 8bittree Jun 09 '16 at 16:54
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    @FrankShearar, I would say there are only two reasonably clear distinctions in computing - data, and instructions. Even that goes out the window in the (unusual) case of self-modifying code. In OO, what is generally referred to as a "method" (or a "member function") need not be either a method or function by your definition - all these words in general use are effectively synonyms with a general and interchangeable meaning. The "impure function" whose existence you allude to, is simply another (and also most long-winded and jargonistic) synonym for the same general concept of "instructions". – Steve Feb 06 '18 at 17:48
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    @Steve I tried to use standard terminology. I feel that it is important to be precise in my speech. These things are not the same, even though they may be _mostly_ the same. "Pure" and "impure" have very precise meanings. Lastly, there is only data. Data is code. Code is data. There is only code. (Look at the Lisp languages' macro/metaprogramming facilities.) – Frank Shearar Feb 07 '18 at 18:57
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    @FrankShearar, that's my point though, there is no standard terminology - just context-specific, specialised terminology (which is often just another set of more jargonistic synonyms). As you say and as I already said, even the distinction between code and data breaks down (because in Von Neumann architectures there is no distinction at the hardware level). And I'm not trying to argue that these words (function, procedure, method) are mostly the same - I'm saying these words (along with a few others, like "subroutine" mentioned by the OP) basically *are* synonyms in computing... – Steve Feb 07 '18 at 19:39
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    ...I singled out "impure function" for ridicule because it cannot necessarily be distinguished from a "procedure" or "method" by your definition. Nor can a "method" and "procedure" be clearly distinguished, since a method attached to a class may use none of its instance variables, or the class need not have any instance variables, but it would still be called a "method" in OO languages (and it's container would still be called a class). There are no *generally accepted* clear distinctions between these words. – Steve Feb 07 '18 at 19:40
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Bruce has a good answer. I would add, semantically:

  • A procedure should "do something" to the arguments or cause some other side effect (e.g. printf)
  • A function should (a) answer a question about the arguments, or (b) compute a new value based on the arguments
  • A function method should answer a question about the state of the object
  • A procedure method should change the state of the object
Community
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Scott Whitlock
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  • Great answer! Just one tiny addition: `A procedure should "do something" to the arguments` - or cause some other side effect (e.g. `printf`). – Allon Guralnek Nov 23 '10 at 18:05
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    @Allon Note that `printf` returns a value - the number of characters printed - so it is technically a function. – Sjoerd Apr 12 '11 at 20:09
  • @Sjoerd I don't agree that `printf` is a value. It had a specific side effect outside its invocation scope: namely I/O to whatever the standard output its supposed to be. Even though, Scott was not explicit this distinction, in functional programming functions are not supposed to have side effects, and should be able to answer questions as if you had the actual data it returns. – Alan Aug 14 '14 at 17:17
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good detailed answers above; the short story is that they'll all flavors of subroutines; what is meant by each term will vary according to the programming language context

in general, functions return a value, but they don't have to

methods are a generic OOP terms at present

in SQL, stored procedures have outputs but typically only return an error code, while user-defined functions must return a value (which may be a result-set)

again, the precise difference between these terms depends on who you're talking to!

Steven A. Lowe
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2

80% of proficiency is directly related to familiarity with nomenclature,

95% of productivity is the ability to identify what is useful at the moment despite the terms used to describe it

I pretty much prefer to call them all methods in c# except back when I used MSSQL we had sproc's, but of course now we use Postgres and they are called functions.

MvcCmsJon
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    83.5% of all statistics are made up on the spot ;-P – Django Reinhardt Jan 05 '13 at 05:22
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    I have to admit, I get nervous when I hear someone throwing around the term "method" when working in a non-OO language. It seems to be heavily correlated with running into non-idiomatic code. – Racheet Jan 17 '14 at 15:54
  • I use 'Method' when referring to C code because many OO programmers I deal with have a mental breakdown on hearing the terms procedure or function. For fun, if I really want to mess with them, I might interchange the terms randomly. Its not good, kind of like inviting a poltergeist into you home... :) – mattnz Jul 07 '14 at 01:46