What is the difference between a function and a subroutine?

Question

What is the difference between a function and a subroutine? I was told that the difference between a function and a subroutine is as follows:

A function takes parameters

Answer 1

In terms of Visual Basic a subroutine is a set of instructions that carries out a well defined task. The instructions are placed within Sub and End Sub statements.

Functions are similar to subroutines, except that the functions return a value. Subroutines perform a task but do not report anything to the calling program. A function commonly carries out some calculations and reports the result to the caller.

Answer 2

One of the differences could be from the origin where the terminology comes from.

Subroutine is more of a computer architecture/organization terminology which means a reusable group of instructions which performs one task. It is is stored in memory once, but used as often as necessary.

Function got its origin from mathematical function where the basic idea is mapping a set of inputs to a set of permissible outputs with the property that each input is related to exactly one output.

Answer 3

Every subroutine performs some specific task. For some subroutines, that task is to compute or retrieve some data value. Subroutines of this type are called functions. We say that a function returns a value. Generally, the returned value is meant to be used somehow in the program that calls the function.

Answer 4

From the view of the user, there is no difference between a programming function and a subroutine but in theory, there definitely is!

The concept itself is different between a subroutine and a function. Formally, the OP's definition is correct. Subroutines don't take arguments or give return values by formal semantics. That's just an interpretion with conventions. And variables in subroutines are accessible in other subroutines of the same file although this can be achieved as well in C with some difficulties.

Summary:

Subroutines work only based on side-effects, in the view of the programming language you are programming with. The concept itself has no explicit arguments or return values. You have to use side effects to simulate them.

Functions are mappings of input to output value(s) in the original sense, some kind of general substitution operation. In the adopted sense of the programming world, functions are an abstraction of subroutines with information about return value and arguments, inspired by mathematical functions. The additional formal abstraction differentiates a function from a subroutine in programming context.

Details:

The subroutine originally is simply a repeatable snippet of code which you can call in between other code. It originates in Assembly or Machine language programming and designates the instruction sequence itself. In the light of this meaning, Perl also uses the term subroutine for its callable code snippets.

Subroutines are concrete objects.

This is what I understood: the concept of a (pure) function is a mathematical concept which is a special case of mathematical relations with an own formal notation. You have an input or argument and it is defined what value is represented by the function with the given argument. The original function concept is entirely unrelated to instructions or calculations. Mathematical operations (or instructions in the programming world) only are a popular formal representation (description) of the actual mapping. The original function term itself is not defined as code. Calculations do not constitute the function, so that functions actually don't have any computational overhead because they are direct mappings. Function complexity considerations only arrived as there is an overhead to find the mapping.

Functions are abstract objects.

Now, since the whole PC-stuff is running on small machine instructions, the easiest way to model (or instantiate) mathematics is with a sequence of instructions itself. Computer Science has been founded by mathematicians (noteworthy: Alan Turing) and the first programming concepts are based on it so there is a need to bring mathematics into the machine. That's how I imagine the reason why "function" is the name of something which is implemented as subroutine and why the term "pure" function was coined to differentiate the original function concept from the overly broad term-use in programming languages.

Note: in Assembly Language Programming, it is typically said, that a subroutine has been passed arguments and gives a return value. This is an interpretation on top of the concrete formal semantics. Calling conventions specify the location where values, to be considered as arguments and return values, should be written to before calling a subroutine or returning. The call itself takes only a subroutine address, and has no formal arguments or return values.

PS: functions in programming languages don't necessarily need to be a subroutine (even though programming language terminology developed this way). Functions in functional programming languages can be constant variables, arrays or hash tables. Isn't every datastructure in ECMAScript a function?

Answer 5

I disagree. If you pass a parameter by reference to a function, you would be able to modify that value outside the scope of the function. Furthermore, functions do not have to return a value. Consider void some_func() in C. So the premises in the OP are invalid.

In my mind, the difference between function and subroutine is semantic. That is to say some languages use different terminology.

Answer 6

A function returns a value whereas a subroutine does not. A function should not change the values of actual arguments whereas a subroutine could change them.

Thats my definition of them ;-)