# Fortran/Fortran procedures and functions
## Functions and Subroutines

In most programs, a block of code is often re-used at several places. In order to minimize duplicating code and facilitate maintaining the code, such blocks of code should be placed within a function or subroutine. A Fortran function is similar to a mathematical function, which takes one or many parameters as inputs and returns a single output value. A Fortran subroutine is a block of code that performs some operation on the input variables, and as a result of calling the subroutine, the input variables are modified.
An expression containing a function call:
A call to a subroutine:
Many programming languages do not distinguish between functions and subroutines (e.g. C/C++, Python, Java). Pure functional programming languages (e.g. Haskell) only allow functions, because subroutines can, in some case, modify input variables as side-effects, which can complicate the code.
Functions are simpler than subroutines. A function must return a single value, and can be invoked from within expressions, like a write statement, inside an if declaration if (function) then, etc. A subroutine does not return a value, but can return many values via its arguments and can only be used as a stand-alone command (using the keyword call).

## Function

In Fortran, one can use a function to return a value or an array of values. The following program calls a function to compute the sum of the square and the cube of an integer.
The intent (in) attribute of argument i means that i cannot be changed inside the function and in contrast, the return value j has automatic intent (out). Note that the return type of func needs to be declared. If this is omitted, some compilers will not compile. Open64 will compile the resulting code with warning, but the behavior is ill-defined.
An alternative formulation (F77 compatible) is
The return type of the func_name still needs to be declared, as above. The only difference is how the return type of func_name is referenced within func_name. In this case, the return variable has the same name as the function itself.

### Recursion

Recursive functions can be declared , in a way such as the one shown below, in order for the code to compile.

## Subroutine

A subroutine can be used to return several values through its arguments. It is invoked with a call statement. Here is an example.

## Intent

When declaring variables inside functions and subroutines that need to be passed in or out, intent may be added to the declaration. The default is no intent checking - which can allow erroneous coding to be undetected by the compiler.
intent (in) - the value of the dummy argument may be used, but not modified, within the procedure.
intent (out)- the dummy argument may be set and then modified within the procedure, and the values returned to the caller.
intent (inout) - initial values of the dummy argument may be both used and modified within the procedure, and then returned to the caller.

## More on Functions vs. Subroutines
### Different function result definitions

Functions can define the data type of their result in different forms: either as a separate variable or by the function name.
See the examples below

### External

Procedures must be included by module use or by specifying them as external procedures. external supplies only an implicit interface which is inferior as the compiler doesn't know the number of arguments and neither their data types. Thus, it cannot yield warnings at compile time (in contrast to an explicit interface given from a module use, c.f. Fortran/OOP in Fortran).

### Pure procedures

Both functions and subroutines can modify their input variables. By necessity, subroutines modify input variables, since they do not return any output value. Functions do not have to, but are allowed, by default, to modify input variables. A function can be turned into a pure function, which does not have any side-effects through the use of the intent attribute on all input variables, and further enforced through the keyword pure. The pure keyword imposes additional restrictions, which essentially prevents the function from having any side-effects.
An example of a pure function.

### Keyword arguments

One can use any order of the input arguments if one specifies them by their dummy name. That is possible as long as the calling procedure has an interface block of the intended procedure (which is automatically created if one includes the function by module usage uses modules).
There is also a hybrid method where one specifies some parameters by position and the rest by their dummy name.
An example is given

### Optional arguments

Arguments can be set optional. The intrinsic function present can be used to check if a specific parameter is set.
An example is given below.

### Interface block

If a procedure has another procedure as dummy argument then one has to specify its type, just as the type of other parameters. An interface block is used for this case. It consists of the procedure statement with the definitions of its arguments.
Note, that each interface block has its own scope. Thus, if one needs to access outside values one needs to explicitly load them. This can be achieved by the import, or use statements.
An example is given below.

### Save attribute

The value of a variable can be saved in-between procedure calls by explicitly giving the save attribute.
An example is given below.

### Generic

It is possible to create generic functions with the same name for different input arguments, similar to the abs function which works for integer, real, and complex data types.
The following example illustrates how to create a function add which adds either two integers or character strings.

### Deferred

One can set type-bound procedures of an abstract type as deferred such that it needs to be reimplemented in derived types. For more information see the section on abstract types.

## Elemental

One can create procedures that operate parameters of arbitrary dimension. The keyword elemental is used where one defines the operation on a single object (e.g. integer) and the general case is automatically handled.
An example for the addition of arbitrary long integer dimension is given.