Program Explanation

• Observe the first statement, which is

  implicit none
  

  This statement enforces type checking. Otherwise implicit types are assumed, i.e., undefined variables whose names begin with i, j, k, l, m, and n are assumed integers, whereas all the other undefined variables are assumed real. We don't want that, because it can easily lead to programming errors.
• The next step is to define a new derived Fortran type, which we're going to call node. A data of type node will be a node in a linked list. In this case our derived type node has 4 components, of which 3 are reals and one is a pointer that will point to the next occurrence, somewhere in the memory of the computer system, of a data of this type - thus forming a list of elements linked with pointers.
• Observe that the definition of type node is recursive - this is always the case with linked lists and trees.
• We then define two pointers that will eventually point to objects of type node:

    type(node), pointer :: list, first
  

  We must always keep the address of the first element of a linked list stored safely somewhere, because if we lose that, the whole data set will become dereferenced, i.e., lost.
• There is nothing new about the remaining definitions, so we can proceed directly to the first executable statement.
• The program opens a data file, whose name, in this case, is hardwired.
• Assuming that the file has been successfully opened for reading (also observe the required file status 'old': this means that the file must already exist - it is possible to open a new file for reading, although it is hard to tell what one can read from such a file) we allocate the first object of type node and set its pointer component to point nowhere with the nullify statement:

    allocate(list); nullify(list%next)
  

• At the same time we make first point to the same object:

    first => list; count = 0
  

  Observe the new symbol =>, which means that first has been assigned an address of object that is now referenced by list. Once we have allocated list, list is no longer a pointer. It is now an object, which may become a target for another pointer, in this case, first.
• In Fortran pointers once they have been allocated they cease to be pointers and become normal data items instead. So they're really polymorphic creatures, which at various times are different things. Because an allocated pointer is no longer a pointer, you cannot do a thing such as:

    first = list
  

  Instead you have to do:

    first => list
  

  The meaning of this operation is

  a pointer points to ( $\Rightarrow$) an object

• Now we enter a loop. This is a non-terminated loop that loops forever until something special happens inside that gets us out of there.
• What happens inside is that we read lines of input. Every line is supposed to have 3 floating point numbers in it, which we read on

  list%x, list%y, list%sigma
  

  the three floating point number slots of the object that is currently referred to by the name list.
• When the end of file is reached, the end=100 condition fires up and the program jumps to label 100. There is no other portable way to process end of file condition in Fortran.
• Once the record has been read we increment the counter and allocate space for the next node of the list. That object is now placed in list%next so that we can locate it by marching along the list. At the same time we redirect the list pointer itself so that now it points to list%next, and within this newly created object we nullify the pointer slot. We need to do that, so that when the list ends we have some means of ascertaining that.
• Having read all the data from the file, we close it, and, since now we finally know how much data we have, we allocate arrays for it.

       allocate(x(count)); allocate(y(count)); allocate(sigma(count));
  

• Next we redirect list to point back at the first object, and march along the list. This time hitting a pointer that is no associated with any object terminates the march. That's why we've been always careful to nullify the pointer in a newly allocated object.
• As we march through the list, we write the records on standard output, and, at the same time copy the data from the list to appropriate slots in the arrays x_i, y_i, and $\sigma _i$.
• The last loop simply writes the arrays on standard output.
• Like in C and in C++ there is no garbage collection in Fortran. If you dereference accidentally or on purpose the beginning of the list, you'll lose access to the data, but you will not be able to reclaim that space either. To reclaim it, you must walk backwards through the list deallocating every node. For that to work the nodes should have a pair of pointers, pointing in both directions of the list, for every node.