Language Reference

How to Read Syntax Diagrams

Throughout this book, the syntax of Fortran statements and elements is illustrated by diagrams, using notation often called "railroad tracks".

If a variable or user-specified name ends in _list, it means that you can provide a list of these terms separated by commas.

Punctuation marks, parentheses, arithmetic operators, and other special characters must be entered as part of the syntax.

Syntax Diagrams

• Syntax diagrams are read from left to right and from top to bottom, following the path of the line:

  The >>--- symbol indicates the beginning of a statement.

  The ---> symbol indicates that the statement syntax is continued on the next line.

  The >--- symbol indicates that a statement is continued from the previous line.

  The --->< symbol indicates the end of a statement.

  Program units, procedures, constructs, interface blocks and derived-type definitions consist of several individual statements. For such items, a box encloses the syntax representation, and individual syntax diagrams show the required order for the equivalent Fortran statements.

• Required items appear on the horizontal line (the main path):

  >>-keyword--required_argument----------------------------------><
   
   
  

• Optional items appear below the main path:

  >>-keyword--+-------------------+------------------------------><
              '-optional_argument-'
   
   
  

  Note:

  Optional items (not in syntax diagrams) are enclosed by square brackets ([ and ]). For example, [UNIT=]u

• If you can choose from two or more items, they appear vertically, in a stack.

  If you must choose one of the items, one item of the stack appears on the main path:

  >>-keyword--+-required_argument-+------------------------------><
              '-required_argument-'
   
   
  

  If choosing one of the items is optional, the entire stack appears below the main path:

  >>-keyword--+-------------------+------------------------------><
              +-optional_argument-+
              '-optional_argument-'
   
   
  

• An arrow returning to the left above the main line (a repeat arrow) indicates an item that can be repeated, and the separator character if it is other than a blank:

              .-,-------------------.
              V                     |
  >>-keyword----repeatable_argument-+----------------------------><
   
   
  

  A repeat arrow above a stack indicates that you can make more than one choice from the stacked items.

              .-,---------------------.
              V                       |
  >>-keyword----+-required_argument-+-+--------------------------><
                '-required_argument-'
   
   
  

Example of a Syntax Diagram

                                         .-,-.
                                         V   |
>>-EXAMPLE--char_constant--+-a-+--+---+----e-+--name_list------><
                           '-b-'  +-c-+
                                  '-d-'
 
 

Interpret the diagram as follows:

• Enter the keyword EXAMPLE.
• Enter a value for char_constant.
• Enter a value for a or b, but not for both.
• Optionally, enter a value for c or d.
• Enter at least one value for e. If you enter more than one value, you must put a comma between each.
• Enter the value of at least one name for name_list. If you enter more than one value, you must put a comma between each. (The _list syntax is equivalent to the previous syntax for e.)