Fortran 90 Tutorial

4. Basic program structure

Key features

Free-form source code is allowed. Statements may begin in any column.
Lines may extend to 132 characters.
More than one statement may be placed on a line. A semicolon is used as the delimiter to separate statements.
In-line comments are allowed. These comments begin with an exclamation point and extend to the end of the line.
Continuation statements are indicated by the placement of an ampersand (&) as the last non-blank character in the line being continued. Up to 39 continuations are allowed.
Variable names may consist of up to 31 characters. Valid characters include all letters, digits, and the underscore symbol (_). The first character of a variable name must be a letter.

Declarations

The command IMPLICIT NONE is allowed. This feature cancels the default naming convention. When invoked, program variables MUST be specified explicitly.
Modified type specification statements may be used to initialize variables or parameters, e.g.,
```
   REAL :: Area = 0.
   INTEGER :: Num_Months = 12
   REAL, PARAMETER :: Pi = 3.1415927
```
declares Area to be a real variable with initial value 0., Num_Months to be an integer variable with the initial value 12, and Pi a real parameter with the value 3.1415927.
The separator :: is required in a type specification statement whenever it is used to initialize a variable or to declare a special attribute (such as PARAMETER). It may be omitted in other cases. For example, to declare the variable Area_circle to be real, we may say
```
   REAL Area_circle
```
or, equivalently
```
   REAL :: Area_circle
```
The latter form is preferred for the purpose of uniform programming style.
The precision of a real, integer, complex, or logical variable may be specified using a kind type parameter. For example, to declare a variable A to be a real number with at least 10 decimal places of precision with a range of at least -10^34 to 10^34, do the following:
```
   REAL(KIND = SELECTED_REAL_KIND(10,34)) :: A
```
If the processor you are using is unable to support a variable with this type specification, a compile-time error will result.

To declare a real variable to have the equivalent of Fortran 77 accuracy DOUBLE PRECISION, simply do this:
```
   INTEGER, PARAMETER :: DP = SELECTED_REAL_KIND(14)
   REAL(KIND = DP) :: A
```
This declares the variable A to be of type real and have at least 14 decimal places of accuracy.

Program composition

The basic form of a Fortran 90 program is as follows:

PROGRAM name
   declarations
   executable statements
END PROGRAM