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Variables and Statements

Variables and Statements

Topics

• Numeric Data.

• Variables.

• Parameters.

• Arithmetic Expressions.

• Assignment Statement.

• Simple Input and Output.

• Comments.

• Program layout.

• Derived Data Types.

Numeric data

Real and integer

• Numbers represented as characters

• Identifying name tags

• Integer numbers:

• Real numbers:

2.576x1032 1.3x10-22

2.576E32 1.3E-22

Naming Convention

• Names upto 31 characters long:
  • Letters a...z, A...Z,

  • Numerals 0, 1, 2...9,

  • Underscore _

• First character must be a letter.

• No case sensitivity.

• No reserved words.

• Examples:
  • Valid:

• Invalid:

Variables

Declaration

• Declared at the start of the program.

• General form:

• Examples

INTEGER :: count, hours, minutes

• Variables may be given a value on declaration:

INTEGER :: months=12, weeks=52

Explicit vs implicit declarations

• All variables must be declared if the IMPLICIT NONE statement is used.
  • Otherwise a type is implied by the first letter

i,j,k,l,m,n are INTEGER

Parameters

Specification

• Parameters have a constant value.

• Requires the PARAMETER attribute.

• Examples:

INTEGER, PARAMETER :: maxvalue=1024

INTEGER, PARAMETER :: repeat=1000

• Parameters must be given an initial value.

Arithmetic Expressions

Operators

• Variable, parameters and literal numeric constants may be combined with the arithmetic operators:

- Subtraction

* Multiplication

/ Division

** Exponentiation

• Examples:

cost * number + postage

10 + 3

4 * pi

1 / pressure

pi * radius * radius

Operator Precedence

• Expressions in brackets evaluated first

• Operators executed in order of ascending precedence

• Operators of equal precedence evaluated from left to right

• Example
  • pi*radius**2

  • (pi*radius) **2

  • pi*(radius**2)

  • a+b+c-d

  • x*y/z+a

Assignment Statement

=

• General form:

• Used to assign a value to a variable:

radius = 10.0

• Used to assign the result of an expression to a variable:

• The result of the expression should be the same type as the variable.

Simple Input and Output

READ and WRITE

• Input from keyboard via READ statement:

• Output to screen via WRITE statement:

• Mutliple variables may be specified on the same READ/WRITE statement:

WRITE(6,*) temp, pressure, mass

WRITE(6,*) pi*radius**2, 2.0

Comments

• Aid to understanding complex program sections.

• Characters following ! are ignored by the compiler.
  • May be used to temporarily remove statements when debugging by "comment out".

• Examples:

area = pi*radius**2 !Calculate area

!WRITE(6,*) temp, radius*radius

Program Layout

Sample program

PROGRAM Circle_area

IMPLICIT NONE

!This program reads a value

!representing the radius of a circle,

!then calculates and writes out the

!area of the circle.

REAL :: radius, area

REAL, PARAMETER :: pi=3.141592

READ(5,*) radius

area = pi*radius*radius

WRITE(6,*) area !answer

END PROGRAM Circle_area

General form

• Maximum of 132 characters per line.
  • 80 may be a more practical limit.

• Extend on to next line using &

pi*radius*radius,&

length, breadth

• Multiple statements per line using semicolon as a statement separator.
  • poor programming practice.

area= length*breadth

Derived Data Types

Definition and specification

• An aggregate data structure, composed of:
  • intrinsic data types,

  • other derived types.

• General form:

component definition statements

...

END TYPE [name]

• Variables of type name can then be declared, general form:

Example

• To declare a data type representing a rectangle:

REAL :: length, breadth, area

END TYPE rectangle

• The following would declare two variables of type rectangle

Nested definitions

• Consider a type point:

REAL :: x_coord, y_coord

ENDTYPE point

• The previous definition of type rectangle may be modified to include a spacial coordinate, thus

TYPE (point) :: centre

REAL :: length, breadth, area

ENDTYPE rectangle

Accessing Components

• Each component is accessed using:

• For example

rect_a%breadth = 20.0

rect_a%area = rect_a%length * &

rect_a%breadth

• Nested elements are accessed thus:

rect_a%position%y_coord = 6.0

• A derived data type may be given an initial value: