Fortran 90 Program Structure and Layout

AS3013: F90 lecture 2
Fortran 90
Program Structure and Layout
Modular Structure
Split total program into modular units.
Any number (including 0) subprograms.
One (and only one) main program.
Each program/subprogram should have
one major task
Input/Output
Calculations
Now let’s examine the sample program.
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Declaration section (non-executable)
Program name PROGRAM first
IMPLICIT NONE
Type declarations REAL x, y, z
must precede
Execution section ** starts at the beginning **
Assignment
and
Input/Output
Statements
AS3013: F90 lecture 2
Forces variables
to be declared
Data types of Variables
PRINT *, 'Enter two numbers to add:'
READ *, x, y
Read/Write values using default devices
(keyboard/screen) in a default (*) format
z = x + y
Assign to z the sum of x and y
PRINT *, 'Result =', z
** finish at the end **
Termination section END PROGRAM first last statement in unit
Comments start with ! anywhere on a line, anywhere before END
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Subprograms: Same structure as main program
CALLed from main program or another subprogram
First statement: SUBROUTINE name
or: FUNCTION name
Statements: are written in lines.
Most can be written on one line.
May be continued onto more lines if
last character of line is an ampersand (&)
Use to improve layout.
Maximum 40 lines in a statement.
A line may contain 0 to 132 characters.
Recommendation: no more than about 70 characters on a line
keeps program readable and manageable
In Fortran 90 a statement may be anywhere on a line
AS3013: F90 lecture 2
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Comments:
• Important for understanding code
Even if you have written it yourself
• An exclamation mark (!) anywhere in any line
starts a comment to the end of the line.
• Comments have no effect on execution
Blank lines and blanks in lines improve readability.
ADD COMMENTS WITHIN THE PROGRAM TO
DELINEATE SECTIONS AND EXPLAIN ACTIONS
THE FIRST LINES OF EVERY COMPUTER PROGRAM
SHOULD CONTAIN THE PROGRAMMER’S NAME,
THE DATE, AND THE PURPOSE OF THE PROGRAM.
AS3013: F90 lecture 2
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Fortran 90: Data types and constants
CHARACTER constant: a string of one or more characters
between single or double quotes.
e.g. "Fred", ' ' (a blank)
Numerical constants may be positive, negative or zero.
If positive the sign is optional.
Embedded commas (e.g. 1,000,000) or spaces (1 000 000)
are not permitted.
INTEGER constant: any number not containing a
decimal point and/or exponent.
e.g. 2153 -36 0 123456 999
Range is machine dependent.
On SUN, PC and many others to > |2 000 000 000|
Complete accuracy. (32 bits = 4 bytes)
AS3013: F90 lecture 2
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REAL constant: any number containing a decimal point
and/or exponent (“scientific” notation)
e.g. 21.362 -9.6405 0.0 -25. .00125
The exponent is the letter E followed by an integer constant
representing a power of 10
e.g. -3.5E4( ! " 35 # 10 ) 0.263E-6
4
.
# ( ! 0 263" 10 ) 6
.
"
! 10 8
! 5" 10 12
AS3013: F90 lecture 2
( )
5E+12 1E-8
( )
decimal point not required if exponent present.
Precision depends on computer and compiler.
SUN, PC: 6 to 7 decimal digits Exponent range is ±38 ~ ± ±
10 38
REAL numbers: Beware of rounding errors and truncation.
DOUBLE PRECISION: for greater range and precision (64 bits total).
15-17 digits, range ~ ± . Constants must be in exponent form using D.
±
10 308
e.g. 5D12, 3.141592653589793D0, 1D0
STRONGLY RECOMMENDED FOR SCIENTIFIC WORK
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Fortran 90: Standard character set
A to Z (upper case), digits 0 to 9, underscore _
special characters:
space = + - * ( ) , . ' : " ! % & ; < > ? $
For portability it is advisable to stick to these characters even in character
constants, but most computers now use the ASCII character set to represent
characters as 8 bit numbers (0-255).
additional characters:
a to z (lower case), # @ [ \ ] ^ ` { | }
Any character acceptable to the computer may be used in CHARACTER
constants.
WARNING: Fortran compilers do not distinguish between upper
and lower case except in character constants.
i.e. ABCDE ≡ abcde ≡ AbCdE ≡ aBcDe, etc.
Other data types: LOGICAL, COMPLEX
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Variable names: (and most other names, e.g. for PROGRAM)
• First character must be a letter.
• Remaining characters may be any combination of
letters, digits or underscore ( _ ) characters.
• Maximum length is 31 characters.
e.g. A A12 Alpha_3 Next_Month width
Variables are names of ‘boxes’ in computer memory where
numerical or other data values are stored.
We must tell the compiler what type of data they will hold by
declaring all variables in Type statements.
e.g. old style new style
REAL a, b, c or REAL :: a, b, c
INTEGER count, year INTEGER :: count, year
Use IMPLICIT NONE at the start of each program unit.
Use comments to create a variable dictionary.
AS3013: F90 lecture 2
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