4.3 Concept of a computer variable
4.5 Implicit declarations in FORTRAN
4.8 FORTRAN 90 - PRINT statement
4.9 Design and setup of a complete program
The Four Basic FORTRAN Data Types
1. INTEGERS
+ add
- subtract
* multiply
/ integer
divide : 17/5 = 3 (remainder of 2 thrown away)
** raise
to power = exponentiation
2. REALS
+ add
- subtract
* multiply
/ integer
divide : 17./5. = 3.4
** raise
to power = exponentiation
3. CHARACTER DATA
4. LOGICAL DATA
.AND. logical
AND -- result true if both are true
.OR.
logical OR -- result true
if at least one is true
.NOT. changes
.TRUE. to .FALSE. and vice versa
Data Types
.16 0. 00014 -0.3 6.02E23 (means 6.02 x l023)
'HELLO, WORLD' 'PROF. G. RATZER' '398-7082'
'"HOW''S IT GOIN'', EH?" '
.TRUE. .FALSE.
1 BYTE | ||
FORTRAN Also Allows
.
.
.
MORE
FORTRAN Variables
1- A quantity that can vary.
2- A location in memory.
3- A name used to refer to.
Rules for Choosing a Name
A) 1 to 6 characters ( FORTRAN 77 )
or
1 to 31 characters ( FORTRAN 90 )
B) First character must be A
... Z, rest A...Z plus 0...9
EXAMPLES :
TODAY SINGLE MAX MIN X ZMAT APOLLO13 -> NOT VALID IN FORTRAN 77 THISISTOOLONGTOBEAGOODVARIABLENAME -> NOT VALID 7UP -> NOT VALID !ALLO -> NOT VALID 22CATCH -> NOT VALID
Declaration Statements
For each data type and corresponding
constant there are what are called declaration statements, which
are used to define the type of data to be associated with
a chosen FORTRAN variable. Thus
INTEGER :: K, PTR REAL :: SUM, COST, VALUE CHARACTER(LEN=20) :: NAME, ADDRSS LOGICAL :: SINGLEThe declaration statements should be placed at the beginning of a program, before any executable statements like a PRINT statement. The above declarations define K and PTR to be INTEGER variables and any data assigned to these variables will be stored as a whole number, without any fractional part.
Likewise the variables SUM, COST
and VALUE should be used in calculations which may require a numeric
value with a fractional part.
The CHARACTER statement usually
has a length attribute - namely 20. This specifies that both the
variables NAME and ADDRSS may be used to store a string of up
to 20 alphanumeric characters.
The LOGICAL variable SINGLE may only be assigned the value .TRUE. or .FALSE.
Implicit Declaration in FORTRAN
You should ALWAYS declare all
the variables you will use in your program.
If you forget, FORTRAN will assume
that variable names starting with one of these letters
(FIRST letter of variable name):
are INTEGER variable otherwise, they are implicitly
declared as REAL VARIABLE.
EXAMPLE :
1) INTEGER :
INUM INDEX LSUM MONTH
2) REAL :
TOTALSUM
VALUE
TITLE
Common FORTRAN ''Gotcha's''
1) Integer conversions
2) Undeclared variables
3) Priority of operators
2**2**3 = 223 = 28 = 256
B/C*D = (B*D)/C
4) Watch your punctuation !
DO J=1.10 PRINT *,J END DO
The rules governing Mixed
Mode expressions are as follows.
1. Apply the priority
rules for the selection of the arithmetic operations to be performed
first.
( ) High priority
** Medium priority
* / Same priority
+
Low priority
Operators with the same priority
in an assignment are processed from left to right.
2. If both of the selected variables
or constants are INTEGER, do the operation in INTEGER.
3. IF both of the selected variables
or constants are REAL, do the operation in REAL.
4. If one is INTEGER and one
is REAL, convert the INTEGER data to REAL and do the operation
in REAL.
Repeat the process until the expression is reduced to a single value. Note that internally, the computer must do arithmetic between data of the same type, both INTEGER or both REAL. Apart from these rules which apply to the evaluation of expressions on the right of the equal sign in an Assignment statement, there may still be conversion to a different type on the left of the equal sign.
! Arithmetic in print statements ! PROGRAM ARITHMETIC ! IMPLICIT NONE PRINT *, '2+2 = ', 2+2 PRINT *, '2 ** 3 = ', 2**3 PRINT *, 'WITH REALS : ', 2.**3 PRINT *, 'PARENTHESES : (-2+3)*(4+5) = ', (-2+3)*(4+5) PRINT *, 'WITHOUT : ', -2+3*4+5 PRINT *, 'LOGIC : ', .TRUE., .FALSE. PRINT *, 'CONTRADICT : (NOT TRUE) = ', .NOT. .TRUE. PRINT *, 'AND : (TRUE AND FALSE) = ', .TRUE. .AND. .FALSE. STOP END PROGRAM ARITHMETIC
PROGRAM OUTPUT :
2+2 = 4 2 ** 3 = 8 WITH REALS : 8.00000 PARENTHESES : (-2+3)*(4+5) = 9 WITHOUT : 15 LOGIC : T F CONTRADICT : (NOT TRUE) = F AND : (TRUE AND FALSE) = F
Assignment Statement
General form :
Variable
= Expression
Expression can be :
1)
Constant
2)
Variable
3) Combination of constants and variables
( combined with operators, ref.
to data type operators )
EXAMPLE :
PROGRAM EXAMPLE IMPLICIT NONE INTEGER :: AGE,PDS REAL :: TOTAL, COST CHARACTER (LEN=20) :: NAME LOGICAL :: LOVED
AGE = 20 AGE = 20 + AGE PDS = 10 TOTAL = (PDS*30.)/3 COST = TOTAL*1.13955 NAME = 'LUCIE' LOVED = .TRUE.
STOP END PROGRAM EXAMPLE
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