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Fundamentals of Fortran 90
Basic Building Blocks
Identifiers
An identifier is a name that a programmer creates
for items within a program including: variables,
constants, and subprograms.
2
Rules for Identifiers
1 The first character must be a letter.
2 The remaining characters may be letters,
digits, or the underscore.
3 There may be no more than 31 characters in
an identifier.
3
Guidelines for Identifiers
Capitalize the first letter
If the identifier is composed of two words
which have been concatenated (run together)
Capitalize the first letter of each word
Optionally, place an underscore between the
words
Select identifiers that have some significance
to the problem being solved
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Identifiers
Valid:
TaxRate Last_Page Radius
Not Valid
Time-secs 3rdYear
5
Variables
A variable is a memory location in the primary
memory of a computer.
Each variable can store a single value
Since computers use several different internal
representations for various types of data, a
specific data type must be associated with
each variable
The value can be changed while the program
is running, but the data type cannot be
changed 6
Data Types
Fortran 90 supports five basic data types
INTEGER
REAL
CHARACTER
LOGICAL
COMPLEX
Fortran 90 allows a programmer to define additional
data types to fit a specific application.
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Declarations
Syntax
type_name :: list of identifiers separated by commas
A declaration reserves variables (memory
locations) for each of the identifiers and
associates a data type with each variable
A declaration DOES NOT provide an initial
value for any variable
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Examples of Declarations
REAL :: A, B
INTEGER :: K
INTEGER :: N
LOGICAL :: IsThere
CHARACTER (6) :: W, T, H*17
• W, T and H are character variables
• W and T will hold exactly 6 characters
• H will hold exactly 17 characters
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Default Types
Fortran does not require that variables be
declared
If a variable is referenced without being
declared, Fortran will give that variable a data
type based on the first letter of its identifier
I through N ==> INTEGER
otherwise ==> REAL
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The IMPLICIT Statement
The IMPLICIT statement can be used to
override Fortran‟s rules for assigning data
types to undeclared variables
Good programming style is to declare all
variables in a program
If the statement IMPLICIT NONE precedes
the first declaration, the Fortran 90 compiler
will ensure that every variable which is used
in the program has been explicitly declared
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Writing an INTEGER Value
String of digits
No commas
Optional + or - sign
Examples:
21856 -89 0
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Integers: Valid & Invalid
Valid Not Valid
234 5-
+236 --5
-225 17.0
0 14,555
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Writing a REAL Value (1)
Fixed Point Form ( xxx.xx )
String of digits with decimal point
No commas
Optional + or - sign
Valid Real Values:
-54.873 0.0 6.5 18.0 0.56
NOT Valid Reals:
12,345.67 12
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Writing a REAL Value (2)
Floating Point Form
Analogous to scientific notation
Basic form: fEn
Fixed point real ( f ) “Mantissa”
letter E
integer exponent ( n )
Interpretation f*10n
Examples:
6.57E-3 for 0.00657
-45.6E4 for -456000.0
0.7206E+3 for 720.6
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Writing a Character Value
Two forms
„string‟
“string”
Either form can be used
Examples
“This is easy”
„What is your name?‟
“That‟s all folks”
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Declaring a Constant
Syntax
type_name, PARAMETER :: name = value
This variant on the basic declaration is used
to give a symbolic name to a constant
The use of declared constants is considered
to be a better programming practice than
writing the value of the constant in the code
itself unless the constant is an integral part of
a formula and will never change value or
precision
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Examples Using PARAMETER
INTEGER, PARAMETER :: NumStates = 50
REAL, PARAMETER :: PI = 3.14159
CHARACTER (*), PARAMETER :: Prompt = “next>“
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Assignment Statement Syntax
Syntax
target_variable = expression
The value represented by the expression on the right
hand side of the equal sign is stored in the target
variable, replacing its previous value
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Assignment Statement Uses
Store a specific value in a variable
N = 32 ; X = -3.5 ; W = „cat‟
Copy a value from one variable to another
T=N
Store a computed value in a variable
X = 3.5 + W
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Numeric Conversions on
Assignment
INTEGER :: K, N, M
REAL :: W, C, Y
Integer value 6 will be stored in N
N = 6.8 (truncation rather than rounding)
X = 53 Real value 53.0 will be stored in X
K = -6 Integer value -6 will be stored in K
W=K Real value -6.0 will be stored in W
Y = 7.8 Real value 7.8 will be stored in Y
M =N Integer value 6 will be stored in M
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Character Padding and
Truncation on Assignment
CHARACTER (5) :: A, B
CHARACTER (8) :: C
frogwill be stored in A
A = „frog‟ (padded out to five characters with blank at end)
eleph will be stored in B
B = “elephant” (extra characters are truncated from the right end)
will
eleph be stored in C
C=B (B contained eleph which must be padded to 8 characters)
Note: The symbol represents a blank space
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Numeric Expressions
A numeric expression is any mathematically valid
sequence of operators, operands, and parentheses.
An operand is a constant, variable, or value returned by a
function.
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Arithmetic Operators
+ add
- subtract
* multiply
/ divide
** exponentiation
A**B means AB
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Result Types
The result type is determined for each
mathematical operation
The type is based on the type of the
operands
If both operands are INTEGER, the result is
an integer
If at least one operand is REAL, the result is a
real number
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Precedence Rules
Evaluate subexpressions in parentheses
Determine the value returned by any function
call
Evaluate ** (right to left)
2**3**5 is the same as 2**(3**5)
Evaluate * and / from left to right
Evaluate + and - from left to right
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Precedence Rules
All exponentiations are performed first;
consecutive exponentiations are performed
from right to left
2 ** 3 ** 2 = 2 ** 9 = 512
All multiplications and divisions are
performed next, in the order in which they
appear from left to right.
10 / 5 * 2 = 2*2= 4
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Precedence Rules
The additions and subtractions are
performed last, in the order in which they
appear from left to right
10 + 8 – 2 + 3 = 19
Another example
2 + 4 ** 2 / 2
2 + 16 / 2
2+8
10
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Characteristics of the /
Operator
If numerator and denominator are integers,
the quotient is truncated to an integer
• 23/5 is 4 rather than 4.6
If either the numerator or denominator is a
real number, the quotient is a real number
• 12/9.0 is 1.3333
• 3.6/1.2 is 3.0
• 8.6/2 is 4.3
• 23.0/5.0 is 4.6
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Characteristics of the ** Operator
Consider A**B
If B is a nonnegative integer, Fortran uses
repeated multiplication to evaluate A**B
If B is a real number, Fortran uses the
expression eB*ln(A) to approximate A**B.
Ex. (-4.0) ** 2.0 is undefined, because the
logarithms of negative values are not defined.
30
Type Conversions
Using both integers and reals in a computation
is a poor programming practice.
Fortran provides some type conversion
functions to convert a value to a different data
type.
Type conversions should be used to avoid
mixed data types in an expression.
The original value is not changed, but the
converted value is placed in a temporary
(unnamed) variable.
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Type Conversion Examples
INTEGER ==> REAL REAL ==> INTEGER
INTEGER N INTEGER K
REAL X REAL Y
N = 15 Y = 2.8
X = 3.7 * REAL(N) K = INT(Y) + 5
X = 3.7 * 15.0 K=2+5
N is unchanged Y is unchanged
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A Few Functions
A good programmer makes use of functions that are provided
as part of the programming language. Fortran 90 has an
extensive collection of intrinsic functions. (See Appendix D)
SQRT(X) Preferred to X**(0.5)
ABS(X) Absolute value
SIN(X), COS(X), TAN(X) Angles must be in radians
EXP(X) eX
LOG(X) ln X
LOG10(X) log10X
MOD(A,B) A mod B
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Form of a Program
------------ Header portion -------------
Initial comment block
PROGRAM statement
--------- Specification portion ---------
IMPLICIT NONE
Type specification statements
-------- Execution portion ------------
READ, PRINT, and assignments
-------- Ending portion ---------------
END PROGRAM statement
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Structure of a Line of Source Code
Maximum of 132 characters.
Can begin anywhere on the line.
Two or more statements can appear on a single line if
they are separated by semicolons.
Blanks can be inserted to improve comprehension.
A comment can appear at the end of the line.
If a statement label is used, it must appear first.
A label is an unsigned integer with at most 5 digits.
If a statement is too long, place an ampersand, &,
at the end and continue on the next line.
35
Comments
Syntax
! comment
Everything from the exclamation mark (!), to
the end of the line is a comment
A comment can appear on a line by itself or
at the end of some other line
Comments are included in a program to
provide a person with information about the
program
Comments are ignored by the compiler 36
The PROGRAM Statement
Syntax
PROGRAM program_name
Provides a name (identifier) for the program
Optional but strongly recommended
Required by most programming standards
Must be the first statement, but can be
preceded by comments
37
The END PROGRAM Statement
Syntax
END PROGRAM program_name
This is the only required statement
Must be the last line in the source code file
program_name must match that on the
PROGRAM statement
Omit the program-name if there is no
program statement
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The STOP Statement
Syntax
STOP
or STOP n !n is an integer value
Causes the program to stop its execution at
that point
More than one STOP statement can be
included
This statement is not required, but is
recommended by some coding standards
39
Simple Input (list-directed)
Syntax
READ *, input_list
input_list is a list of variables which will
receive the values that are read
Each READ statement begins reading a new
line of data
A READ statement can read more than one
line of data
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Simple Output (list-directed)
Syntax
PRINT *, output_list
output_list is a list of items to be printed
character string literals will be printed exactly
variables will have their values printed
Each PRINT statement starts a new line of
output
Fortran will use its internal rules to determine
the location and appearance of values on
each line
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