Fundamentals of Fortran 90

1
Fundamentals of Fortran 90

• Basic Building Blocks

2
Identifiers
An identifier is a name that a programmer creates
for items within a program including variables,
constants, and subprograms.
3
Rules for Identifiers

• The first character must be a letter.
• The remaining characters may be letters, digits,
  or the underscore.
• There may be no more than 31 characters in an
  identifier.

4
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

6
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

7
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.
8
Declarations

• 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

9
Examples of Declarations
REAL A, B INTEGER K INTEGER
N LOGICAL IsThere CHARACTER (6) W, T, H17

• W, T and H are character variables
• W and T will hold exactly 6 characters
• H will hold exactly 17 characters

10
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 gt INTEGER
• otherwise gt REAL

11
The IMPLICIT Statement

• The IMPLICIT statement can be used to override
  Fortrans 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

13
Integers Valid Invalid

• Valid
• 234
• 236
• -225
• 0

• Not Valid
• 5-
• --5
• 17.0
• 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 f10n
• Examples 6.57E-3 for 0.00657 -45.6E4
  for -456000.0 0.7206E3 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?
• Thats all folks

17
Declaring a Constant

• 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

18
Examples Using PARAMETER
INTEGER, PARAMETER NumStates 50 REAL,
PARAMETER PI 3.14159 CHARACTER (),
PARAMETER Prompt nextgt
19
Assignment Statement Syntax
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 N
6.8 X 53 K -6 W K Y 7.8 M N
Integer value 6 will be stored in N
(truncation rather than rounding)
Real value 53.0 will be stored in X
Integer value -6 will be stored in K Real value
-6.0 will be stored in W
Real value 7.8 will be stored in Y 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 A
frog B elephant C B
frog will be stored in A (padded out to five
characters with blank at end)
eleph will be stored in B (extra characters
are truncated from the right end)
eleph will be stored in C (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 AB 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)235 is the same
  as 2(35)
• 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 AB

• If B is a nonnegative integer, Fortran uses
  repeated multiplication to evaluate AB
• If B is a real number, Fortran uses the
  expression eBln(A) to approximate AB.
• Ex. (-4.0) 2.0 is undefined, because the
  logarithms of negative values are not defined.

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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 gt REAL
REAL gt INTEGER
INTEGER N REAL X N 15 X 3.7 REAL(N)
INTEGER K REAL Y Y 2.8 K INT(Y) 5
X 3.7 15.0 N is unchanged
K 2 5 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.

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Comments

• 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

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The PROGRAM Statement

• 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

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The END PROGRAM Statement

• 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

• 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

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Simple Input (list-directed)

• 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

41
Simple Output (list-directed)

• 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