Fundamentals of Fortran 90

Fundamentals of Fortran Fundamentals of Fortran 9090

Basic Building Blocks

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Identifiers

An identifier is a name that a programmer creates for items within a program including: variables, constants, and subprograms.

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Rules for Identifiers1 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.

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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

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Variables

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

A variable is a memory location in the primary memory of a computer.

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Data TypesFortran 90 supports five basic data types

Fortran 90 allows a programmer to define additionaldata types to fit a specific application.

INTEGER REAL CHARACTER LOGICAL COMPLEX

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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

type_name :: list of identifiers separated by commas

Syntax

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Examples of DeclarationsREAL :: A, BINTEGER :: KINTEGER :: NLOGICAL :: IsThereCHARACTER (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

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 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

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

type_name, PARAMETER :: name = value

Syntax

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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

target_variable = expression

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, MREAL :: W, C, Y

N = 6.8

X = 53

K = -6W = K

Y = 7.8M =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 KReal value -6.0 will be stored in W

Real value 7.8 will be stored in YInteger value 6 will be stored in M

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Character Padding and Truncation on Assignment

CHARACTER (5) :: A, BCHARACTER (8) :: C

A = ‘frog’

B = “elephant”

C = B

frog’ will be stored in A (padded out to five characters with blank at end)

Note: The symbol ’ represents a blank space

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)

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Numeric ExpressionsA 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

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.

Consider A**B

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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 ==> REAL REAL ==> INTEGERINTEGER N

REAL X

N = 15

X = 3.7 * REAL(N)X = 3.7 * 15.0N is unchanged

INTEGER K

REAL Y

Y = 2.8

K = INT(Y) + 5K = 2 + 5Y is unchanged

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A Few FunctionsA good programmer makes use of functions that are providedas 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

! comment

Syntax

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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

PROGRAM program_name

Syntax

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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

END PROGRAM program_name

Syntax

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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

STOP

or STOP n !n is an integer value

Syntax

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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

READ *, input_list

Syntax

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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

PRINT *, output_list

Syntax