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Week 2

Organizational matters Fortran 90 (subset F): Basics Example programs in detail

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Top-down programming

4 basic steps– Specify the problem clearly– Analyse the problem and break it down into

its fundamental elements– Code the program according to the plan

developed at step 2– Test the program exhaustively, and repeat

steps 2 and 3 as necessary until the program works in all situations that you can envisage

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Program = Data Types +

Algorithms Data types:

what you work on Algorithms:

what you do with them

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Structure of a program(in Fortran 90) Heading (program, module, etc.) specification part execution part subprogram part end program statement

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program Radioactive_Decay!----------------------------------------------------------------------------! This program calculates the amount of a radioactive substance that ! remains after a specified time, given an initial amount and its ! half-life. Variables used are:! InitalAmount : initial amount of substance (mg)! HalfLife : half-life of substance (days)! Time : time at which the amount remaining is calculated (days)! AmountRemaining : amount of substance remaining (mg)!! Input: InitialAmount, HalfLife, Time! Output: AmountRemaining!-----------------------------------------------------------------------------

implicit none real :: InitialAmount, HalfLife, Time, AmountRemaining

! Get values for InitialAmount, HalfLife, and Time.

print *, "Enter initial amount (mg) of substance, its half-life (days)" print *, "and time (days) at which to find amount remaining:" read *, InitialAmount, HalfLife, Time ! Compute the amount remaining at the specified time. AmountRemaining = InitialAmount * 0.5 ** (Time / HalfLife)

! Display AmountRemaining. print *, "Amount remaining =", AmountRemaining, "mg"

end program Radioactive_Decay

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Data Types

Five basic types:– integer– real– complex– character– logical

Data types of ‘container’ classes

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Integers

a whole number (positive, negative or zero)

no decimal point Examples0123-23456+123789

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Reals

Numbers with decimal fractions There has to be decimal point Examples1.23456-0.0019875678.

Another representation:1.952e30.1952e4123.456e-8

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Character

Sequence of symbols from the Fortran character set

Enclosed between double quotes Examples"This is a string""I do, I don't""1234abc345"

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Logical

Can take only two values:•.TRUE.•.FALSE.

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Identifiers

Names used to identify programs, constants, variables, etc.

Identifiers must Begin with a letter This can be followed by up to 30

letters, digits, undescores Be careful with the case: lower or

upper case letters

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Identifiers

ExamplesCurrentDecay_Ratepressurean_identifier_with_a_long_namethe_best_program

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Constants

10293845 is an integer constant 12.3456 is a real constant "What a nice day!" is a character

constant

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VariablesVariables:: It is usual for people to associate a name or phrase with a piece of information. For example, the phrase "today's date" has an associated numeric value which varies day by day. This is similar to the concept of a program variable; a program variable is some object (named by the programmer) which uniquely identifies a piece of data stored in memory.

Variables are value containers Compiler associates with a variable

a memory location Value of a variable at any time is

the value stored in the associated memory location at that time

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Variables

123.5623456

MEMORY

Hello World1234567

Message

Payment

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Declarations of Variables

Form:type-specifier :: list

Declares that the identifiers in the list have the specified type

Type statements must appear in the specification part of the program

Examplesinteger :: number_years, counts, monthsreal :: Mass, Velocity, Accelerationcharacter (len=12) :: MyName, YourName

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implicit none

It should be placed at the beginning of the specification part

You have to declare all variables you will be using in the program!

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Variable initialization

All variables are initially undefined Initialization in the declarations

Examples:real :: W=1.2, z=5.678, mass=4.56integer :: year=1998, count=0

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Named constants

Form:type-specifier, parameter :: list

Examplesinteger, parameter :: INITCOUNT = 30real, parameter :: G = 9.81

It's a good idea to write named constants in upper case

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Arithmetic operations

Variables and constants can be processed by using operations and functions appropriated to their types.

Operations

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Operator Operation

+ Addition, unary plus

- Subtraction, unaryminus

* Multiplication

/ division

** exponentiation

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Operations

ExamplesTo calculate B2 - 4ACB**2 - 4*A*C

Types are important:9/4 = 29.0/4.0 = 2.25

Mixed-mode expressions:3 + 8.0/5 3 + 8.0/5.0 3 + 1.6 3.0 + 1.6 4.6

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Priority rules

All exponentiations are performed first; consecutive exponentiations are performed from right to left

All multiplications and divisions are performed next; in the order in which they appear from left to right

Additions and subtractions are performed last, in the order in which they appear from left to right

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Some examples

2 ** 3 ** 2 = 512 10/5 *2 = 2 * 2 = 4

To calculate 51/3

5.0**(1.0/3.0) but not

5.0**(1/3) 5.0**0 1.0

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Library functions

abs(x) Absolute value of x cos(x) Cosine of x radians exp(x) Exponential function int(x) Integer part of x sqrt(x) Square root of x

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Assignment statement Form:variable = expression

Assigns the value of expression to variable Assignment is not a statement of algebraic

equality; it is a replacement statement ExamplesDensity = 2000.0Volume = 3.2Mass = Density*VolumeWeightRatio = log(Mass/90.)

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Programs need to communicate with users! Two kinds of I/O (for the moment!):

– Formatted I/O– List-directed I/O

List-directed outputprint *, output-listwrite (unit=*, fmt=*) output-list

List-directed inputread *, input-listread (unit=*, fmt=*) input-list

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List-directed I/O

Examplesprint *, "Tell me your birthday"write (unit=*, fmt=*) a, b, c**2read *, day, month, year

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