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Week 2

Week 2. Organizational matters Fortran 90 (subset F): Basics Example programs in detail. 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

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Week 2

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  1. Week 2 • Organizational matters • Fortran 90 (subset F): Basics • Example programs in detail

  2. 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

  3. Program = Data Types + Algorithms • Data types: what you work on • Algorithms: what you do with them

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

  5. Data Types • Five basic types: • integer • real • complex • character • logical • Data types of ‘container’ classes

  6. Integers • a whole number (positive, negative or zero) • no decimal point • Examples0123-23456+123789

  7. Reals • Numbers with decimal fractions • There has to be decimal point • Examples1.23456-0.0019875678. • Another representation:1.952e30.1952e4123.456e-8

  8. Character • Sequence of symbols from the Fortran character set • Enclosed between double quotes • Examples"This is a string""I do, I don't""1234abc345"

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

  10. 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

  11. Identifiers • ExamplesCurrentDecay_Ratepressurean_identifier_with_a_long_namethe_best_program

  12. Constants • 10293845is an integer constant • 12.3456 is a real constant • "Whataniceday!" is a character constant

  13. Variables • 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

  14. Variables Payment 123.56 23456 Hello World 1234567 MEMORY Message

  15. Declarations of Variables • Form:type-specifier :: list • Declares that the identifiers in the list have the specified type • Type statements must appear in the specificationpart of the program • Examplesinteger :: number_years, counts, monthsreal :: Mass, Velocity, Accelerationcharacter (len=12) :: MyName, YourName

  16. 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!

  17. Variable initialization • All variables are initially undefined • Initialization in the declarationsExamples:real :: W=1.2, z=5.678, mass=4.56integer :: year=1998, count=0

  18. 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

  19. Arithmetic operations • Variables and constants can be processed by using operations and functions appropriated to their types. • Operations

  20. 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 à 3 + 8.0/5.0 à 3 + 1.6 à 3.0 + 1.6 à 4.6

  21. 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

  22. 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) à 5.0**0 à 1.0

  23. 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

  24. 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.)

  25. 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

  26. List-directed I/O • Examplesprint *, "Tell me your birthday"write (unit=*, fmt=*) a, b, c**2read *, day, month, year

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