Fortran: Genel Bilgiler

1
Fortran Genel Bilgiler

• Dr. Mürtezaoglu

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ENF 102 Bilgisayar Programlama Dili

• Fortran 90

3
Introduction

• Introduction
• What is a Computer?
• What is Hardware and Software?
• Telling a Computer What To Do
• Some Basic Terminology
• How Does Computer Memory Work?
• Numeric Storage
• Programming Languages
• High-level Programming Languages
• An Example Problem
• An Example Program
• Analysis of Program
• A Closer Look at the Specification Part
• A Closer Look at the Execution Part
• How to Write a Computer Program
• A Quadratic Equation Solver - The Algorithm
• A Quadratic Equation Solver - The Program
• A Quadratic Equation Solver - The Testing Phase

4
Introduction

• What is a Computer?
• A simple computer may look like this
•                                                 
                       
• memory (RAM) -- used to store values during
  execution of a program,
• CPU (Central Processor Unit) -- does the work',
• disc drive -- permanently' stores files,
• keyboard -- allows user to input information,
• VDU -- visually outputs data,

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Introduction

• What is Hardware and Software?
• A computer system comprises hardware and
  software.
• Hardware is the physical medium, for example
• circuit boards
• processors
• keyboard
• Software are computer programs, for example
• operating system
• editor
• compilers
• a Fortran 90 program

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Introduction

• Telling a Computer What To Do
• To get a computer to perform a specific task it
  must be given a sequence of unambiguous
  instructions or a program.
• We meet many examples of programs in everyday
  life, for example, instructions on how to
  assemble a bedside cabinet. These instructions
  are generally numbered, meaning that there is a
  specific order to be followed, they are also
  (supposed to be) precise so that there is no
  confusion about what is intended
• insert the spigot into hole A',
• apply glue along the edge of side panel,
• press together side and top panels
• attach toggle pin B' to gromit C'
• ... and so on
• If these instructions are not followed to the
  letter', then the cabinet would turn out wonky.

7
Introduction

• Some Basic Terminology
• It is necessary to cover some terminology.
  Hopefully, much of it will be familiar -- you
  will hear many of the terms used throughout the
  course.
• Bit is short for Binary Digit. Bits have value of
  1 or 0, (or on or off, or, true or false),
• 8 Bits make up 1 Byte, 1024 Bytes make up 1 KByte
  (1 KiloByte or 1K), (Why 1024?'' I hear you
  ask. Because .
• 1024 KBytes make up 1 MByte (1 MagaByte or 1M),
• 1024 MBytes make up 1 GByte (1 GigaByte or 1G),
• all machines have a wordsize -- a fundamental
  unit of storage, for example, 8-bits, 16-bits,
  etc. The size of a word (in Bytes) differs
  between machines. A Pentium based machine is
  32-bit.
• a flop is a floating point operation per second.
  A floating point operation occurs when two real
  numbers are added. Today, we talk of megaflops or
  even gigaflops.
• parallel processing occurs when two or more CPUs
  work on solution of the same problem at the same
  time.

8
Introduction

• How Does Computer Memory Work?
• Here the wordsize is 8-bits
• A computers memory is addressable,
• each memory location will contain some sort of
  value',
• Each location has a specific number'
  (represented as hexadecimal base-16, e.g.,
  3F2C),
• Fortran 90 allows (English) names to be given to
  memory locations,
• the value of a location can be read from or
  written to.
• The CPU can say, fetch the contents of memory
  location 3F2C' or write this value to location
  3AF7'.

9
Introduction

• Numeric Storage
• In general, there are two types of numbers used
  in Fortran 90 programs INTEGER s (whole numbers)
  and REAL s (floating point numbers).
• INTEGER s are stored exactly, often in range
  (-32767, 32767).
• REAL s are stored approximately.
• They are partitioned into a mantissa and an
  exponent,
• Exponent can only take a small range of values.
• You can get numeric exceptions
• overflow -- exponent is too big,
• underflow -- exponent is too small.
• In Fortran 90 you can decide what numeric range
  is to be supported.
• CHARACTER s are stored differently.

10
Introduction

• Programming Languages
• Programming languages must be
• totally unambiguous (unlike natural languages,
  for example, English),
• expressive -- it must be fairly easy to program
  common tasks,
• practical -- it must be an easy language for the
  compiler to translate,
• simple to use.
• All programming languages have a very precise
  syntax (or grammar).
• This ensures all syntactically-correct programs
  have a single meaning.

11
Introduction

• High-level Programming Languages
• Assembler code is a Low-Level Language.
• Fortran 90, FORTRAN 77, ADA, C and Java are
  High-Level Languages.
• a program is a series of instructions to the CPU,
  
• could write all programs in assembler code but
  this is a slow, complex and error-prone process,
• high-level languages are more expressive, more
  secure and quicker to use,
• the high-level program is compiled (translated)
  into assembler code by a compiler.

12
Introduction

• To convert from F (Fahrenheit) to C
  (Centigrade) we can use the following formula
• To convert from C to K (Kelvin) we add 273.
• The program would accept a Fahrenheit temperature
  as input and produce the Centigrade and Kelvin
  equivalent as output.

• PROGRAM Temp_Conversion
• IMPLICIT NONE INTEGER Deg_F, Deg_C, K
• PRINT, "Please type in the temp in F
• READ, Deg_F Deg_C 5(Deg_F-32)/9
• PRINT, "This is equal to", Deg_C, "C" K Deg_C
  273 PRINT, "and", K, "K"
• END PROGRAM Temp_Conversion
• This program, called Temp.f90, can be compiled
• chad2-13adamm 26gt f90 Temp.f90 NAg Fortran 90
  compiler v2.2. New Debugger 'dbx90' and run
• chad2-13adamm 27gt a.out Please type in the temp
  in F 45 This is equal to 7 C and 280 K

13
Introduction

• Analysis of Program
• The code is delimited by PROGRAM ... END PROGRAM
  statements. Between these there are two distinct
  areas.
• Specification Part
• specifies named memory locations (variables) for
  use,
• specifies the type of the variable,
• Execution Part
• reads in data,
• calculates the temp in C and K and
• prints out results.

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Introduction

• A Closer Look at the Specification Part
• IMPLICIT NONE -- this should always be present.
  Means all variables must be declared.
• INTEGER Deg_F, Deg_C, K -- declares three
  INTEGER (whole number) variables. Other variable
  types
• REAL -- real numbers, e.g., 3.1459, ,
• LOGICAL -- take vaules .TRUE. or .FALSE.,
• CHARACTER -- contains single alphanumeric
  character, e.g., 'a',
• CHARACTER(LEN12) -- contains 12 alphanumeric
  characters, a string,
• Fortran 90 is not case sensitive.
• K is the same as k and INTEGER is the same as
  integer.

15
Introduction

• A Closer Look at the Execution Part
• This is the part of the program that does the
  actual work'.
• PRINT, "Please type in the temp in F" -- writes
  the string to the screen,
• READ, Deg_F -- reads a value from the keyboard
  and assigns it to the INTEGER variable Deg_F,
• Deg_C 5(Deg_F-32)/9 -- the expression on the
  RHS is evaluated and assigned to the INTEGER
  variable Deg_C,
• is the multiplication operator,
• - is the subtraction operator,
• / is the division operator, (takes longer than )
  
• is the assignment operator.
• PRINT, "This is equal to", Deg_C, "C" --
  displays a string on the screen followed by the
  value of a variable (Deg_C) followed by a second
  string ("C").
• By default, input is from the keyboard and output
  to the screen.

16
Introduction

• How to Write a Computer Program
• There are 4 main steps
• specify the problem,
• analyse and break down into a series of steps
  towards solution,
• write the Fortran 90 code,
• compile and run (i.e., test the program).
• It may be necessary to iterate between steps 3
  and 4 in order to remove any mistakes.
• The testing phase is very important.

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Introduction

•  
• A Quadratic Equation Solver - The Algorithm
• The problem Write a program to calculate the
  roots of a quadratic equation of the form
• The roots are given by the following formula
• The algorithm
• READ values of a, b and c,
• if a is zero then stop as we do not have a
  quadratic,
• calculate value of discriminant
• if D is zero then there is one root ,
• if D is gt 0 then there are two real roots and
  ,
• if D is lt 0 there are two complex roots and
  ,
• PRINT solution.

18
Introduction

• A Quadratic Equation Solver - The Program
• PROGRAM QES
• IMPLICIT NONE INTEGER a, b, c, D
• REAL Real_Part, Imag_Part
• PRINT, "Type in values for a, b and c"
• READ, a, b, c
• IF (a / 0) THEN ! Calculate discriminant
• D bb - 4ac IF (D 0) THEN ! one root
• PRINT, "Root is ", -b/(2.0a) ELSE IF (D gt 0)
  THEN ! real roots
• PRINT, "Roots are",(-bSQRT(REAL(D)))/(2.0a),
  "and", (-b-SQRT(REAL(D)))/(2.0a) ELSE ! complex
  roots
• Real_Part -b/(2.0a)
• ! D lt 0 so must take SQRT of -D
• Imag_Part (SQRT(REAL(-D))/(2.0a))
• PRINT, "1st Root", Real_Part, "", Imag_Part,
  "i"
• PRINT, "2nd Root", Real_Part, "-", Imag_Part,
  "i"
• END IF
• ELSE ! a 0 PRINT, "Not a quadratic equation"
• END IF
• END PROGRAM QES

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Introduction

• A Quadratic Equation Solver - The Testing Phase
• The output from the program is as follows
• uxaadamm 35gt a.out
• Type in values for a, b and c
• 1 -3 2 Roots are 2.0000000 and 1.0000000
• uxaadamm 36gt a.out
• Type in values for a, b and c
• 1 -2 1 Root is 1.0000000
• uxaadamm 37gt a.out
• Type in values for a, b and c
• 1 1 1
• 1st Root -0.5000000 0.8660254 i
• 2nd Root -0.5000000 - 0.8660254 i
• uxaadamm 38gt a.out
• Type in values for a, b and c
• 0 2 3 Not a quadratic equation
• Its can be seen that the test data' used above
  exercises every line of the program. This is
  important in demonstrating correctness.

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Introduction

• Points Raised
• The previous program introduces some new ideas,
• comments -- anything on a line following a ! is
  ignored,
• -- means the line is continues,
• IF construct -- different lines are executed
  depending on the value of the Boolean expression,
  
• relational operators -- (is equal to') or gt
  (is greater than'),
• nested constructs -- one control construct can be
  located inside another.
• procedure call -- SQRT(X) returns square root of
  X.
• type conversion -- in above call, X must be REAL.
  In the program, D is INTEGER, REAL(D) converts D
  to be real valued.
• To save CPU time we only calculate the
  discriminant, D, once.

21
Introduction

• Bugs - Compile-time Errors
• In previous program, what if we accidentally
  typed
• Rael_Part -b/(2.0a)
• the compiler generates a compile-time or syntax
  error
• uxaadamm 40gt f90 Quad.f90
• NAg Fortran 90 compiler v2.2.
• Error Quad.f90, line 16
• Implicit type for RAEL_PART
• detected at RAEL_PART_at_
• Error Quad.f90, line 24
• Symbol REAL_PART referenced but never set
• detected at QES_at_ltend-of-statementgt
• f90 terminated - errors found by pass 1

22
Introduction

• Bugs - Run-time Errors
• If we had typed
• Real_Part -b/(.0a)
• then the program would compile but we would get a
  run-time error,
• uxaadamm 43gt a.out
• Type in values for a, b and c
• 1 1 1
• Arithmetic exception Floating divide by
  zero - aborting
• Abort
• It is also possible to write a program that gives
  the wrong results!

23
Introduction

• Compiler Switches
• Compiler is invoked by f90
• f90 Quad.f90 This
• checks the program syntax
• generates executable code in a file a.out
• Switches or flags can be supplied to compiler
• -o lt output-filename gt gives executable a
  different name, for example, Quad
• f90 Quad.f90 -o Quad-time report execution
  times f90 Quad.f90 -o Quad time For more
  information about the compiler type
• man f90