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Fortran: Genel Bilgiler

Fortran: Genel Bilgiler. Dr. Mürtezaoğlu. ENF 102 Bilgisayar Programlama Dili. Fortran 90. 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

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Fortran: Genel Bilgiler

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  1. Fortran: Genel Bilgiler Dr. Mürtezaoğlu

  2. 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 • Points Raised • Bugs - Compile-time Errors • Bugs - Run-time Errors • Compiler Switches

  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,

  5. 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

  6. 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. 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 becompiled: chad2-13{adamm} 26> f90 Temp.f90 NAg Fortran 90 compiler v2.2. New Debugger: 'dbx90' and run: chad2-13{adamm} 27> a.out Please type in the temp in F 45 This is equal to 7 C and 280 K Introduction

  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.

  14. 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(LEN=12) -- 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.

  17. 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 > 0 then there are two real roots: and , • if D is < 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 = b*b - 4*a*c IF (D == 0) THEN ! one root • PRINT*, "Root is ", -b/(2.0*a) ELSE IF (D > 0) THEN! real roots • PRINT*, "Roots are",(-b+SQRT(REAL(D)))/(2.0*a),& "and", (-b-SQRT(REAL(D)))/(2.0*a) ELSE ! complex roots • Real_Part = -b/(2.0*a) • ! D < 0 so must take SQRT of -D • Imag_Part = (SQRT(REAL(-D))/(2.0*a)) • 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

  19. Introduction • A Quadratic Equation Solver - The Testing Phase • The output from the program is as follows: • uxa{adamm} 35> a.out • Type in values for a, b and c • 1 -3 2 Roots are 2.0000000 and 1.0000000 • uxa{adamm} 36> a.out • Type in values for a, b and c • 1 -2 1 Root is 1.0000000 • uxa{adamm} 37> 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 • uxa{adamm} 38> 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.

  20. 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 > (`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/(2.0*a) • the compiler generates a compile-time or syntax error: • uxa{adamm} 40> f90 Quad.f90 • NAg Fortran 90 compiler v2.2. • Error: Quad.f90, line 16: • Implicit type for RAEL_PART • detected at RAEL_PART@= • Error: Quad.f90, line 24: • Symbol REAL_PART referenced but never set • detected at QES@<end-of-statement> • [f90 terminated - errors found by pass 1]

  22. Introduction • Bugs - Run-time Errors • If we had typed • Real_Part = -b/(.0*a) • then the program would compile but we would get a run-time error, • uxa{adamm} 43> 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.f90This: • checks the program syntax • generates executable code in a file a.out • Switches or flags can be supplied to compiler: • -o < output-filename >: gives executable a different name, for example, Quad • f90 Quad.f90 -o Quad-time: report execution times f90 Quad.f90 -o Quad –timeFor more information about the compiler type: • man f90

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