MIS 4395.A Structured Programming Language

1. MIS 4395.AStructured Programming Language Ahmed Imran Kabir Week 1, Introductory Class

2. Outline of Topics • Hardware/Software interface • Layers of the Machine • Kinds of Software • Computer Languages • Syntax, Semantics, Grammars • What happens to your program? • Compilation, Linking, Execution • Program errors • Compilation vs. Interpretation etc.

3. Software Categories • System SW • Programs written for computer systems • Compilers, operating systems, … • Application SW • Programs written for computer users • Word-processors, spreadsheets, & other application packages

4. A Layered View of the Computer Application ProgramsWord-Processors, Spreadsheets, Database Software, IDEs,etc… System SoftwareCompilers, Interpreters,Preprocessors, etc.Operating System, Device Drivers Machine with all its hardware

5. Operating System (OS) • Provides several essential services: • Loading & running application programs • Allocating memory & processor time • Providing input & output facilities • Managing files of information

6. Programs • Programs are written in programming languages • PL = programming language • Pieces of the same program can be written in different PLs • Languages closer to the machine can be more efficient • As long as they agree on how to communicate • A PL is • A special purpose and limited language • A set of rules and symbols used to construct a computer program • A language used to interact with the computer

7. Computer Languages • Machine Language • Uses binary code • Machine-dependent • Not portable • Assembly Language • Uses mnemonics • Machine-dependent • Not usually portable • High-Level Language (HLL) • Uses English-like language • Machine independent • Portable (but must be compiled for different platforms) • Examples: Pascal, C, C++, Java, Fortran, . . .

8. Machine Language • The representation of a computer program which is actually read and understood by the computer. • A program in machine code consists of a sequence of machine instructions. • Instructions: • Machine instructions are in binary code • Instructions specify operations and memory cells involved in the operation Example:

9. Assembly Language • A symbolic representation of the machine language of a specific processor. • Is converted to machine code by an assembler. • Usually, each line of assembly code produces one machine instruction (One-to-one correspondence). • Programming in assembly language is slow and error-prone but is more efficient in terms of hardware performance. • Mnemonic representation of the instructions and data • Example: Load Price Add Tax Store Cost

10. High-level language • A programming language which use statements consisting of English-like keywords such as "FOR", "PRINT" or “IF“, ... etc. • Each statement corresponds to several machine language instructions (one-to-many correspondence). • Much easier to program than in assembly language. • Data are referenced using descriptive names • Operations can be described using familiar symbols • Example: Cost := Price + Tax

11. Syntax & Semantics • Syntax: • The structure of strings in some language. A language's syntax is described by a grammar. • Examples: • Binary number <binary_number> = <bit> | <bit> <binary_number> <bit> = 0 | 1 • Identifier <identifier> = <letter> {<letter> | <digit> } <letter> = a | b | . . . | z <digit = 0 | 1 | . . . | 9 • Semantics: • The meaning of the language

12. Syntax & Grammars • Syntax descriptions for a PL are themselves written in a formal language. • E.g. Backus-Naur Form (BNF) • The formal language is not a PL but it can be implemented by a compiler to enforce grammar restrictions. • Some PLs look more like grammar descriptions than like instructions.

13. Compilers & Programs • Source program • The form in which a computer program, written in some formal programming language, is written by the programmer. • Can be compiled automatically into object code or machine code or executed by an interpreter. • Pascal source programs have extension ‘.pas’

14. Compilers & Programs • Object program • Output from the compiler • Equivalent machine language translation of the source program • Files usually have extension ‘.obj’ • Executable program • Output from linker/loader • Machine language program linked with necessary libraries & other files • Files usually have extension ‘.exe’

15. Running Programs • Steps that the computer goes through to run a program: Memory Machine language program (executable file) C P U Input Data Data entered during execution Computed results Program Output

16. Program Execution • Steps taken by the CPU to run a program (instructions are in machine language): • Fetch an instruction • Decode (interpret) the instruction • Retrieve data, if needed • Execute (perform) actual processing • Store the results, if needed

17. Program Errors • Syntax Errors: • Errors in grammar of the language • Runtime error: • When there are no syntax errors, but the program can’t complete execution • Divide by zero • Invalid input data • Logical errors: • The program completes execution, but delivers incorrect results • Incorrect usage of parentheses

18. Brief History of Python • Invented in the Netherlands, early 90s by Guido van Rossum • Named after Monty Python • Open sourced from the beginning • Considered a scripting language, but is much more • Scalable, object oriented and functional from the beginning • Used by Google from the beginning • Increasingly popular

19. Python’s Benevolent Dictator For Life “Python is an experiment in how much freedom program-mers need. Too much freedom and nobody can read another's code; too little and expressive-ness is endangered.” - Guido van Rossum

20. Installing • Python is pre-installed on most Unix systems, including Linux and MAC OS X • The pre-installed version may not be the most recent one (2.6.2 and 3.1.1 as of Sept 09) • Download from http://python.org/download/ • Python comes with a large library of standard modules • There are several options for an IDE • IDLE – works well with Windows • Emacs with python-mode or your favorite text editor • Eclipse with Pydev (http://pydev.sourceforge.net/)

21. IDLE Development Environment • IDLE is an Integrated DeveLopment Environ-ment for Python, typically used on Windows • Multi-window text editor with syntax highlighting, auto-completion, smart indent and other. • Python shell with syntax highlighting. • Integrated debuggerwith stepping, persis-tent breakpoints,and call stack visi-bility

22. Python Scripts • When you call a python program from the command line the interpreter evaluates each expression in the file • Familiar mechanisms are used to provide command line arguments and/or redirect input and output • Python also has mechanisms to allow a python program to act both as a script and as a module to be imported and used by another python program

23. The Basics

24. A Code Sample (in IDLE) x = 34 - 23 z = 3.45 print x print z print (x + z) print (z*x)

25. Enough to Understand the Code • Indentation matters to code meaning • Block structure indicated by indentation • First assignment to a variable creates it • Variable types don’t need to be declared. • Python figures out the variable types on its own. • Assignment is = and comparison is == • For numbers + - * / % are as expected • Special use of + for string concatenation and % for string formatting (as in C’s printf) • Logical operators are words (and, or, not) not symbols • The basic printing command is print

26. Naming Rules • Names are case sensitive and cannot start with a number. They can contain letters, numbers, and underscores. bob Bob _bob _2_bob_ bob_2 BoB • There are some reserved words: and, assert, break, class, continue, def, del, elif, else, except, exec, finally, for, from, global, if, import, in, is, lambda, not, or, pass, print, raise, return, try, while

27. Assignment • You can assign to multiple names at the same time >>> x, y = 2, 3 >>> x 2 >>> y 3 This makes it easy to swap values >>> x, y = y, x • Assignments can be chained >>> a = b = x = 2

28. Accessing Non-Existent Name Accessing a name before it’s been properly created (by placing it on the left side of an assignment), raises an error >>> y Traceback (most recent call last): File "<pyshell#16>", line 1, in -toplevel- y NameError: name ‘y' is not defined >>> y = 3 >>> y 3