Transcript cs1102_12B_lec07 - Department of Computer Science

CS1102 Lec06 –
Programming Languages
& Program Development
Semester B, 2012-13
Computer Science Department
City University of Hong Kong
Objectives
 Differentiate between machine and assembly languages
 Differentiate between markup languages and general-purpose
programming languages
 Differentiate between scripting languages and general-purpose
programming languages
 Identify the steps in a typical program development cycle
 Explain the basic control structures in structural design
 Use the basic control structures (sequence, selection, repetition) to
design solutions for simple problems
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Programming Languages
 Computer program – a series of instructions that that directs a
computer to perform tasks
 The instructions that make up a computer program are sometimes referred to
as code
 Programming language - a set of keywords and grammar rules for
creating program code
 E.g., BASIC, C, C++, Pascal, FORTRAN, Java, COBOL
 Low-level languages typically include commands specific to a
particular CPU or microprocessor family
 High-level languages use command words and grammar more similar
to human languages
 Programming - the process of writing/coding a program in a
specific programming language to solve some problem
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Program Code
 Program code contains
 Keyword: a reserved word that carries a pre-defined meaning
 E.g., var function if
else for return
 Data types: data of a certain type
 E.g, int float string
 Variables: a name refers to the memory space allocated to store a
value
 E.g., var i = 2; var h = “Hello!”; var f = 6.95;
 Statements: one action that the CPU will take
 E.g., i = i + 1; f = Math.floor(6.95);
 Program code needs to follow certain Syntax: grammar rules that
dictate how code is written
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Low-Level Programming Language
 Machine language
 Assembly language
 The first generation of
programming language
 The ONLY language CPU
directly recognizes
 Instructions, memory
addresses, numbers,
characters and all other data
are represented by 0s and 1s
Machine code
0
001 1 000010
1
010 0 001101
2
001 1 000101
3
010 0 001110
4
001 0 001101
5
011 0 001110
6
010 0 001111
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111 0 000000
 The second generation of
programming language
 Represent machine
instructions in symbolic
form(usually are meaningful
abbreviations)
 Need an assembler to
translate each assembly
instruction into a
Assembly code
corresponding machine
LOAD #2
instruction
STORE 13
LOAD #5
STORE 14
LOAD 13
ADD 14
STORE 15
HALT
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High-Level Programming Language
 Types of high-level programming languages
 Structural / procedural: the program is a collection of step-by-step
instructions
Most widely used examples include: BASIC, COBOL, and C
 Object-oriented: the program is a collection of objects which contain
both data and instructions and the program is event-driven
 Most widely used examples include: Visual Basic, C++, Java, C#
 From high-level to low-level
 Compiler
translates a program from high-level language (source code) into lowlevel instructions (object code)
 Interpreter
translates program codes one-by-one for immediate execution
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Scripting and Markup Languages
 HTML - markup language composed of special tags that instruct the
web browsers to format and display the Web page content
 XML - method for putting structured data into a text file
 Extensible: developers can create their own meaningful tags
 Mostly used for data sharing across networks and different systems
 Scripting language - a programming language that is used to
manipulate, customize, and automate the operations of an existing
system
 Not intended for writing applications from scratch; they are intended
primarily for plugging components
Always embedded in the application with which they are associated
E.g., client-side scripts (such as JavaScript) are executed by the browser
 Most widely examples : JavaScript, Perl, PHP, Unix Shell Script
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Software Development Cycle
 Programming is a specialized form of problem solving
 by programmers or software engineers or software designers
 A typical software development cycle looks like this:
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Software Development Cycle
 1. Program specification
 Meet with customers to review the requirements, identify input,
output, processing and data
 2. Program design
 Devise the solution algorithm, i.e., a set of step-by-step procedures
for solving the problem
 Such as top-down design
 Programmer may need to check the correctness of the logic and
attempts to uncover logic errors (design flaw that causes inaccurate
results)
 Programmers may use test data to verify solution through logic
 3. Program code
 Programmers write the code that translate the design into the actual
program
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Software Development Cycle
 4. Program test
 By running the program with testing data, programmers ensure that
the program runs correctly and is error-free
 Debugging - locating and correcting the errors (also called bugs)
found in the programs
 Program errors include syntax error, logic error, runtime error
 5. Program documentation
 Programmers write down the description, reference manual, and user
help files of the program
 6. Program maintenance
 Completed programs are periodically reviewed to evaluate their
accuracy, efficiency, and east of use. Updates will be made to the
program code when needed.
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Step 1 – Analyze Requirements
 A number-guessing game
An elementary-school teacher needs a program that is a
number-guessing game so students can learn to develop
logical strategies and practice their arithmetic. In this
game, the computer picks a number between 1 and 100
and gives the player 7 turns to guess the number.
After each incorrect try, the computer tells the player
whether the guess is too high or too low.
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Step2 - Design Solution
 Divide & Conquer approach to devise an algorithm (i.e., a step-by-step
procedure)
 Initially, the original problem is divided into several smaller parts
 Each of these parts represents a smaller programming problem to solve
 The next refinement fills in a few details for each part
1. Begin the game
1.1 display instructions
1.2 generate a number
between 1 and 100
2. Repeat guessing process
until the number is guessed
Or seven turns are completed
2.1 input guess from user
2.2 respond to the guess
2.3 if seven turns are used up,
then end the game
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2.2.1 if guess == number, then
output “win” & end the game;
else-if guess < number, then
output guess-small-msg;
else output guess-big-msg
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Step 2 – Design Solution
 Programmers may perform a Desk Check to verify the algorithm
logic by using real test data to step through
 Test data: sample data that the program will process once it is in
production
Develop
various
sets of test
data
Determine
the expected
result
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Step through
the
algorithm
Compare
the results
Repeat steps
for each set
of test data
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Step 3 – Programming
 Implementation or coding of the design includes using a program
development tool that assists the programmer by:
 Generating or providing some or all code
 Writing the code that translates the design into a computer program
 Creating the user interface
A Notepad-like text editor
allows programmers to
enter lines of code
Or
an IDE (Integrated
Development Environment)
tool provides programmers
with tools to build the program
(interface + code)
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Step 4 & 5 – Test and
Documentation
 The goal of program testing is to ensure the completed program
runs correctly and is error free
 Errors include syntax errors, logic errors, runtime errors
Syntax error: occurs when the code violates the grammar requirements of
the programming language
Logic error: a flaw in the algorithm design that causes inaccurate result
Runtime error: an error that causes the program to stop running
 Debugging the program involves removing the errors (bugs)
 Documentation is the accompanying text either explains how the
program operates or how to use it
 Important for people who may be involved with the program in the
future
 May include a user manual, all charts, solution algorithm, test data,
etc.
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Structural Design
 Structural design - conceptualizes the solution as a sequence of steps
 Any computer program can be built from these three control
structures:
 Sequence
Tells computer to take actions in sequential order
 Selection
Tells computer which action to take, based on a certain condition
 Repetition
Tells computer to perform one or more actions repeatedly, until a certain
condition is satisfied or dissatisfied
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Flowchart
 Flowchart - a graphical representation of an algorithm/a program
Selection Control Structure
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While-Do Control Structure
Do-Until Control Structure
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Flowchart Example: Number-Guessing game
The program runs to “END”
for each repetition, waiting for
user’s input to “Begin” again.
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Another Example: Run-Length
Encoding Decompression
Run-length encoding (RLE) is a simple form of data compression in
which sequences of the same data value occurs consecutively are
stored as a single data value and count, rather than as the original
data sequence.
Sample Input: W5B1W2B3W4
Sample output: WWWWWBWWBBBWWWW
Sample Input: R1B1G1W8B7
Sample output: RBGWWWWWWWWBBBBBBB
Draw the flowchart for the RLE decompression algorithm, which
reads in a RLE-compressed sequence and outputs the original long
character sequence.
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Flowchart for RLE Decompression
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Lesson Summary
 Programmers(or software designers) write programs in programming
languages to instruct the computer to perform some task
 Computer programming is a specialized form of problem solving that
involves devising an algorithm
 Early generation of programming languages are low-level ones that are
different for programmers to write and read; while newer generation
of programming languages are high-level ones that are more close to
human natural languages
 To convert programs written in high-level languages to low-level
computer instructions, a compiler or interpreter is needed
 Markup language and scripting language are distinct from generalpurpose programming languages
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Lesson Summary (cont'd)
 All programs follow the similar basic structure, but may use
different design methods and languages
 Six steps in the program development life cycle used to make this
process efficient and systematic
 Three basic control structures (sequence, selection and repetition)
are used in structural design and any complex program can be built
from these three control structures
 Structural programmer often use flowchart to plan the sequence of an
algorithm
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Careers in the Computer Industry
 An information technology (IT) department is
 Department in business or government agency that employs people in
computer-related jobs, responsible for keeping all computer
operations and networks operating smoothly
 Jobs available in an IT department include…
Management
System
development
and programming
Technical services
Chief Information Officer /
E-commerce administrator Project leader / manager
VP of IT
Chief Security Officer
Network administrator
Database analyst
Systems programmer
Software engineer
Technical writer
Computer scientist
Systems analyst
Web page author
Computer forensics
specialist
Desktop publisher
Quality assurance
specialist
Computer technician
Graphic designer /
illustrator
Security administrator
Application programmer
Database administrator Network security specialist
Operations
Computer operator
Data
communications analyst
Training
Corporate trainer
Help desk specialist
Web administrator /
Webmaster
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Preparing for a Career in the
Computer or IT Industry
 Computer Science (CS) includes…
 Programs that stress the technical and theoretical side of
programming and operating systems
 Courses cover almost every aspect of the computer system
Computer Core
Information
Security
Multimedia
Computing
Software
Engineering
Systems and
Networks
Problem Solving and
Programming
Object-oriented
Programming
Software Design
Computer Organization
Database System
Operating Systems
Topics on Computer
Security
Information Security and
Management
Internet Security and
E-commerce Protocols
Internet Application
Development
Computer Graphics
Multi-modal Interface
Design
Multimedia Technologies &
Computer Games Design
Applications
Object-oriented
Methodology
Software Testing and
Maintenance
Software Quality
Management
Performance Evaluation
Computer Architecture
Software Testing and
Maintenance
Distributed Systems
Mobile Computing
Computer Vision & Image
Processing
High Speed Multimedia
Network
Web Usability Design and
Engineering
Formal Methods in
Software Engineering
Internet and Distributed
Systems Programming
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Pervasive Computing