How Are Computers Programmed? CPS120: Introduction to Computer Science Lecture 5.
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Transcript of How Are Computers Programmed? CPS120: Introduction to Computer Science Lecture 5.
How Are Computers Programmed?How Are Computers Programmed?
CPS120: Introduction to Computer ScienceLecture 5
Problem SolvingProblem Solving
Is an art, not a science It must be experienced, it can't be taught
Guidelines can be provided– Based on experience– Sound like platitudes– Are internalized as we learn to solve problems
It requires patience
Problem Solving TasksProblem Solving Tasks
Establish the context of the problemUnderstand the problemDetermine the primary goals of the solutionCreate a solution
Problem Solving GuidelinesProblem Solving Guidelines One big problem is unsolvable
Many small problems can be managed Understand the problem: What is needed?
Write on paper what results are expectedWhat functions (procedures) will be required to generate these resultsWhat information will need to be given to these functionsDescribe what the functions do before you write a single line of code
Programs
A program is a set of step-by-step instructions that directs the computer to do the tasks you want it to do and produce the results you want.
The Program Development CycleThe Program Development Cycle
Get the program into machine-readable form
Test and debug the program
Translate the program
Understand the problem
Code the program
Plan the program's logic
Document the program
Understand the problemUnderstand the problem
Become familiar with what the program is trying to accomplish
Read the specificationsAsk questionsUnderstand the specifications
Eliminate extraneous information
Plan the Program’s LogicPlan the Program’s Logic Decide how to best meet the program’s
specifications Often uses a flowchart or pseudocode Divide the program into subroutines, functions or
modules Subroutines cost less and take less time to maintainCan be used in other programsSeveral programmers can work on the same project
Code the ProgramCode the Program
Put the solution, generally documented with pseudocode or a flowchart into a programming language
Machine-Readable FormMachine-Readable Form
Key the program into a computer
Translate the programTranslate the program
Translated into machine language using an assembler, a compiler or an interpreter
Includes the elimination of syntax errors Generate executable file
Test the ProgramTest the Program Logic errors are not listed during the translation
projects Only way to find logic errors is to do program
testingInvolves using sample data as inputExecuting the programChecking the results manually
Know as debugging
Document the ProgramDocument the Program Internal Documentation
Comments External Documentation
Program specificationLayout chartHierarchy chartProgram flowchartPseudo-codeData Dictionary
– Input, Output, Work Area– Name, description, type, initial value, calculation
Source listingTest Plan
Programming LanguagesProgramming Languages
A programming language is a set of rules that provides a way of telling a computer what operations to perform.
Levels of Programming LanguagesLevels of Programming Languages
Machine language Assembly Language High Level Languages Fourth Generation Languages (4GL)
Machine Languages
different for each computer processor
0100001101 100000 001101 11000100101 10001 1000001110111001. . .
Assembly Languages
different for each computer processor
main proc paymov ax, dsegmov ax, 0b00hadd ax, dxmov a1, b1mul b1, axmov b1, 04h
High-Level LanguagesHigh-Level Languages
Higher Level LanguagesUse traditional programming logic where the programming instructions tell the computer what to do and how to perform the required operations.
4GLsUse high-level English-like instructions to specify what to do, not how to do it .
Interpreter vs. CompilerInterpreter vs. Compiler
InterpreterTranslates instructions to machine code line-by-line.
CompilerTranslates the entire program to machine code before running it.
Types of Programming LanguagesTypes of Programming Languages
Machine language Procedure-oriented languages Object-oriented languages Event-driven languages
Early Language HistoryEarly Language History FORTRAN (short for Formula Translator, developed in the 1950s by IBM In 1958, a language called ALGOL (Algorithm Language) was developed COBOL (Common Business Oriented Language) was created in 1960 to serve
as the primary language for large-scale programs In 1964, the BASIC language (Beginners All-Purpose Symbolic Instruction
Code) was first used In 1965, a language called PL/I was developed in hopes of being everything to
everyone. PL/I proved to be too complex. In the late 1960s, Niklaus Wirth developed a teaching language called Pascal.
Later Language HistoryLater Language History Ada, which was developed in 1983, is large and complex. Smalltalk is graphical and object-oriented. Concepts developed
with Smalltalk were important to the development and continued development of languages like C++ and Java
The C language was derived from ALGOL. C++ is C with the addition of object-oriented concepts.
Procedure-Oriented LanguagesProcedure-Oriented Languages
FORTRAN COBOL Pascal C Ada
OOED LanguagesOOED Languages
Object-oriented languagesSmalltalkC++Ada 95
Event-driven languagesVisual Basicmost Visual languages
What Can a Program Do?
A program can only instruct a computer to:Read InputSequenceCalculateStore dataCompare and branchIterate or LoopWrite Output
Fundamental Programming ConceptsFundamental Programming Concepts
Assignment of values to a variable Iteration (Looping)
Over a set of set of statementsWith respect to a logical expressions (conditions)
Delegation of sub-tasks to functions / procedures
The Structure TheoremThe Structure Theorem
The Structure Theorem states that any algorithm can be built from three basic control structures.
One-after-another (Sequence) Decision-making (Selection)
Making choices between 2 or more alternatives Repetition (Iteration)
Concerned with repetitive tasks (and the termination conditions of loops)
C++ Control StructuresC++ Control Structures
1. "Sequence statements" are imperatives2. "Selection" is the "if then else" statement
– AND, OR, NOT and parentheses ( ) can be used for compound conditions
3. "Iteration" is satisfied by a number of statements– "while" – " do " – "for"
4. The case-type statement is satisfied by the "switch" statement.
– CASE statements are used for most non-trivial selection decisions
Sequence Control Structures Sequence control structures direct the order of
program instructions. The fact that one instruction follows another—in
sequence—establishes the control and order of operations.
Calculate
A program can instruct a computer to perform mathematical operations.
Add 1 to
Counter
Store
A program will often instruct a computer to store intermediate results.
Place 1 in
Counter
Compare and Branch
A program can instruct a computer to compare two items and do something based on a match or mismatch which, in turn, redirect the sequence of programming instructions.
There are two forms:IF-THENIF-THEN-ELSE
IF-THEN
Test Test condition pcondition p
falsefalse truetrue
EntryEntry
ExitExitTrue True
statement astatement a
IF-THEN-ELSE
falsefalse truetrue
EntryEntry
ExitExit
Test Test condition pcondition p
““true” true” statement astatement a
““false” false” statement astatement a
Iterate
A program loop is a form of iteration. A computer can be instructed to repeat instructions under certain conditions.
No
Iteration Control Structures
Iteration control structures are looping mechanisms.
Loops repeat an activity until stopped. The location of the stopping mechanism determines how the loop will work:
Leading decisions Trailing decisions
Leading Decisions
If the stop is at the beginning of the iteration, then the control is called a leading decision.
The command DO WHILE performs the iteration and places the stop at the beginning.
DO WHILE Loop
NoNo
YesYes
EntryEntry
ExitExit
Test Test condition pcondition p
Loop Loop statement astatement a
Trailing Decisions
If the stop is at the end of the iteration, the control mechanism is called a trailing decision.
The command DO UNTIL performs the iteration and puts the stop at the end of the loop.
DO UNTIL Loop
Loop Loop statement astatement a
NoNo YesYes
EntryEntry
Test Test condition pcondition p
ExitExit
Programmer Productivity ToolsProgrammer Productivity Tools
Modular Programming Structured Programming Object-oriented Programming
Modular ProgrammingModular Programming
Large programs are divided by functional parts into subroutines
Strong cohesionLoose coupling
Structured Programming
Composed of sequence, decision (selection), and repetition (looping or iteration) structures
Structured program languages lend themselves to flowcharts, structure charts, and pseudocode.
Structured programming languages work best where the instructions have been broken up into small, manageable parts.
Looks at a problem as proceduresData are maintained separately from data
Structured Program RulesStructured Program Rules
1. Use only sequence, decision, and repetition2. Only one entrance into and one exit from a structure3. Connectors only allowed when continuing processing
from one column or page to another4. Decision and repetition structures can be nested5. Only one STOP instruction is permitted. It must be in
the MAINLINE routine
Structured Programming AdvantagesStructured Programming Advantages
Standard method for solving problems GO-TO less Easier to test and debug Written by more than one programmer Reusability Thrashing minimized
Object-Oriented ProgramsObject-Oriented Programs Developed to respond to programming issues that
structured programming did not adequately address1. Rarely possible to anticipate the design of a completed
system before implementation2. GUIs were difficult to develop in traditional procedure-
oriented languages3. Sharing data across routines is error prone
Information hiding allows programmers to determine what data is exposed to various routines