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