Problem Solving via Problem Solving via ComputersComputersCS 1428 Week 1

Texas State UniversityTom O’HaraSpring 2011

Chapter 1 SynopsisChapter 1 SynopsisImportant

◦Sections 2, 3Useful

◦Sections 1, 4, 5, 6Ignore

◦Section 7◦Table 1-1 (programming languages)◦Serendipity Booksellers project

OverviewOverviewSyllabus and textProblem solving via algorithmsModels for computersHardware and softwareFeatures of C++ programming

languageGenerating machine code from

C++

Computer ScienceComputer ScienceComputers are general-purpose

machines for performing calculations◦Originally just a glorified calculator◦Now quite versatile and pervasive

Computer science studies of how to solve problems on computers◦Focused on abstract model of computers◦Avoids vendor specifics (e.g., IBM vs.

Apple)◦More than just computer programming

Problem Solving via Problem Solving via ComputerComputerAlgorithms: precise specification of how

to solve a particular problemFlowcharts: schematic representation

of steps in solving a problem (old school)

Pseudo-code: instructions that combine aspects of programming languages and natural language

Program: instructions in artificial language designed for computer problem solving

Sample algorithmSample algorithm

For each hour of dayLet temp be current temperatureIf temp less than 32 then

print "Below freezing"Otherwise

print "At or above freezing"Wait one hour

Note: displays hourly status about freezing temperature

Sample flow chartSample flow chart

Sample pseudo-codeSample pseudo-code

hour = 0while (hour < 24) do

temp = read_value() if (temp < 32) then

print("Below freezing") else

print("Above freezing") hour = hour + 1

end while

Sample C++ ProgramSample C++ Program#include <iostream> // standard I/O operations

using namespace std; // import standard names

int main () {

double temperature; // temperature in Farhenheit

for (int hour = 0; hour < 24; hour++) {

cout << "Enter current temperature: ";

cin >> temperature;

if (temperature < 32) {

cout << "It is below freezing" << endl;

}

else {

cout << "It is at or above freezing" << endl;

}

}

return (0);

}

AlgorithmAlgorithmPrecise specification on solving a

problemCentral to computer science

◦First solve problem via algorithm◦Then implement algorithm via program

Example◦Goal: Drive from South Austin to Texas State◦High-level Algorithm

Get on I35 at exit 226 (Slaughter Lane)Drive 20 miles southTake right on exit 206 (Aquarena Springs Drive)

Characteristics of Bad Characteristics of Bad AlgorithmsAlgorithms

Vague descriptionsex: Take I35 to San Marcos

Omits important detailsNot mentioning exit 206 in San Marcos

Overly wordy descriptionsex: Get on I35 by Walmart, taking access road along South Park Meadows, passing I30 toll, ...

In C++ or another programming language

This is a program not an algorithm

Characteristics of Good Characteristics of Good AlgorithmsAlgorithms

Mostly in a natural language (e.g., English)

Precise specification of problemGet on I35 at exit 226, drive for 20 miles south, get off I35 at exit 206, ...

Avoids technical terms and jargonDepress the decelerator if state officer

detected=> Hit the breaks if you spot a trooper

Minimal use of "programeese"Not written using C++ syntax

Algorithm DevelopmentAlgorithm DevelopmentApproach

◦Top-level from high-level description of task

◦Stepwise refinement from English into high-level pseudo-code (but not program)

Example:◦Task: Convert dollars to pesos◦Initial algorithm

Get amount of dollarsGet currency exchange for pesosDivide dollars by exchange rate

Group Exercise on Group Exercise on AlgorithmsAlgorithmsDone in separate groups

◦Have one person record algorithm to hand in

Task: Write algorithm to do following◦Convert Celsius to Fahrenheit

Fahrenheit = 9/5 Celsius + 32

◦Also give clothing advice for temperature ex: Wear heavy parka if temperature below 32

Tip:◦Be specific but not overly wordy◦Write as if explaining to a younger sibling

Computer HardwareComputer HardwareCPU: combines control unit and ALURegisters: high speed storage in CPU

◦Usually just few dozen given expense Main Memory: temporary storage for

dataSecondary Storage: persistent

memory for dataInput Devices: keyboards, mice, etc.Output Devices: monitors, printers,

etc.

Computer SoftwareComputer SoftwarePrograms and associated dataTypes

◦System Operating System Device Drivers System utilities

◦Application Word processing and other office

applications Web browsers

Turing MachineTuring MachineNote: For CS1428, this is mainly FYI material

Early model of computerDesigned by English mathematician Alan

Turing◦Famous for cracking German Enigma machine

Important in computational theoryFeatures

◦Infinite tape◦Operates only on current cell based on contents◦Can just change symbols or erase

Turing Machine ExampleTuring Machine Example

von Neumann Machinevon Neumann MachineModel developed by John von Neumann

◦Mathematician at Princeton◦Pronounced as /noy man/

Foundation for modern computers◦Thus very important (see CS 1428 objectives)

Features:◦Programs and data both reside in memory:

“stored program computer”◦Control unit for fetch/decode/execute cycle◦Separate unit for basic operations (add, etc.)

Example von Neumann Example von Neumann MachineMachine

Programming Language Programming Language TypesTypesMachine: based on CPU

operations10001001000011011100000010000011

Assembly: symbolic version of aboveADL $13, %eax

High-level: usual mathematical notationx + 13

C LanguageC LanguageDeveloped at Bell Labs in early

70’sHigh-level languageSome lower-level features

Direct access to memorySomewhat cryptic

*p++; // result = *p; p = p + 1;

rate = (age>17) ? adult : child;

C++ LanguageC++ LanguageDeveloped circa 1980 by Bjarne

StroustrupAdds additional features to C

◦Object-oriented programming◦Namespaces◦Templates◦Exception handling◦Operator overloading

Not covered in depth until CS 2308

Program Elements in C++Program Elements in C++Variables: user names for

memoryConstants: numbers and stringsOperators: mathematical and

logicalComments: notes about programKeywords: reserved wordsFunctions: user-defined

operations

Currency Conversion Currency Conversion ProgramProgram

#include <iostream> // standard I/O operations

using namespace std; // simplify access to internals

int main ()

{

double rate = 13.07;

double dollars, pesos;

// Get number of dollars and convert to pesos

cout << "Amount in dollars: ";

cin >> dollars;

pesos = dollars * rate;

// Output result with optional qualification

cout << "In pesos: " << pesos << endl;

if (pesos < 100) cout << "Pobrecito!" << endl;

return (0);

}

Miscellaneous C++ IssuesMiscellaneous C++ IssuesLines versus statements

Statements don’t end until semicolon occursEx: value = 1 * digit4 + 2 * digit3

+ 4 * digit2 + 8 * digit1;Syntax versus Semantics

Syntax indicates format of expressions, etc.; semantics determines how interpreted.

Procedural versus Object-orientedStep by step specification versus use of agent-like

objects that encapsulate their behavior.

From C++ to Machine From C++ to Machine CodeCodePreprocessor

◦Loads common definitions: Ex: #include <iostream>

◦Performs miscellaneous text substitutions

Compilation◦Creates relocatable machine code

Linking◦Adds library code and creates

executable

Student ExerciseStudent ExerciseDone by each studentConvert Celsius to Fahrenheit

◦Fahrenheit = 9/5 Celsius + 32 Tip

◦Use currency conversion program as sample

◦Don't panic: just to get you thinking in C++