Algorithms and Problem Solving

Lec01

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Outline1.1 What is a Computer?1.2 What is Programming?1.3 The Anatomy of a Computer (schematic)1.4 Translating Human-Readable Programs to Machine Code (use high level language)1.5 Programming Languages1.6 The Evolution of C++1.7 Becoming Familiar with Your Computer (save ur works)

1.8 Compiling a Simple Program1.9 Errors1.10 The Compilation Process1.11 Algorithms?

Why are we here?

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Why are we here?

• To study “Computer Programming”Simple answer right?

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Why do we study computer programming ?

• To solve problems using algorithms.

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Why do we study computer programming ?

• Subject name: Computer Programming I

• What is a computer?

• What is programming?

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What is computer programming?

1.1 What Is a Computer?

• Good at repetitive tasks• Faster than humans; not smarter• Stores data• Interacts with devices• Flexible: can handle a wide range of tasks.

Runs programs to handle different tasks:– Record daily expenses and incomes– Create/View/Edit presentation slides– Play games

What is a Computer?

• Executes a huge number of primitive operations

• Gives the illusion of smooth interaction because it executes these operations at great speed

Fact: History of “Computer” nameThe ENIAC and the Dawn of Computing• electronic numerical integrator and computer• First usable electronic computer• Designed by J. Presper Eckert & John Mauchly, at Upenn• Completed in 1946 – 2 years before transistors• Many cabinets, in a large room• 18,000 vacuum tubes• Programmed by connecting wires on panels (patch panels)• Supported by Navy, for ballistic calculations• Before ENIAC, people who did these calculations were called

"computers"• Later used for tabulating US Census data, etc.

1.2 What is Programming?

• Programming — act of designing and implementing programs

• Most users (word processing, spreadsheeting) are not programmers

• Programming is an essential skill for a computer scientist and IT professional

1.3 The Anatomy of a Computer (Schematic)

The Anatomy of a ComputerCPU• Central Processing Unit• Performs program control, arithmetic, and data

movement• One operation at a time• Locates and executes program instructions• Consists of a single, or small number of, chip(s) made

of plastic, metal and mostly silicon• Composed of several million transistors• Enormously complicated wiring

CPU Chip/Processor Detail

The Anatomy of a Computer

Memory• RAM (Random Access Memory): read-write memory, the

opposite is the sequential memory• ROM (Read Only Memory): contains certain programs that

must always be present, the opposite is read/write memory• Secondary storage (e.g., a hard drive) provides persistent

storage

Bus• Bus: a set of electrical lines that connect the CPU, RAM, slots

and other connectors

The Anatomy of a Computer

Motherboard• The motherboard holds the CPU, memory, and bus,

as well as card slots and other connectors

The Anatomy of a ComputerPeripherals• Allow the computer to interact with user and

other computers:– Display / Monitor / LCD– Printer– Mouse– Keyboard– Speakers– Network card– Modem / WiFi Modem

The Anatomy of a Computer

Networks• Allow a computer to talk to other devices:

– Other computers– Central data storage– Shared printer

1.4 Translating Human-Readable Programs to Machine Code

Machine instructions:• Human language:

– Move memory contents location 40000 to a register– Subtract the value 100 from a register– If result is positive, jump (go) to another instruction at location 11280

• Extremely primitive. Encoded as numbers: 161 40000 45 100 127 11280

where 161 = move, 45 = substract, 127 = jump if >0• Thousands of instructions for a simple program• Each processor has its own set of machine instructions• Adding numeric codes manually is tedious and error prone

Translating Programs to Machine CodeAssembler• Use computer to translate• Assigns short names (mnemonic) to commands: mov 40000, %eax

sub 100, %eax

jg 11280• Makes reading easier for humans• Translated into machine instructions by the assembler• Can give names to memory locations• -'ve: Still processor dependent• -'ve: Still a great many instructions

// move memory contents location 40000 to a register// eax = processor register

// subtract the value 100 from a register

// if result is positive, jump to another instruction at location 11280// jg = jump if greater than 0

Translating Programs to Machine CodeHigher-level languages:• Easiest for humans to read and write: int eax = loc40000; (int loc40000 = 150;)eax = eax – 100;if (eax > 0) cout << “Positive number”;

• Helped/Translated by compilers into machine instructions– Very sophisticated programs– Translate logical statements into sequences of computations 161 40000 45 100 127 11280 – Find memory locations for all variable names

• Independent of the underlying hardware

1.5 Programming Languages• Designed for a variety of purposes• Machine independent (generally)• E.g. of Programming Languages: C++(also C),

Java, C#, Python• Much easier to read, but...• Much stricter than spoken languages.

– Compilers are less forgiving. For eg, if you omit the the second quotation mark of a C++ statement, the C++ compiler will get confused and complain that it cannot translate an instruction containing this error.

1.6 The Evolution of C++• Many languages are created with a specific purpose

– database processing– "artificial intelligence"– multimedia processing

• General purpose languages can be used for any task• C: developed to be translated efficiently into fast

machine code, with minimal housekeeping overhead• C++: adds "object oriented programming" aspects to

C

Programming Language Design and Evolution

History of C• Initially designed in 1972 (Kernighan & Ritchie)• Features were added from time to time in

response to perceived shortcomings• Resulted in different dialects

– Bad for portability

• 1989 — ANSI standard of C completed

Programming Language Design and Evolution

C++• 1979 — Bjarne Stroustrup of AT&T adds

object oriented features to C, called C with Classes

• 1985 — rename to C++• Other features added over the years• 1998 — First ISO C++98 standard published• 2003 — minor revision to the ISO std, ISO C++03

• 2009 — major revision expected, ISO C++0x

Programming Language Design and Evolution

C++• Most commonly used language for systems

such as:– Database– OS

• Growing popularity in embedded systems

Fact: What standard for?Standards Organizations• 2 groups have jointly developed the C++ standard:

– ISO — Int'l Organization for Standardization– ANSI — American National Standards Institute

• Associations of industry professionals• Many standards:

– Car tires– Shape of ATM cards– Programming languages– etc.

Take a Break

1.7 Becoming Familiar with Your Computer

• Log In• Locate the C++ compiler• Understand files and folders• Write a simple program• Save your work

Hint: ProductivityBackup Copies• Use a thumb drive, memory stick, external drive, or Internet

Storage• Back up often• Rotate backups. For eg, use three folders for backups and rotate

them• Back up source files only• Pay attention to the backup direction

– eg: copy from C: drive to D: drive or D to C. If you do it the wrong direction, you will overwrite a newer file with an older version

• Check your backups once in a while• Relax, then restore: when you lose a file and need to restore it from backup

1.8 Compiling a Simple Program// line comment/* block comment */

/* my first program in C++ with block comment */

#include <iostream>

using namespace std;

int main (){ cout << "Hello, World!\n"; // says Hello World!

return 0;}

> g++ “Lec01 hello world.cpp”> a.exe

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C++ Comments ?

C++ Comments are pieces of source code discarded from the code by the compiler. They do nothing. Their purpose is only to allow the programmer to insert notes or descriptions embedded within the source code.

C++ supports two ways to insert comments:

// line comment/* block comment */

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Layout of a Simple C++ Program ?#include <iostream>// other header files

using namespace std;

int main(){

variable_declarations

statement_1statement_2…statement_last

return 0;}

Compiling a Simple Program

• C++ is case sensitive

• When run, yields:Hello, World!

Compiling a Simple Program

• #include<iostream> — read the iostream header file

• using namespace std; — all names in the program belong to the "std" namespace

• int main () { ... } defines the main function– All C/C++ programs have a main() function

Compiling a Simple Program

• statements (instructions) inside the body of the main function:– executed one by one, in order (by sequence

control flow)– Each statement ends with a semicolon ;

• return is the end of the main function– 0 value signals that the program ran successfully

Compiling a Simple Program• Characters enclosed in double quotes ("Hello, World\

n") called a string• \ is the escape character:

• Escape sequence \n indicates a newline• Some others:

– \r = carriage return– \" = print a double quotation mark – \\ = print a single backslash– \t = tab– \a = alarm alert with beep sound

Compiling a Simple Program

Output• cout — the class output stream object (output

onto the screen)• << — the “insert" operator to output onto the

screen– Understands all built-in types– E.g., to print an integer: cout << 3 + 4 ; // 7

1.9 Errors

Syntax (or compile-time) errors• are faulty input, not quite legal C++. E.g., cot << "Hello, World!\n"; cout << "Hello, World!\"; // second double quote is missing

• Something violates the language rules• Compiler finds the errors, reports them: hello.cpp:7: error: 'cot' was not declared in this scope hello.cpp:7: error: missing terminating " character • Cascade: always start fixing errors from the top• Also, do not ignore warnings (ask if students want example)

ErrorsLogic or run-time error• Program compiles fine (input is legal) but Program doesn't do

what it's supposed to do• Much harder to find• Program author must test and find the error

cout << "Hell, World\n";

Overflow errors• Result falls outside computer's numeric range i.e., too big

(many digit) or too small (many decimal points). Eg, 1010000 = Overflow calculator.

Roundoff errors• Occurs when a value cannot be represented precisely.

Eg, 1/3 may be stored in the computer as 0.33333, a value that is close to, but not exactly equal to 1/3.

Errors• Testing

– Validating program correctness – Very important; discussed often in this text

• Debugging (remove bugs)– Finding the source of an error – A debugger is a handy tool

• Defensive Programming– Crafting programs to limit, minimize, localize errors

in one place if they do occur and the programs do not trigger an extremely bad response.

Common Error (syntax error)Don’t omit Semicolon:• Compiler ignores newlines• Uses semicolons to separate statements/instructions• Compiler sees this:

cout << "Hello, World!\n"return 0;as a single statement:cout << "Hello, World!\n" return 0;

• Make sure all your statements end in a semicolon– Even the last statement before a closing brace

Common Error (syntax or logic error)

Misspelling Words:• Misspelled words aren't obvious, make error

messages ambiguous. Eg: main()-> Main() • The compiler will compile your Main function

but the linker will complain about the “missing main function” and refuse to link the program.

• C++ is case-sensitive– All keywords and most library functions are lower-

case

1.10 The Compilation Process

Integrated desktop environments (IDEs) like CodeBlocks, Notepad++ shield user from compilation details:•Enter code in a window•Click a button to "compile" and "run".

So what is actually going on? See next slide…

The Compilation Process

• The compiler translates your C++ source into object code (*.o files)

• The linker takes your object files and code from various libraries, outputs an executable file

• Libraries contain (already translated/compiled) code, such as iostream

Figure 9: From Source Code to Executable Program

The Compilation Process

The process:• Compile• Errors? Edit file. Compile• Run• Errors? Edit file. Compile• Test• Errors? Edit file. Compile

Figure 10: Edit-Compile-Debug loop

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1.11 Algorithms?

• Algorithm– A sequence of precise instructions for solving a

problem or carrying out a procedure that will terminate at some point.

ProblemSolving

Algorithms

Goals Checklist

• To understand the activity of programming• To learn about the architecture of computers• To learn about machine languages and higher-

level programming languages• To become familiar with your compiler• To compile and run your first C++ program• To recognize syntax and logic errors• To understand the notion of an algorithm to

solve problem