Introduction to Computer Science

Raj BhatnagarDepartment of Computer Science

University of Cincinnati

Administrivia• Lecture 1-1:50, M,W 860D (Raj Bhatnagar)• Lab 2-2:50 M,W 860D (Raj and TA)• Office Hours: M,W 11-12 (Raj’s Office – 835 Rhodes)

• Lectures Focus: What is CS; Problems CS seeks to solve;

• Lab Focus: Some programming exercises in MATLAB

• Evaluation: Programs in Labs 30%• Quizzes/Tests/ 30%• HW 25%• Final Exam 15%

What is Computer Science About?

What is Computer Science About?

"Nobody is going to run 100 meters in five seconds, no matter how much is invested in training and machines. The same can be said about using the brain. The human mind is no different now from what it was five thousand years ago. And when it comes to mathematics, you must realize that this is the human mind at an extreme limit of its capacity." --Herbert Robbins, Mathematician

Technology and Innovation

For Traveling from A to B

For Problem Solving, Reasoning, Mathematical Thinking, Calculating, and everything that a brain does

As Commented by Edgar J. Dijkstra, (A well known CS Professor)

What is CS?

Computer Science is no more about computers than astronomy is about

telescopes.

- E. Dijkstra

What is CS?

Computer Science is a science of abstraction -creating the right model for a

problem and devising the appropriate mechanizable techniques to solve it.

- A. Aho and J. Ullman

What is CS?

• Understand how a human solves a problem/puzzle– Chess, Sudoku, image understanding, . . .

• Write a Recipe – a sequence of instructions on how to solve the problem

• Encode the instructions – so that a computer can follow them

Computer Science – Major Tasks

How to Achieve the following:– Represent Knowledge in electronic

machines– Represent Models of problem solutions– Perform Reasoning– Communicate and collaborate with

• Humans• Other Computers

– Transfer information to long distances

What is Computer Science?

What do CS graduates’ jobs look like?– Make computers solve interesting problems

(Applications?)• Most efficiently, effectively

– Design computers (Software + Hardware Design)

• To solve difficult problems • To integrate with business, sciences, arts,

entertainment, and our everyday life• To communicate with humans and with each other

Various Types of Software

Software to control Computers– Device Drivers

• Written in Assembler language

• Every Hardware device needs it

•Phones

•PDAs

•iPODs

• Must be very efficient

• Must know hardware and software very well

Various Types of Software

Operating Systems:Master Control Program

•“Central Nervous System”

•Controls all devices

•Talks to applications, users, devices, network

•Windows, Linux, OS2, etc.

•Written in lower level (Assembler) or kind of mid-level (C) languages

•Need to know about hardware and software

Various Types of Software

System Level Utilities:

On top of the Operating System Layer

• File Editors /Compilers/Development Tools

• Database Software

• Document Editors

• File System Managers

• Communication Software

• Security/Firewall programs

• Mail readers / Web Browsers

Various Types of Software

Networking softwareControl Communication Among Computers

• Networking protocols

• wireless communications

• internet communications

• large scale file systems on internet (music, movie archives)

Various Types of Software

Application Software – Every aspect of society• Banking, finance, investments• Engineering, science, biology, genetics• Architecture, design, art• Music, computer games, animation movies• Education, on-line tutorials• Retail, airlines, supply chain management• Manufacturing control and planning

No domain is untouched!

No aspect of life is untouched

Fastest Growing Professions - BLS

Job Market Projections by BLS

1 million new jobs by 2014

CS(BS) Salaries - 2006

UC Graduates in CS (BS) 2006

Out of 16 Graduates: (14 responded to the survey)

5 - Continuing for Graduate Degrees

9 – Entered Job Market

Avg. Salary: $76,000

Max. Salary: $140,000

Min. Salary: $48000

National Trends - 2006

Codes to Represent Information

• Morse Codes to represent letters– Use “-” and “.” to encode alphabets– Only two symbols in the “Morse-Language”

• Digital computers’ language: (0,1)– Represent all symbols in language (0,1)– Need to encode “A”, “B”. . . in (0,1)– ASCII codes

ASCII Codes

NumberBase

• Number representation in Base 2, 3, 4, . . .10• Convert from Base 10 (Decimal) to Base 2 (Binary)• Convert from Binary to Decimal• Base 16 (Hexadecimal) Numbers

– Need 16 Symbols (0, 1, 2, 3, . . . A, B, C, D, E, F)– Conversion: Hexadecimal <> Decimal– Conversion: Hexadecimal <> Octal (Base 8)– Conversion: Hexadecimal <> Base 3– Conversion: Hexadecimal <> Binary

NumberBase

(1278)10 = 1278

(1278)16 = 4FE

(1278)2 = 010011111110

(1278)8 = 2376

(1278)2 = 010011111110

(1278)4 = 103332

(1278)2 = 010011111110

Binary numbers are easily read as Hexadecimal numbers

Fractions in Binary

What is (0.25)10 in base 2?

Converting fractions to Binary• Step 1: Begin with the decimal fraction and multiply by 2. The whole number

part of the result is the first binary digit to the right of the point.– Because .625 x 2 = 1.25, the first binary digit to the right of the point is a 1.

So far, we have .625 = .1??? . . . (base 2) .• Step 2: Next we disregard the whole number part of the previous result (the

1 in this case) and multiply by 2 once again. The whole number part of this new result is the second binary digit to the right of the point. We will continue this process until we get a zero as our decimal part or until we recognize an infinite repeating pattern.

– Because .25 x 2 = 0.50, the second binary digit to the right of the point is a 0.So far, we have .625 = .10?? . . . (base 2) .

• Step 3: Disregarding the whole number part of the previous result (this result was .50 so there actually is no whole number part to disregard in this case), we multiply by 2 once again. The whole number part of the result is now the next binary digit to the right of the point.

– Because .50 x 2 = 1.00, the third binary digit to the right of the point is a 1.So now we have .625 = .101?? . . . (base 2) .

• Step 4: In fact, we do not need a Step 4. We are finished in Step 3, because we had 0 as the fractional part of our result there.

– Hence the representation of .625 = .101 (base 2) .

Infinite Binary Fractions• Step 1: Begin with the decimal fraction and multiply by 2. The whole number part of the

result is the first binary digit to the right of the point.– Because .1 x 2 = 0.2, the first binary digit to the right of the point is a 0.

So far, we have .1 (decimal) = .0??? . . . (base 2) .• Step 2: Next we disregard the whole number part of the previous result (0 in this case)

and multiply by 2 once again. The whole number part of this new result is the second binary digit to the right of the point. We will continue this process until we get a zero as our decimal part or until we recognize an infinite repeating pattern.

– Because .2 x 2 = 0.4, the second binary digit to the right of the point is also a 0.So far, we have .1 (decimal) = .00?? . . . (base 2) .

• Step 3: Disregarding the whole number part of the previous result (again a 0), we multiply by 2 once again. The whole number part of the result is now the next binary digit to the right of the point.

– Because .4 x 2 = 0.8, the third binary digit to the right of the point is also a 0.So now we have .1 (decimal) = .000?? . . . (base 2) .

• Step 4: We multiply by 2 once again, disregarding the whole number part of the previous result (again a 0 in this case).

– Because .8 x 2 = 1.6, the fourth binary digit to the right of the point is a 1.So now we have .1 (decimal) = .0001?? . . . (base 2) .

• Step 5: We multiply by 2 once again, disregarding the whole number part of the previous result (a 1 in this case).

– Because .6 x 2 = 1.2, the fifth binary digit to the right of the point is a 1.So now we have .1 (decimal) = .00011?? . . . (base 2) .

Infinite Binary Fractions

(0.1)10 = (0.00011001100110011)2

Lab Assignment for Today Write a matlab program that accepts a number with a fraction Part as input and converts both parts to a given base.

Representation?

• How are their representations different?– A character “1” (ascii representation is “01001001”)– A bit “1” (“1”)– A value of 1 (00000001)– A value 0f 1.0( ?? )

• Representation is a mere abstraction– Abstract concepts (character 1, value 1, bit 1)– Each has a different concrete representation

• Can you peek into the bit string in a memory and decide what you are seeing?

We need Compact Code

• An image with 1 million pixels• Each pixel has three color-levels (RGB) (0-255)

– 3 bytes per pixel– 3 million bytes to store an image

• Can we make the code compact?– All 255 values are not used in an image– Some values are used more often than the others– Does this information help in designing a compact

code?

Compact Code– Example: – Alphabet: (A, B, C, D, E, F, G, H)– Bits needed to represent each symbol: 3– A book written with above alphabet has 1 million

letters– Bits needed to store the book: 3 million– Frequency with which letters are used is:– (A B C D E F G H)– (0.2 0.05 0.05 0.15 0.2 0.1 0.20 0.05)– How do we decide codes for these alphabets?

– READ about HUFFMAN CODES on Wikipedia

Huffman Coding

http://en.wikipedia.org/wiki/Huffman_coding

Things you should know by now

• How to convert a number from one base to another base– Bases 2 through 16– Numbers containing fractional parts

• What are ASCII codes• Why fixed-width codes are better than variable-width

codes• Why variable-width codes are better than fixed-width

codes• MATLAB

– Basic statements– Simple programs for base conversion