Computer Science at Oxford

Stephen DrapeAccess/Schools Liaison Officer

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Four myths about Oxford

There’s little chance of getting in It’s expensive College choice is very important You have to be very bright

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Myth 1: Little chance of getting in False!

Statistically: you have around 20% chance

Admissions data for 2009 entry:Applications Acceptances %

Comp Sci 126 23 18.3%

Maths & CS 78 22 28.2%

Maths 917 180 19.6%

Maths & Stats 166 28 16.9%

Physics 765 183 23.9%

Chemistry 474 201 42.4%

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Myth 2: It’s very expensive

False! Most colleges provide cheap

accommodation for three years. College libraries and dining halls also help

you save money. Increasingly, bursaries help students from

poorer backgrounds. Most colleges and departments are very

close to the city centre – low transport costs!

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Myth 3: College Choice Matters

False! If the college you choose is unable to offer

you a place because of space constraints, they will pass your application on to a second, computer-allocated college.

Application loads are intelligently redistributed in this way.

Lectures are given centrally by the department as are many classes for courses in later years.

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Myth 3: College Choice Matters

However… Choose a college that you

like as you have to live and work there for 3 or 4 years – visit if possible.

Look at accommodation & facilities offered.

Choose a college that has a tutor in your subject.

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Myth 4: You have to be bright

True! We find it takes special qualities to benefit

from the kind of teaching we provide. So we are looking for the very best in ability

and motivation.

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Our courses

Computer Science Computer Science firmly based on

Mathematics

Mathematics and Computer Science Closer to a half/half split between CS and

Maths

Courses can be for three or four years

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Entrance Requirements

Essential: A-Level Mathematics Recommended: Further Maths or a

Science Note it is not a requirement to have

Further Maths for entry to Oxford For Computer Science, Further Maths is

perhaps more suitable than Computing or IT

Usual offer is AAA No offers based on A* as yet

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Admissions Process

Fill in UCAS form Choose a college or submit an “Open”

Application Interview Test

Based on common core A-Level Taken before the interview

Interviews Take place over a few days Often have many interviews

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Computer Science

Year 1

Year 2

Year 3

Mathematics Computing

Project work

Year 4

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Mathematics & Computer Science

Year 1

Year 2

Year 3

Mathematics Computing

Year 4

Project work

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Some of the different courses

Functional Programming Design and Analysis of Algorithms Imperative Programming Digital Hardware Calculus Linear Algebra Logic and Proof

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Useful Sources of Information

Admissions: www.admissions.ox.ac.uk/

Computing Laboratory: www.comlab.ox.ac.uk/admissions/

Mike Spivey’s info pages: web2.comlab.ox.ac.uk/oucl/prospective/ugrad/

csatox/

College admissions tutors

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What is Computer Science?

It’s not about learning new programming languages.

It is about understanding why programs work, and how to design them.

If you know how programs work then you can use a variety of languages.

It is the study of the Mathematics behind lots of different computing concepts.

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Simple Design Methodology

Try a simple version first Produce some test cases Prove it correct Consider efficiency (time taken and

space needed) Make improvements (called

refinements)

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Fibonacci Numbers

The first 10 Fibonacci numbers (from 1) are:

1,1,2,3,5,8,13,21,34,55

The Fibonacci numbers occurs in nature, for example: plant structures, population numbers.

Named after Leonardo of Pisa who was nicked named “Fibonacci”

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The rule for Fibonacci

The next number in the sequence is worked out by adding the previous two terms.

1,1,2,3,5,8,13,21,34,55

The next numbers are therefore34 + 55 = 8955 + 89 = 144

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Using algebra

To work out the nth Fibonacci number, which we’ll call fib(n), we have the rule:

fib(n) =

We also need base cases:fib(0) = 0 fib(1) = 1

This sequence is defined using previous terms of the sequence – it is an example of a recursive definition.

fib(n – 1) + fib(n – 2)

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Properties

The sequence has a relationship with the Golden Ratio

Fibonacci numbers have a variety of properties such as fib(5n) is always a multiple of 5 in fact, fib(a£b) is always a multiple of fib(a) and fib(b)

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Writing a computer program

Using a language called Haskell, we can write the following function:

> fib(0) = 0

> fib(1) = 1

> fib(n) = fib(n-1) + fib(n-2)

which looks very similar to our algebraic definition

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Working out an example

Suppose we want to find fib(5)

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Our program would do this…

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What’s happening?

The program blindly follows the definition of fib, not remembering any of the other values.So, for

(fib(3) + fib(2)) + fib(3)the calculation for fib(3) is worked out twice.

The number of steps needed to work out fib(n) is proportional to n – it takes exponential time.

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Refinements

Why this program is so inefficient is because at each step we have two occurrences of fib (termed recursive calls).

When working out the Fibonacci sequence, we should keep track of previous values of fib and make sure that we only have one occurrence of the function at each stage.

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Writing the new definition

We define > fibtwo(0) = (0,1)

> fibtwo(n) = (b,a+b)

> where (a,b) = fibtwo(n-1)

> newfib(n) = fst(fibtwo(n))

The function fst means take the first number

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Explanation

The function fibtwo actually works out:fibtwo(n) = (fib(n), fib(n+1))

We have used a technique called tupling – this means that we keep extra results at each stage of a calculation.

This version is much more efficient that the previous one (it is linear time).

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An example of the new function

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Algorithm Design

When designing algorithms, we have to consider a number of things:

Our algorithm should be efficient – that is, where possible, it should not take too long or use too much memory.

We should look at ways of improving existing algorithms.

We may have to try a number of different approaches and techniques.

We should make sure that our algorithms are correct.

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Comments and Questions

If there’s any comments or questions then please ask.

There should be staff and students available to talk to

Thank You

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Finding the Highest Common Factor

Example:Find the HCF of 308 and 1001.

1) Find the factors of both numbers: 308 – [1,2,4,7,11,14,22,28,44,77,154,308]1001 – [1,7,11,13,77,91,143,1001]

2) Find those in common [1,7,11,77]3) Find the highest

Answer = 77

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Creating an algorithm

For our example, we had three steps:1) Find the factors2) Find those factors in common3) Find the highest factor in common

These steps allow us to construct an algorithm.

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Creating a program

We are going to use a programming language called Haskell.

Haskell is used throughout the course at Oxford.

It is very powerful as it allows you write programs that look very similar to mathematical equations.

You can easily prove properties about Haskell programs.

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Step 1

We need produce a list of factors for a number n – call this list factor(n).

A simple way is to check whether each number d between 1 and n is a factor of n.

We do this by checking what the remainder is when we divide n by d.

If the remainder is 0 then d is a factor of n. We are done when d=n. We create factor lists for both numbers.

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Function for Step 1

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Step 2

Now that we have our factor lists, which we will call f1 and f2, we create a list of common factors.

We do this by looking at all the numbers in f1 to see if they are in f2.

We there are no more numbers in f1 then we are done.

Call this function: common(f1,f2).

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Function for Step 2

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Step 3

Now that we have a list of common factors we now check which number in our list is the biggest.

We do this by going through the list remembering which is the biggest number that we have seen so far.

Call this function: highest(list).

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Function for Step 3

If list is empty then return 0, otherwise we check whether the first member of list is higher than the rest of list.

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Putting the three steps together

To calculate the hcf for two numbers a and b, we just follow the three steps in order.So, in Haskell, we can define

Remember that when composing functions, we do the innermost operation first.

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Problems with this method

Although this method is fairly easy to explain, it is quite slow for large numbers.

It also wastes quite a lot of space calculating the factors of both numbers when we only need one of them.

Can we think of any ways to improve this method?

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Possible improvements

Remember factors occur in pairs so that we actually find two factors at the same time.

If we find the factors in pairs then we only need to check up to n.

We could combine common and highest to find the hcf more quickly (this kind of technique is called fusion).

Could use prime numbers.

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A Faster Algorithm

This algorithm was apparently first given by the famous mathematician Euclid around 300 BC.

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An example of this algorithm

hcf(308,1001)= hcf(308,693)= hcf(308,385)= hcf(308,77)= hcf(231,77)= hcf(154,77)= hcf(77,77)= 77

The algorithm works because any factor of a and b is also a factor of a – b

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Writing this algorithm in Haskell

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An even faster algorithm

hcf(1001,308) 1001 = 3 × 308 + 77= hcf(308,77) 308 = 4 × 77= hcf(77,0)= 77

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Computer Science

Core CS 1(75%)

Core CS 2(50%)

CS Options(50%)

CS Options(25%)

Advanced Options(50%)

Project(25%)

Advanced Options(66%)

Project(33%)

Core Maths(25%)

Year 1

Year 2

Year 3

Year 4(optional)

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Mathematics & Computer Science

Core 1(100%)

Core 2(58%)

CS Options(25%)

CS Options(25-75%)

OptionalProject(33%)

MathsOptions(17%)

Year 1

Year 2

Year 3

Year 4(optional)

MathsOptions

(25-75%)

MathsOptions

(25-75%)

CS Options(25-75%)