5. Fundamentals of Data Representation AQA A Level Computer Science

Natural number

A positive whole number including zero.

Integer

Any whole positive or negative number including zero.

Rational number

Any number that can be expressed as a fraction or ratio of integers.

Irrational number

A number that cannot be represented as a fraction or ratio as the decimal form will contain infinite repeating values.

Real number

Any positive or negative number with or without a fractional part.

Ordinal number

A number used to identify position relative to other numbers.

Cardinal number

A number that identifies the size of something.

Well-ordered set

A group of related numbers with a defined order.

Unsigned binary

Binary that represents positive integers only.

Two's complement

A method of working with signed binary values.

Fixed point

Where the decimal/binary point is fixed within a number.

Floating point

Where the decimal/binary point can move within a number.

Signed binary

Binary with a positive or negative sign.

Overflow

When a number is too large to be represented with the number of bits allocated.

Underflow

When a number is too small to be represented with the number of bits allocated.

Normalisation

A process for adjusting numbers onto a common scale.

Precision

How accurate a number is.

Mantissa

The significant digits that make up a number.

Exponent

The 'power of' part of a number indicating how far a binary point should be shifted left or right.

Character code

A binary representation of a particular letter, number or special character.

ASCII

A standard binary coding system for characters and numbers.

Unicode

A standard binary coding system that has superseded ASCII.

Parity bit

A method of checking binary codes by counting the number of 0s and 1s in the code.

Majority voting

A method of checking for errors by producing the same data several times and checking it is the same each time.

Check digit

A digit added to the end of binary data to check the data is accurate.

Bit-mapped graphic

An image made up of individual pixels.

Pixel

An individual picture element.

Resolution

Width x height or pixels per inch.

Colour depth

The number of bits or bytes allocated to represent the colour of a pixel in a bit mapped graphic.

Vector graphic

An image made up of objects and coordinates.

Compression

The process of reducing the number of bits required to represent data.

Run-length encoding

A method of compressing data by eliminating repeated data.

Dictionary-based encoding

A method of compressing text files.

Encryption

The process of turning plaintext into scrambled ciphertext, which can only be understood if it decrypted.

Decryption

The process of deciphering encrypted data or messages.

Plaintext

Data in human-readable form.

Ciphertext

Data that has been encrypted.

Caesar cipher

A substitution cipher where one character of plaintext is substituted for another, which becomes the ciphertext.

Substitution cipher

A method of encryption where one character is substituted for another to create ciphertext.

Key

In cryptography it is the data that is used to encrypt and decrypt the data.

Polyalphabetic

Using more than one alphabet.

Frequency analysis

In cryptography it is the study of how often different letters or phrases are used.

Transposition cipher

A method of encryption where the characters are rearranged to form an anagram.

Railfence cipher

A type of transposition cipher that encodes the message by splitting it over rows.

Route cipher

A type of transposition cipher that encodes the message by placing it into a grid.

Vernam cipher

A method of encryption that uses a one-time pad (key) to create ciphertext that is mathematically impossible to decrypt without the key.

One-time pad

A key that is only used once to encrypt and decrypt a message and is then discarded.

Baudot code

A five-digit character code that predates ASCII and Unicode.

Computational security

A concept of how secure data encryption is.

Computational hardness

The degree of difficulty in cracking a cipher.