AQA AS Computing - Unit 1

Computation: The act or process of calculating or determining something by mathematical, logical or interactive methods.

Computability: Measures what can and cannot be computed.

Computing: The study of natural and artifical information processes.

Artificial Intelligence: A branch of computing that studies the use of computers to perform computational processes normally associated with human intellect.

Algorithm: A description, independant of any programming language, of a process that achieves some task. It is a step-by-step procedure for solving a problem.

Deterministically: Without guessing a solution before confirming it.

Program: A description in a programming language of a process that achieves some useful result.

Problem: A given where it is not immediately obvious how to reach the goal.

Ownership: Someone has to take ownership and responsibility for designing a solution.

Given: An initial solution.

Goal: A desired outcome.

Resources: Things that can be used to reach a goal and impose constraints.

Defining Boundaries: Establishing the limits or rules about what can and cannot be done when solving a problem. These limits are types of constraints.

Lateral Thinking: Used to challenge assumptions, establish facts and rules and define the bounderies of problem solving.

Top-Down Design: Breaks a problem into smaller problems that are easier to work on.

Module: A self contained entity that results when a problem is devided into sub-problems; each module corresponds to a sub-problem.

Stepwise Refinement: The process of breaking a problem down through successive steps into smaller problems.

Structure Table: Indented number list of steps produced by stepwise refinement.

Finite State Machine: A machine that consists of a fixed set of possible states with a set of allowable inputs that change the state and a set of possible outputs.

State Transition Diagram: A way of describing an FSM graphically. Each state is represented by a circle and each transition by an arrow labelled with the input that causes the transition plus any output resulting from the transition.

State Transition Table: Shows the effect on the current state of an FSM of particular inputs and any corresponding output.

Decision Table: A table that shows the outcome for a given logical condition.

Algorithm: A description, independent of any programming language, of a process that achieves some task. It is a step-by-step procedure for solving a problem.

Program: A description in a programming language of a process that achieves some useful result.

Sequence: Consecutive steps or groups of steps processed one after another in the order that they arise.

Selection: A decision-making step.

Repetition/Iteration: A step or sequence of steps that are repeated until some condition is satisfied or while some condition is satisfied.

Assignment: An operation that assigns a value to a variable.

Structured English: A very restricted subset of the English language.

Pesudocode: Code that resembles a programming language but uses less strict syntax to express an algorithm and is independent of any programming language.

Hand Trace, Desk Check or Dry Run: A careful step-by step simulation on paper of how an algorithm would be executed by a computer.

Key Point: The purpose of an algorithm is to communicate a computation to humans not to computers.

Key Point: Programs are not algorithms. An algorithm is a description of a process apart from any programming language. The same algorithm may be programmed in many different languages.

Key Point: Understanding a problem means turning an ill-defined problem into a well-defined problem stated clearly and ambiguously.

Key Point: A well defined problem has four components: given, resources and constraints, a goal and ownership.

Key Point: Identify assumptions and challenge them. Establish the real facts.

Key Point: One reason why FSM are so useful is that they can recognise sequences. The set of recoginisible sequences can be legal and valid programs given in a programming language.

Key Point: An FSM with no outputs is called a Finite State Automation (FSA).

Key Point: FSAs have an initial state and one or more accepting or goal states. State transition diagrams use a special arrow to indicate the initial state and a double circle to indicate the accepting state, or goal state.

Key Point: FSMs without outputs have an initial state and usually no accepting states. They are called Mealy Machines. The initial state is indicated with a special arrow labelled start.

Key Point: There are four major steps for algorithm design (Identify the problem inputs, identify the problem outputs, Determine the variables & derive the algorithm that transforms inputs into outputs).

Key Point: A left-pointing arrow is used to assign a value to a variable in assignment.

Key Point: Wherever possible, use meaningful names for variables, etc. Use indentation to show structure.

High-Level Programming Language: A language that has English like keywords and commands to express the basic constructs of sequence, selection and repetition.

Variable: A location in memory that contains one or more data values.

Identifier: A unique name given to a variable, a procedure, a function, etc.

Integer Division: Returns only the whole number part of a division.

Compiler: A program that translates source code into executable object code.

Console Application: A program that runs in a text based window into which the user types and which displays text output fro the computer.

Global Variable: A variable declared at the beginning of a program and accessible from anywhere in the program.

Local Variable: A variable declared in a program block and accessible only within the program block.

Constant: A value that does not change throughout the program.

Integer: A whole number

Real Number: A number with a decimal point and a fractional part.

String: A sequence or zero or more characters.

Character (Char): Any letter, numeral or symbol in the give character set, such as the ASCII character set.

Boolean/Boolean Data: Can be one of two possible values, True or False.

Record: A data structure that groups a number of variables.

Field: A variable that is part of a record.

Enumerated Type: An ordered set of values.

Set: A collection of values of the same ordinal type with no associated order.

Ordinal Type: Defines an ordered set of values such as integer, character, boolean and enumerated type; real numbers are not of ordinal type.

Boolean Expression: Returns True or False.

Boolean Operators: NOT, AND, OR.

Function: A routine that is called as part of an expression and returns a value.

Procedure: A routine that may or may not return a value; a procedure call is a statement in its own right, not part of an expression.

Routine Interface: The part of the routine that lists the parameters used to pass data into and out of the routine.

ByRef: The variable inside the routine uses a reference to the same memory location. The reference is the variable passed as the parameter.

ByVal: The value of the calling code's variable is copied to the routines parameter.

Parameters In Routines: Placeholders for variables supplied at call time.

Validation Check: A check made by a program to see that the data entered is reasonable.

Robust System: A system that is unlikely to malfunction.

Test Plan: A list of planned tests with their data or referring to a set of data.

Evidence Of Testing: Something that shows the data entered and the result from the program, e.g. a screenshot.

Binary: A number system that used two digits, 0 and 1.

Bit: A binary digit; it can be 0 or 1.

Byte: A group of 8 bits.

Word: A group of bytes.

Denary Number System: A number system that uses the digits 0 to 9.

Two's Complement: A system for representing negative numbers.

Hexadecimal Number System: A number that uses the digits 0 to 9 and the letter A to F.

Pixel: The smallest addressable area or smallest block of colour in an image.

Bitmap: Created when the pixels of an image are mapped to position in memory that store binary codes representing the colour of each pixel.

Resolution Of A VDU Screen: A quantity expressed as the number of pixels per row X the number of pixels per column.

Resolution Of An Image: A quantity expressed as the number of pixels per inch or centimeter.

Colour Depth: The Number of bits used to represent the colour of a single pixel.

Calculation For Number Of Colours: 2 to the power of colour depth. (2^32 for 32 bit colour allows you to have 4,294,967,296 colours).

Vector Graphic: Records geometric and other information about the objects that make up an image.

Object: A component of a vector graphic, such as a line, a rectangle  or a circle.

Drawing List: The list of drawing commands that create a vector graphic.

Property: The property of a vector graphic object describes things such as the size, direction, thickness, shading, font size or typeface.

Data Compression: Squeezes data into a smaller number of bytes that the data would occupy if it was uncompressed.

Analogue Data: Data that varies in a continuous manner.

Digital Data: Data that takes the form of discrete values.

Analogue Signal: An electrical signal that varies in a continuous manner.

Digital Signal: An electrical signal with voltage changes in discrete steps.

ADC: Analogue To Digital Converter; converts an analogue signal into an equivalent digital signal.

DAC: Digital To Analogue Converter; converts a digital signal into an equivalent analogue signal.

PCM: Pulse Code Modulation; A process of coding sampled analogue signals by recording the height of each sample in a binary electrical equivalent.

PAM: Pule Amplitude Modulation; A process that samples analogue signals at regular intervals and produces electrical pulses of height proportional to the original signal's amplitude at the instant of sampling.

Quantisation Noise: The difference between the original amplitude and its sampled value.

Nyquist Theorem: States we must sample the sound at a frequency at least twice the rate of the highest frequency in the sampled signal.

MIDI: Musical Information Digital Interface; a way of representing the sounds made by an instrument.

Synthesises Sound: Uses digital means to generate audio signals resembling instrument sound or the human voice.

System: A whole composed of parts in orderly arrangement.

System Analysis: The study of a complex process in order to improve its efficiency.

Manual System: A system that does not involve computers.

Requirements Specification: A set of system objectives.

Software Maintenance: Updating a program to correct faults or improve features.

There are 6 stages in a system life cycle:

• Analysis: A system may be perceived to be in need of change for a variety of reasons such as technical development or outdated practices.
• Design: A new system must be designed carefully before it can be developed. The system components need to be specified.
• Implementation: This stage involves development of software by coding and testing, installation of software and hardware, preparation of data files, training the users and writing the system documentation. 
• Testing: Put the system to use and seeing if there are any faults or errors which can be corrected.
• Evaluation: Analyses whether the system is successful and whether any changes could be or need to be made.