AQA Computer Science Theory

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What is the job of a computer?
To store, process and execute.
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What 4 things do all computers have?
An input, Processer, Storage and an output.
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What are some examples of input devices?
Keyboard, microphone and a camera.
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What are some examples of output devices?
Computer Monitor and Speaker.
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What is Avail?
If a computer system is down for 1 hour in 100 hours then the system availabilty is 99%.
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What is MTTF?
It is means mean time to failure. This is a record of the average amount of times it takes before a system fails.
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What is Data redundancy?
It is when data is duplicated in a number of places so if it becomes corrupted, data will not be lost.
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What is Software redunancy?
This is when 3 software routines are in place which are written by 3 different programmers so if one fails the other two are in place. All programs have bugs and could fail because of these so different programs means different bugs.
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What are some advantages of cloud computing?
Automatic backup, Device independent, Improved performance, Instant Software update, universal access and unlimited storage (in theory).
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What are some disadvantages of cloud computing?
Can be slower, Limited features, Lack of security, needs a reliable and fast internet connection.
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What is the Cpu?
The brain of the computer. It has millions of tiny switches caled transitors.
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On average how much can the hard drive hold?
4 Tb
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How much can the biggest memory stick hold?
256 Gb
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How much can a blu-ray disk hold?
128 Gb
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How much can a DVD hold?
4.7 Gb
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How much can a CD hold?
700 Mb
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What is a nibble represented with in hexadecimal?
An integer between 1 and 9 or a letter between A and F (represent 10 through 15)
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What does ASCII stand for?
American Standard code for information interchange
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What Character is assigned to 65 in ASCII
A
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What Character is assigned to 97 in ASCII
a
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What Character is assigned to 48 in ASCII
0
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How does ASCII represent characters?
A character is represented by a number which the computer would see in binary.
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How many characters can ASCII include?
128
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What is a pixel?
A Picture Element that is one particular colour.
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What is data about the datafile called?
MetaData.
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What is meant by sampling rate?
How close together the samples are taken of the sound.
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What converts soundwaves into a form that can be used by digital computers?
Analog to digital converters
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What is meant by the bit rate?
How often the sound is sampled.
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What is one advantage of using a higher sampling rate?
Better sound sampling
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What is one disadvantage of using a higher sampling rate?
Lots of memory is used.
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What is meant by compression?
When a file is made smaller than its original file.
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What is meant by 'lossless' compression?
Reduces a file size with no reduction of quailty.
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What is meant by 'lossy' compression?
The original data can be recovered when the file is uncompressed
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What compression would be used to compress an audio file and why?
W.A.V format, it reduces the file size without hurting the quality.
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What is the binary of 4F
0100 0111
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What does CPU stand for?
Central processing unit
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What is clock speed?
How many times the CPU turns on and off in a second. This is measured in Giga-Hertz
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What is the fetch execute cycle?
It is when a set of instructions are loaded and they are fetched in little sections and then decoded in the CPU. They are executed one bit at a time. The main memory is the start address.
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What is Hertz the name for?
The number of Electrical cycles per second
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What does it mean if the clock of a CPU is fast.
More instructions can fetched, decoded and executed quicker.
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How can you improve the speed of your processor?
Place frequently used instructions in the faster cache memory.
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Name each level of cache.
L1- primary cache. L2- secondary cache. l3- cache found in the mother board.
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Write each type if memory in increasing distance from the processor.
Cache, RAM, Main Memory.
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What is one advantage of having a bigger RAM.
Can have more apps running at once without loss of speed.
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Name two characteristics of RAM.
Write and read. Volatile
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Name two characteristics of ROM
Read only, Not Volatile
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What is virtual memory?
This is when part of the hard drive is given up to add storage to the RAM.
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What are some differences between RAM and ROM.
RAM can be altered whilst ROM cannot. As soon as you turn off the computer, the RAM is emptied. ROM is not emptied. The ROM is non-volatile, the RAM is volatile
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How does magnetic storage work?
Has tiny magnetic dots with different charges that represent 1 or 0.
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What is the Capacity of magnetic storage?
Theoretically infinite.
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What is the speed of magnetic storage?
3GB/S
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How portable is magnetic storage?
Not very portable.
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How durable is magnetic storage?
Not very it breaks easily
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How reliable is magnetic storage?
Very reliable.
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How does optical storage work?
A laser burns onto the disk and creates bumps and dots which then can be read.
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What is the capicty of optical storage?
Cd- 800mb, dvd- 4.7gb blu ray- 25-50gb
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Speed of optical storage?
7.6 mb/s
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How portable is optical storage?
very portable
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How durable is optical storage?
Not very snapped and scratched easily.
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How does flash/solid state storage work?
Made up of floating gate transistors that have charges the can be read as 0 or 1
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Capcity of solid state?
1gb-5tb
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Speed of solid state?
400 mb/s
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How durable is solid state?
5-10 times more durable than hard drive
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Name the six stages of the software development lifecycle (sdlc)
Feasabilty study, Analysis, desgin, implementation, evaluation, maintenance
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What is an anagram for remembering the reason for the feasibilty study>
TELOS
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What does TELOS stand for?
Technical, Economical, Legal, operational and Schedule
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What happens the analysis stage of the sdlc?
Data dictionaries are used to caputre the data you will need and what it will look like. Interviews/questionares etc and diagrams etc that identify processes.
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What happens the design stage of the sdlc?
A detailed data dictionary is created that stores any data in the system. Input screens/user interface. Output screens/reports. Psuedocode, flow charts, algorithms. How teh software will be tested.
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What happens in the implementation stage of the sdlc?
Coding and tetsing of sfotware, writing documentation - techinal documentation for supporting and maintaining. Installing the software for the user.
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What happens in the evaluation stage of the sdlc ?
software must be evaluated against the original specification document. This is also know as acceptance testing. If everything works the development crew gets paid
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What happens in the maintance stage of the sdlc?
Corrective maintenance- Fixing any bugs Adaptive- Chnaging software for new user requirements Perfective- Request performance improvements or made easier to use.
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What does CAP stand for?
Corrective maintenance. Adaptive maintenance. perfective maintenance
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What is the waterfall sdlc design ?
A sequential process. That is step by step
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Advantages of waterfall design?
Self contained steps that are easy to manage, Defined process and output per step.
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Disadvantages of waterfall design?
Changes can require a lot of time and money. Requirement changes mean going back to an earlier stage that has laready been completed.
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What is sprial sdlc model?
After each run through of the cycle a new prototype is created.
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What are some advantages of the sprial sdlc model?
Keeps improving the software. Shows the buyer the prototype so that they can make improvements.
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What are some disadvantages of the sprial sdlc model?
Costs a lot of money. It takes time to build and get feedback therefore long process
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What is the agile sdlc model?
Every project can be handled differently and the exsiting models need to taliored accordingly. Tasks are divided up to provide specific features.
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What are some advantages of the agile sdlc model?
Small multitasking groups make the project flexible to changes. Good if requirements keep changing
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What are some disadvantages of the agile sdlc model?
Only good for small developers not for larger groups.
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What are some advantages of prototyping?
Helps to indentify problems early on. Helps ensure the solution does what it is supposed to do. Gives the client a greater sense of involvement. Indentify user requirements.
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Disadvantages of prototyping?
Goal posts are constantly moving, Costs a lot of money and time so not good for small companies.
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What is Tracing algorithims?
Test tables to test the output of a block of code.
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What is unit/module testing?
A complex piece of software is broken down. Each unit is then tested.
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Whar is intergration testing?
All the seperate modules are brought together. They should all work together
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What is Alpha testing?
Colleagues will give improvements and test your code,
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What is Beta testing?
Small group of public test te code.
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What is acceptance testing?
When the buyer pays the developer after the code is tested successfuly.
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What are some advantages of networks?
Sharing an internet connection, Users can use their files on any PC on the network, software can be distributed across the network and computers can communicate and transfer files
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What are some disadvantages of networks?
Danger of hacking and lots of damage. If the server file breaks down files can become inaccessible. Viruses can spread to other computers through network. Expensive to set up- cables and servers. complicated to manage
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Differences between wired and wireless network
Wired is more reliable, more secure and quicker in dpped
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Advantages of Bus network?
Easy to install and add other workstations. Uses less cable. cheaper then other network types. Transmitting data is quicker than ring
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Disadvantages of Bus network?
If there is a problem with the central cable the whole network stops working. If there are a lot of workstations the data will travel slowly. Low security as every station can see all the data
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Advantages of Ring Network?
Token passing protocol is simple and reliable, not dependant on central computer, consistent performance even with a lot of traffic.
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Disadvantages of Ring Network?
If one machine/link fails the network doesn't work. if cable breaks the whole network doesn't work, Data passes through every computer so not secure
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Advantages of Star Network?
Very reliable as if one connection fails no others are affected, Good security as no computer can interact with another without going through the hub, consistent performance and easy to add other computers
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Disadvantages of Star Network?
Very expensive as a lot of cables are required. If the servers crashes then nobody can access any files on the network
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What is a router and what is its role in a network?
It Transfers packets of data along networks. It transfers data across two or more networks
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What is a Hub and what is its role in a network?
It is a common connection point in a network. They handle data types called frames which carry your data.
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What is a Switch and what is its role in a network?
They allow and filter data packets. Forwards packets between LAN segments.
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What is a CAT 5 CABLE and what is its role in a network?
Short for category 5 network and consists of four twisted pairs of copper wire. They connect computers and other stuff to the lan
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What is a Server and what is its role in a network?
A server manages the Network resources. That's about it really
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What is a Node and what is its role in a network?
A node can be a computer or other device. It is a device that can be used and connected to the network.
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What characteristics does a Server computer have?
They have many processors and Large amounts of memory and hard drive space.
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What is a client?
A normal pc
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Name the three components in the client-server model.
Home.html, Web Server, Website.html
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What happens when a PC wants to see a webpage
It will request a webpage and a data packet is sent to the server requesting to see it.
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What kind of service are offered by a server?
File sharing, Web pages and printing
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What is peer to peer?
When a computer shares files to nother computer
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How does bit torrent work?
It is when you download small parts of a big package unitl you get the full package
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Arguements for file sharing.
It is publicy for Artist's work, You should be able to share what you buy. Basic Humn right for fredom speech
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Arguements against file sharing.
Creators lose profits from what they make. Your stealing work from other people
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What are the three steps in the ******** handshake?
1.Syn 2.syn + ack 3. Ack
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What is the TCP/IP?
transfer control protocol/ internet protocol. They are used to set up communication between the client and server.
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What is the most protocol?
TCP/IP
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What is unique to a device?
MAC address
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Why is IVP4 going to updated?
It will be updated to IVP6 as there are not enough unique combonations.
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What is thr basic principle of a client server system?
The client requests data and the server processes the request and returns a response
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State some characteristics of Stand-alone application.
All data stored locally, No network location to manage, single userm used as needed, modular structure is less important,
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State some characteristics of Client-server application?
Data must be passed between the client and server, must be available all the time, must manage a network link, clients have to be authenticated before data is sentm must be desgined and created in modular fashion
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What does hard coded mean?
Cod is executed on the client computer, shown using the browser
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In HTMl what is the opening and closing tag for a hyperlink?
and
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In HTMl what is the opening and closing tag for a list?
and
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What is a field a database?
one piece of information about one person/thing
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What is a Primary key?
A unique way of identifying a particular record.
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What is a table in a database?
Information stored here.
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What is the command in SQL for creating fields in tables?
INSERT INTO
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What is the command for selecting data in SQL
SELECT
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What is * in SQL?
A wildcard that means all.
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What is the difference between MOD and DIV?
MOD returns a remainder, DIV returns an integer value.
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What is concatenating?
when two strings are joined together.
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What are teh three basic ways of controlling the order in which program statements are carried out?
Sequence, Selection and Iteration.
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What is a linear search?
Each item is examined in sequence until the required item is found.
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What is an array?
A data structure containing mutliple data values of the same data type.
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What is a data structure?
A collection of elementary data containing data types such as an integer float or boolean
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What is a record?
A data structure consisting of a number of different of fields which can all be different types.
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Does python use functions or procedures?
Functions only !!!
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What is the difference between Functions and Procedures?
Functions return something, procedures do something
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What are three keywords you must use when defining a function in psuedocode?
FUNCTION, RETURN and ENDFUNCTION
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Why use subroutines?
Useful to break up a complex, large program. Each subroutine can be tested individually. Many programmers can work ont eh same program at the same time. Sub routines can be copyed to save time
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What can a data validation not do?
Check that the user has not entered a reasonable value e.g. if they are lying.
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What is a range check?
A number or data is within a reasonable range
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What is a Type check?
Data is of the right type
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What is a length check?
Text entered is not to long or short
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What is a presence check?
Checks that data has been entered
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What is a format check?
Checks the format of the data.
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Why is low level code better in some aspects than high level?
Faster and takes up less storage
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Why is high level code better in some aspects than low level?
Easier to learn, programs can be written faster, easier to understand debug and maintain
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What is the scope of a variable?
It defines the parts of a program in which the variable can be used.
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Card 2

Front

What 4 things do all computers have?

Back

An input, Processer, Storage and an output.

Card 3

Front

What are some examples of input devices?

Back

Preview of the front of card 3

Card 4

Front

What are some examples of output devices?

Back

Preview of the front of card 4

Card 5

Front

What is Avail?

Back

Preview of the front of card 5
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