Support for Paper 2 – See Revision website links for questions and help powerpoints on Technical Topics

Computational Thinking:

1. Abstraction: Looking at a set of information or text and removing unwanted information to leave only relevant information or the information asked for.

2. Decomposition:

Breaking a difficult or complex problem into sub problems and then into smaller parts to be solved individually.

3. Algorithm:

A logical way of getting from the problem to the solution either using a flowchart, set of instructions or pseudocode.

Algorithm symbols:

Start/stop Input/output Process/calculation

Decision – is …..? Sub routine –

a sub program within main program

Sequence, Selection + iteration

Sequence: A set of instructions or commands where there is only one possible route from start to finish.

Selection

There are multiple ways from start to end.

This usually involves decision boxes in an algorithm or

using IF THEN ELSE when programing

Iteration

This would include a loop which will allow you to repeat a task eg counters

Searching Algorithm methods

Binary Search:

Only looks for items in an ordered list eg 7 21 36 46 62 71

Method (Find the value 46 in the list):

1. Find the middle value eg 36

2. Compare 46 to middle value - it is smaller so remove all values lower than 36.

3. Values left are 46 62 71

4. Repeat Steps 1, 2 and 3 ( eg middle value is now 62 – compare – it is higher so remove all higher values

5. You are now left with 46.

Searching Algorithm methods

Linear Search:

Mainly looks for items in an unordered list eg 7 35 18 62 46 71

Method (Find the value 71 in the list):

1. Look at the 1st item – 7 - stop if it is the value you are looking for.

2. It is not so look at next item – 35 - stop if it is the value you are looking for.

3. Repeat step 2 until you find the value you’re looking for.

It is simpler than a binary search, but not as efficient and can be much slower if the value is towards the end of a list.

Only useful in small lists!!

Sorting Algorithm methods - Bubble Sort:

• This compares 2 values or a pair of numbers at a time.

• Used to sort an unordered list of items.

• It is very simple but can take a long time.

Method:

Sort these numbers in numerical order –

2 1 6 3 5 4

Sorting Algorithm methods - Merge Sort:

Method:

• This uses a method of splitting a large list in half each time using sub lists.

• Compare 2 values and rearrange theme in size or alphabetical order.

• Merges back together and then check if in order ?

• Repeat process again until list is sorted

• It is more efficient than a bubble sort for large lists.

Sort these numbers in numerical order –

7 2 1 6 3 5 4 8

Sorting Algorithm methods – Insertion Sort: Method:

• This takes an item(2nd in list) and compares against others in the list and just simply places it in the correct place.

• Repeats this action with the next value in the list(3rd value).

• Repeats until all values are in the correct order.

• It is a very quick method on small lists,

but would not be as efficient

as a merge sort on large lists.

Data Types + Casting In programming we need to store data as different types. You need to know these types:

integer – whole numbers

real – numbers with a decimal part

Boolean – 2 values – T/F, Y/N, 1/0

character – 1 letter, number, symbol -

string – text – Gh3r%

What is Casting:

When you convert between data types - We have used this when saving to file changing the integer value to a string

eg asking for age: age=int(input(“what is your age”)) THEN BECOMES str( age)

Then when we may need to save to file we would change the data type:

File.write ……………..(name+ gender + str(age))

Operators:

These are simply special characters to perform calculations or functions in maths.

The 4 standard ones are +, - , * and / .

Equally as important and used in Pseudocode questions are :

^ or ** Powers eg 2 ^ 4 = 16 ( 2⁴) (Exponentiation)

DIV eg 20 DIV 6 = 3 ( 6 goes into 20 3 times) (Quotient)

MOD eg 11 MOD 5 = 1 ( Remainder is 1 – 5 goes into 11 twice rem 1)

Comparison Operators:

These are used to compare values on the left and right of the expression.

== is equal to eg 5 == 5 is True

!= or <> is not equal to

< is less than

> is greater than

<= is less than or equal to

>= is greater than or equal to

these are used to produce a True or False outcome eg Boolean result

SQL: (Structured query language)

• This is used in tables or databases of information.

• It is mainly used for searching for information

• Can also create a table and update information.

Example 1 : Looking for all the hotel names and ratings:

SELECT hotel_name, hotel_rating FROM hotels (table)

Part 2 – Filter the results:

WHERE price < = 80 AND rating > = 4

( price is less than or equal to 80 and Rating is 4 or more)

Part 3 – Sort results - Price highest first

ORDER by price DESC

Arrays :

An array is a data structure that allows you to hold several variables, all of the same type, with one name.

Each item is called an element with a position in the list called an index.

The 1st item in the list has position 0

Eg five different resorts with great beaches, you could write the pseudocode statement

resort [“Margate”, “Swansea”, “Brighton”, “St Ives”, “Cromer”]

so resort [ 0 ] = Margate

resort [4] = Cromer

2D Arrays :

Suppose you wanted to hold in a program, 5 marks for each of 3 students

Marks

Student1 12 14 8 7 17Student2 6 8 5 3 13Student3 16 15 9 10 18

These marks could be put into a 2D array

The position of each element or item is usually written [ a ] [ b ] where a is the row b is the column

You can set individual items in the array like this: mark [2] [4] =18 eg Row 3, column 5

example 2: What is the value of mark[1][0]? = 6 eg Row 2, column 1

Questions

What is the value of mark [0] [0]?

What is the value of mark [0] [2]?

What is the value of mark [1] [3]?

String manipulation:

Remember strings are a data type made of mixed characters.

Concatenation – strings can be joined together

eg fullname = firstname + surname

Useful types of manipulation:

.upper – changes characters to uppercase

.lower - changes characters to lowercase

.length – counts the number of characters in a string

x[ i ] – Finds the position of i from string x

town.substring (0,3) - this finds the 1st 3 characters of the town.

File handling:

This deals with how a program can access and change data in an external file.

newfile = open Read(“newfile.txt”) - opens the file in read mode

newfile = open Write(“newfile.txt”) - write data to the file

newfile.close() - closes the file

variable.writeLine( name + str(age) + …..) - writes variables to file

first_line = newfile. readLine() - reads the 1st line of the file

endoffile() - used to stop reading all lines of file

TestingTesting:

This is needed to make sure that a program is fully working, bug free and meets the client’s needs.

2 main types:

Iterative testing – When the program goes through the system life cycle a few times to meet the customers needs egRequirements/Design/implementation/Testing

Final testing – usually when the program has gone through iterative testing and has one final run through the system life cycle. If all requirements are met the program is ready for the customer.

Constants and variables

Data values can be stored as a constant or a variable

• Constant values are assigned and can’t be changed • If you attempt to change a constant it may return an error when being compiled.

Eg CONST pressure as INT = 30

A variable is a value that can be changed eg Temperature, name, weight

Variables can either be classed as LOCAL or GLOBAL.

• Local variables are only ever contained inside the function or procedure

• Global variables can be used anywhere else in the program

Switch – Case statements

This is used when you want to use the same variable for different actions for different values.

Voting/Election program• 3 people – John, Sue, Alan• All start with 0 votes• A variable called vote is used • If John, Sue or Alan is entered the totalof votes goes up by 1 for that person• It saves using a different variable for the 3 different options

Validation and Sanitisation

This is used to make sure that when a user inputs data they are not allowed to enter unwanted or invalid data.

Programmers will try to predict how users may misuse a program and try to prevent it from crashing.

Input Sanitisation – Remove any unwanted characters in a program

Input Validation – Checks if data meets certain criteria eg @ included in email address and ends with .com, co.uk

White list – All data that the program should accept

Black list – All data that the program should reject

Functions

• A function will always return a value eg Area

• It will also need at least one parameter eg pi and radius

Example of a function:

FUNCTION area_circle ( pi, radius)

area=pi*radius*radius

return area

END FUNCTION

Procedures

• A procedure is a set of instructions stored under one name eg hello()

• to repeat these set of instructions you only need to type in hello() at a different place in the program

• A procedure doesn’t have to take a parameter but could!

• It doesn’t return a value

Example of a Procedure: A parameter or argument could be added here

PROCEDURE hello() variable added here

print(“ Hello Year 11”)

print(“this is a procedure”)

END PROCEDURE

Examples of Pseudocode

Iteration example:

FOR k = 1 to 100

Normal program text in here

NEXT K

Pseudocode:

Try to use key phrases like the following:

password = INPUT(“Enter your password”)

IF length of password <=6 THEN

print(“Password is too short”)

ELSE

print(“Password is valid”)

ENDIF

While loop example:

WHILE total < cost

coin = INPUT(“Enter coin value)total = total + coin

ENDWHILE

Repeat loop example:

REPEAT

coin = INPUT(“Enter coin value)total = total + coin

UNTIL total > cost

Examples of Pseudocode

Iteration example: FOR Loop

FOR k = 1 to 100

Normal program text in here

NEXT K

Pseudocode:

Try to use key phrases like the following:

password = INPUT(“Enter your password”)

IF length of password <=6 THEN

print(“Password is too short”)

ELSE

print(“Password is valid”)

ENDIF

While loop example:

WHILE total < cost

coin = INPUT(“Enter coin value)total = total + coin

ENDWHILE

Repeat loop example:

REPEAT

coin = INPUT(“Enter coin value)total = total + coin

UNTIL total > cost

Types of Languages and Translators:

Machine code is an instruction that can run inside a CPU - binary or hex

Assembly code refers to mnemonic instructions eg keyterms that do instructions

eg ADD, SUB, STORE

Code could be either high level or low level language.

Examples of high level language – Python, Java

Examples of low level language – Assembly code

Translators:

A translator is a piece of software that converts programming code to machine code.

There are 3 types of translator:

Assembler

Compiler

Interpreter

Assembler – converts assembly language to machine code

Compiler – converts HLL source code into machine code in one operation and stores it in an exe file

Interpreter – converts HLL source code into machine code one line at a time and runs it on the CPU.

More detail on Translators:

Compiler: Will return a list of errors

Can take a long time to compile the program into one file before converting it

Interpreter: Needed every time you want to run the program

Program would stop every time it finds the 1st error

Programs run slowly since the code is being translated one line at a time.

IDE(Integrated Development Environment):

This provides programmers with handy tools to help when programming.

An example would be pyscripter

Features:

Line numbering

Auto colour coding eg print, input, variables

Auto correct of variables or commands , Auto indents

Tools :

GUI – easier navigation

Code editor – shows source code

Run-time environment - Runs the code without compiling – shows the outputs

Debugger – Tests the code for errors

Translator – compiler or interpreter to run the source code

Logic Gates(Binary Logic):

The 3 main logic operations a CPU can perform are:

a) The AND operation ‘Logic Gates’

b) The OR operation

c) The NOT operation

These can be combined to perform complex calculations in the CPU.

Logic gates are created from a number of transistor switches – very small

Truth tables: AND gate

Each gate produces an output from different inputs. A truth table can show these combinations of results.

"If both A and B are True then the output is also True, otherwise it is False”.

a) The AND operation Boolean expression:

(The shorthand for this) is Q = A AND B

Truth tables: OR gate

This gate needs 2 inputs to produce 1 output -"If either or both A, B are True then the output is also True”.

a) The OR operation Boolean expression:

(The shorthand for this) is Q = A OR B

Truth tables: NOT gate

This operation produces an output that is opposite to the input.

If the input is 1 (Logic True) then the output is 0 (Logic False).

If the input is 0 (Logic False) then the output is 1 (Logic True)

a) The NOT operation Boolean expression:

(The shorthand for this) is Q = NOT A

More complex Logic circuits

In this example we have an AND gate with inputs A and B where the result Q is then taken as one of the inputs to an OR gate

Boolean expression: P = (A AND B) OR C

Units of data

Binary – computers use 1’s(on) and 0’s(off) to represent flow of electricity.

Sizes: Bit(b) single binary digit 1 or 0 -

Nibble 4 bits - 2⁴ = 16 different values

Byte(B) 8 bits - 2⁸ = 256 different values

Kilobyte(kB) 1000 bytes

Megabyte(MB) 1000 kB

Gigabyte(GB) 1000 MB

Terabyte(TB) 1000 GB

Petabyte(PB) 1000 TB

Binary representationDenary to Binary - egs of 4bit numbers

0 = 0 4= 100 8 =1000 12 =11001 = 1 5 = 101 9 =1001 13=11012 = 10 6 =110 10=1010 14 =11103 = 11 7=111 11=1011 15=1111

8 bit number could be: 0011 0101

128 64 32 16 8 4 2 1

0 0 1 1 0 1 0 1

32 + 16 + 4 + 1 = 53

Adding Binary using column addition – (right to left)

1 0 0 0 1 1 0 1

+ 0 1 0 0 1 0 0 0

______________________________________1 1 0 1 0 1 0 1

Rules are: 0 + 0 = 0, 1 + 0 = 1, 0 + 1 = 1

and 1 + 1 = 1 0 so it is 0 carry 1 to next column on left

If you end up with a value greater than 8 bit eg 9 bit you get an overflow error – can cause software to crash.

Binary shifts – aka Logical Shift

If you are asked to do a binary or logical shift it will either be:

Left or Right

Each shift to the left multiplies the value by 2

Each shift to the right divides the value by 2

example a logical shift of 2 places to the left would:

Increase the number by 2² = 4 times

Hexadecimal numbersHexadecimal – uses a combination of numbers and letters to represent a number. Base 16

The numbers 1-15 become 1-9 followed by A - F

Reasons why:• Simpler to remember large numbers in hex

• Less chances of input errors

• Easier to convert between binary and hex than with denary.

Hex to Binary:

0 = 0 4= 100 8 =1000 C =11001 = 1 5 = 101 9 =1001 D=11012 = 10 6 =110 A=1010 E =11103 = 11 7=111 B=1011 F=1111

Hexadecimal numbers to denaryHex values:

Hex increases by a power of 16 eg: 4096 256 16 1

So Hex number 87 to denary is:8 7(8x16) (7x1)= 128 + 7 = 135

Hex number 6A to denary is:6 A(6x16) (10x1)= 96 + 10= 106

Binary to Hex

Convert 11001001 to hexadecimal

Split the number into 4 bit numbers – nibbles 1100 1001

so 8 4 2 1 8 4 2 1

1 1 0 0 1 0 0 1

= 12 = C =9

so answer is C9 in hexadecimal

Hex to Binary

Convert 6B to binary

Find the number in denary 6 = 6 and B =11

so 8 4 2 1 8 4 2 1

0 1 1 0 1 0 1 1

check =6 check =11

so answer is 0 1 1 0 1 0 1 1 in binary

Error Checking

There are 2 types of errors –

The compiler or interpreter will not run the program if it finds one of these errors

Syntax –

an error due to a grammar mistake or doesn’t follow (python) command language

Logical –

an error which stops the program running properly – inaccurate formulae or inequality

Error Checking

Check digits:

These are used to make sure that data has been entered and read correctly.

They are digits at the end of numbers eg ISBNs on books.

Eg ISBN 978 1 78294 602 1

The check digit is 1

ASCII

What is meant by a character set?All of the characters recognised/represented by a computer system. Every character on the keyboard is represented by a binary value

What does ASCII stand for?American Standard Code for Information Interchange

What is ASCII used for?Developed for representing the English alphabet. Characters include 0-9, upper case, lowercase a-z, punctuation and space.

How many characters can ASCII represent?128 characters – standard ASCII256 characters – Using 8 bits – extended ASCII

UNICODE

What is a problem with ASCII?

It doesn’t have enough characters to deal with other languages and extra symbols like accents .

A different character set was invented called UNICODE

This uses 16 and even 32 bit character sets so it can cover languages that have completely different letters to english.

ImagesStored as pixels. Colours relate to number of bits.

Black and white – 2 colours – 1 bit – 0 –white1- black

2 bit images have 4 colours 00, 01, 10, 11

4 bit = 16 colours (2⁴)

More bits - greater colour range or depth

Most devices have 24 bit, 8bits for red, green ,blue

Resolution – number of pixels per inch – more pixels greater quality of images.Larger file size

Metadata – information stored about the image – file format, height, width, colour depth, resolution. Also date and time information

Without this information the image cannot be displayed.

SoundSound is recorded by a microphone in Analogue format (constantly changing value)

Needs to be changed to Digital for a computer to use(made up of bits)

An analogue to digital converter is used to record sound – this is called sampling! See page 74 of

revision book

Sample intervals – the gaps between each of the points where the analogue recording is sampled eg

5milliseconds.

Sampling frequency – How many samples you take in a second eg 44KHz – 44, 000 samples a second

Sample size – number of bits available for each sample.

Bit rate = sampling frequency x sample size

Increase the frequency – better quality sound –will pick up the quieter sounds even if they happen

at the same time as the louder ones.

Obviously this increases the bit rate, number of bits stored and makes the file size large.

Types of compression Data compression is when we make files smaller to save space but try not to lose too much quality with the new compressed file.

Reasons: 1.Less storage, 2. streaming and downloading is quicker, 3. web pages load quicker4. email attachments can be sent - limit on file sizes.

2 types of compression:

Lossy – permanently removes data so file is smaller – quality will be less eg mp3, jpeg. The difference in quality is only slight though.

Lossless – temporarily removes data while compressed - eg zip file. When extracted can be turned back to original eg png image.