Computer Science


Revision notes for 2.1 Algorithms on paper 2 of the OCR computer science 1-9 scale course.
Alex Rollin
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Alex Rollin
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Resource summary

Computer Science
  1. 2.1 Algorithms
    1. Computational Thinking
      1. There are four elements of computational thinking, they are:
        1. Decomposition
          1. Simple tasks have simple solutions. You can break large problems that are complex into several smaller simpler tasks, this is known as decomposition
            1. For example you have been tasked with creating a game and you have broke it down into smaller tasks
              1. What type of game is it going to be?
                1. What is the style of the game?
                  1. Who is the audience of the game?
                    1. Does it have challenge and what are the rewards?
                      1. How does the game end?
                        1. What is the name of the game?
                        2. This is what decomposition is about, defining the problem in more detail and making it easier to solve
                        3. Top Down design
                          1. Complex problem
                            1. Element of problem 1
                              1. Element of problem 2
                                1. Sub element of this part
                                  1. Sub element 2 of this part
                                  2. Element of problem 3
                                  3. With the top down design the decomposition element can be shown
                                2. Pattern Recognition
                                  1. Being aware of any repetition in solving the task, this pattern can simplify the steps needed to solve the problem and furthermore, if the pattern can be made general, it can be used to solve other problems as well.
                                  2. Abstraction
                                    1. Removing unnessisary information is called abstraction.
                                      1. Only leaves the data that is required
                                        1. For example a designer needs to make a T Junction sign
                                          1. The sign must follow certain rules:
                                            1. Must take no more that 1 second for a driver to look at it and understand its meaning
                                              1. Must NOT contain any words
                                                1. It MUST use the standard shape and colours of all UK road signs
                                          2. The first attempt is shown to the right and as you can see it is very cluttered and will take more than a second for a driver to recognise the sign
                                            1. It also doesn't follow the rules for the shape and colour
                                            2. The second attempt is shown to the right and as you can see it is less cluttered than version 1, however it still breaks the rules as it says that there must not be any words
                                              1. It also doesn't follow the rules for the colour
                                              2. The third and final attempt follows all the rules, the shape, the colour and doesn't contain any words
                                                1. Also any unessisary clutter has been removed
                                                2. The same method of abstraction can be used for many other problems including those in Computer Programming
                                                3. Algorithmic Thinking
                                                  1. There are 2 popular methods to set out an algorithm:
                                                    1. Pseudocode - Text statements written for each step of the algorithm
                                                      1. An example of Psudocode
                                                        1. Psudo code is not a formal computer language and has no particular rules governing how it should be written. What is important is that Psudocode should be easy to understand, unambihiousous and error free. Especially because programmers often use it to form the basis of their programming code
                                                        2. Flow chart - This uses a set of standart shapes and arrows (Shown to the right-->) tO make a diagram of the algorithm
                                                        3. Algorithms often include a decisaion to be made at some point in the task. In order to handle this, a conditional statement is used.
                                                  2. 1.1 Architecture
                                                    1. CPU
                                                      1. The 3 States of a CPU
                                                        1. Fetch
                                                          1. Decode
                                                            1. Execute
                                                            2. The central processing unit is the central brains of the computer
                                                              1. The CPU is used to process data so no matter what you do on your device it will go through the CPU
                                                              2. 1.6 Network Security
                                                                1. Malware
                                                                  1. This is a type of Software that is used for mallicious software.
                                                                  2. Botnets
                                                                    1. These usually start as worms or viruses which build up a network of computers that are under control of the hackers, this can be any Os such as Windows, Linux or Mac OS
                                                                    2. Security Misconceptions
                                                                      1. It is a common misconseption that Mac's are immune to this problem. This is simply not the case with many MacOS malware coming out everyday
                                                                        1. All PC's and internet connected devices can be vunerable and if you do get hacked it is not the machines fault it is the operators fault
                                                                        2. It has got to the point that 86% of the worlds iPhones can be hacked with just a simple text, which means that these users are vunerable to a software called Pegasus which is impossible to detect.
                                                                          1. That means in my class of 34, if 46.% percent of people in the UK have an iPhone, which is 15, only 1 would not have the Pegasus virus, that's 14 unsecure devices that will leak phone numbers photos, key strokes and everything that you do. They will even have access to your location and friends and family's location s through apps like Snapchat and Find my friends and Iphone. They can also view your browsing History and many other aspects of your iPhone send messages on your phone and get contacts and make your phone an hive for Criminal activity
                                                                        3. Viruses
                                                                          1. These can
                                                                          2. Ransomeware
                                                                            1. Ransomeware is the software that will encrypt files with a code and not allow them to be unlocked and threatens to delete them within a certain ti,me, they will demand money to release your files or if not, if you have photos that you want nobody else seeing on your phone they will probably be shared on your Social Media which they will take control of.
                                                                              1. You can protect yourself from randsomeware by not clicking on suspicous web pages and links. Only look through your spam filter if you are missing an important email.
                                                                              2. White hat and Black hat Hackers
                                                                                1. White Hat
                                                                                  1. These are also refered to as Ethical Hacking as they hack for good. They do this by stress testing networks and try to find vunerabilities by breaking into the network and leaving their calling card without doing any phyical irriversible damage.
                                                                                  2. Black Hat
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