Topics to revise for your Year 10 Computer Science Exam

Please check your School Email for a link to your own set of revision cards



Unit 1 Computer systems

Elements of a computer system
Computer systems in the modern world
Ethical, environmental and legal considerations

Unit 2 Hardware

The CPU
Memory
Secondary Storage
How storage devices work
Input and output devices
Developments in hardware technologies

Scratch Lesson 3: Variables

Create and program variables using Scratch

Task 1: Creating a Variable

a) Open your Scratch game and create a 'score' variable

Task 2: Enemy Score

a) Add script to your main character that sets the score to '0' when the game starts

b) Add script to your 'enemy' that will decrease the score by 10 points (-10) when you hit it

Task 3: Collecting Points

a) Load or create a sprite to act as way to collect points (coin, heart, jewel etc.)

b) Add script to your 'good item' that will increase the score by 10 points

Try adding a life or health variable into the game

a) Open your Scratch Evidence Document

b) Complete the Lesson 3 section

c) Save your work

Unit 2 Revision Computer Science Theory

Use these slides to make your own revision cards on Unit 2

Please check your School Email for a link to your own set of revision cards

Hardware L1 The CPU

Hardware L2 Memory

Hardware L3 Secondary Storage

Hardware L4 How storage devices work

Hardware L5 Input and output devices

Hardware L6 Developments in hardware technologies

SID 2016 E-safety Quiz

It is Safer Internet Day on 9th February 2016. To show our support you will all be having one ICT lesson on e-safety. We are also running a competition all you need to do it take part in an e-safety quiz.

Just click on this link and answer the questions. Make sure you login using your school email address and select your House.

E-SAFETY QUIZ

Winners will be selected by a prize draw and announced after half term.

Click on the Safer Internet Day 2016 website for help and advice and to see what events are taking place around the country.

TWEET how you will Play Your Part #SID2016 #SHAREAHEART
@WildernEsafety

Scratch Challenges

Challenge 1: Importing and Programming

a)  Can you import your two characters into your Scratch - a hero and villain
b)  Can you add a suitable backdrop?  

c)  Can you add aad script to your main character (hero) to so that the character will always follow the mouse

If you need help with any of this - click here! But only if you're really stuck! 

Challenge 2: Selection on your Baddy ! 

Watch this video..... it explains Selection - in Scratch IF, THEN, ELSE

Click the image to watch an introduction to 'selection' and 'if'

a)  Can you add script to your baddy or villain that programms him or her to do the following?

When the game is activated, he/she turns 15 degress
Moves 1o steps
If he/she touches the edge then he/she bounces off
He or she does all of the above until you programme him or her to do something else.

Test to see if it works...

If you need help with any of this - click here! But only if you're really stuck! 

 Challenge 3 : If Else  - GOODY

Click here to read and learn about If Else

a) ON YOUR GOODY (HERO) SPRITE
b) Edit the code blocks of your main character sprite to include an 'If Else' statement which does the following: 

When the game is started...

If the mouse button is held down, your hero/goody follows your mouse pointer

Else (if it isn't)  then the game speaks to you go say 'Hold Down the Left Mouse Button to Start for 3 secons)

This happens forever until you programme it to do something else..

If you need help with any of this - click here! But only if you're really stuck! 

Unit 1: Scratch - Lesson 2

Demonstrate Selection programming using 'if' in Scratch

Click the image to watch an introduction to 'selection' and 'if'

Task 1: 'If' in Scratch

a) Launch your Scratch game from last lesson

b) Add some code blocks to your game for the enemy sprite that:

• makes the enemy sprite move in a random direction
• if touching the edge bounce away

These are the codes blocks you could use

Task 2: If Else

Click here to read and learn about If Else

Task 3: 'If Else' in Scratch

a) Edit the code blocks of your main character sprite to include an 'If Else' statement

These are the code blocks you could use

Add in some If and If Else statements of your own into the game

a) Open your Scratch Evidence Document that you started last lesson

b) Complete the Lesson 2 section

c) Save your work

Developments in Hardware Technolgies

Errors and Exceptions

Until now error messages haven’t been more than mentioned, but if you have tried out the examples you have probably seen some. There are (at least) two distinguishable kinds of errors: syntax errors and exceptions.

8.1. Syntax Errors

Syntax errors, also known as parsing errors, are perhaps the most common kind of complaint you get while you are still learning Python:

>>>

>>> while True print 'Hello world'
  File "<stdin>", line 1, in ?
    while True print 'Hello world'
                   ^
SyntaxError: invalid syntax

The parser repeats the offending line and displays a little ‘arrow’ pointing at the earliest point in the line where the error was detected. The error is caused by (or at least detected at) the token preceding the arrow: in the example, the error is detected at the keyword print, since a colon (':') is missing before it. File name and line number are printed so you know where to look in case the input came from a script.

8.2. Exceptions

Even if a statement or expression is syntactically correct, it may cause an error when an attempt is made to execute it. Errors detected during execution are called exceptions and are not unconditionally fatal: you will soon learn how to handle them in Python programs. Most exceptions are not handled by programs, however, and result in error messages as shown here:

>>>

>>> 10 * (1/0)
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
ZeroDivisionError: integer division or modulo by zero
>>> 4 + spam*3
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
NameError: name 'spam' is not defined
>>> '2' + 2
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
TypeError: cannot concatenate 'str' and 'int' objects

The last line of the error message indicates what happened. Exceptions come in different types, and the type is printed as part of the message: the types in the example are ZeroDivisionError, NameError and TypeError. The string printed as the exception type is the name of the built-in exception that occurred. This is true for all built-in exceptions, but need not be true for user-defined exceptions (although it is a useful convention). Standard exception names are built-in identifiers (not reserved keywords).

The rest of the line provides detail based on the type of exception and what caused it.

The preceding part of the error message shows the context where the exception happened, in the form of a stack traceback. In general it contains a stack traceback listing source lines; however, it will not display lines read from standard input.

Built-in Exceptions lists the built-in exceptions and their meanings.

8.3. Handling Exceptions

It is possible to write programs that handle selected exceptions. Look at the following example, which asks the user for input until a valid integer has been entered, but allows the user to interrupt the program (using Control-C or whatever the operating system supports); note that a user-generated interruption is signalled by raising the KeyboardInterrupt exception.

>>>

>>> while True:
...     try:
...         x = int(raw_input("Please enter a number: "))
...         break
...     except ValueError:
...         print "Oops!  That was no valid number.  Try again..."
...

The try statement works as follows.

• First, the try clause (the statement(s) between the try and except keywords) is executed.
• If no exception occurs, the except clause is skipped and execution of the try statement is finished.
• If an exception occurs during execution of the try clause, the rest of the clause is skipped. Then if its type matches the exception named after the except keyword, the except clause is executed, and then execution continues after the try statement.
• If an exception occurs which does not match the exception named in the except clause, it is passed on to outer try statements; if no handler is found, it is an unhandled exception and execution stops with a message as shown above.

A try statement may have more than one except clause, to specify handlers for different exceptions. At most one handler will be executed. Handlers only handle exceptions that occur in the corresponding try clause, not in other handlers of the same try statement. An except clause may name multiple exceptions as a parenthesized tuple, for example:

... except (RuntimeError, TypeError, NameError):
...     pass

Note that the parentheses around this tuple are required, because except ValueError, e: was the syntax used for what is normally written asexcept ValueError as e: in modern Python (described below). The old syntax is still supported for backwards compatibility. This means exceptRuntimeError, TypeError is not equivalent to except (RuntimeError, TypeError): but to except RuntimeError as TypeError: which is not what you want.

The last except clause may omit the exception name(s), to serve as a wildcard. Use this with extreme caution, since it is easy to mask a real programming error in this way! It can also be used to print an error message and then re-raise the exception (allowing a caller to handle the exception as well):

import sys

try:
    f = open('myfile.txt')
    s = f.readline()
    i = int(s.strip())
except IOError as e:
    print "I/O error({0}): {1}".format(e.errno, e.strerror)
except ValueError:
    print "Could not convert data to an integer."
except:
    print "Unexpected error:", sys.exc_info()[0]
    raise

The try ... except statement has an optional else clause, which, when present, must follow all except clauses. It is useful for code that must be executed if the try clause does not raise an exception. For example:

for arg in sys.argv[1:]:
    try:
        f = open(arg, 'r')
    except IOError:
        print 'cannot open', arg
    else:
        print arg, 'has', len(f.readlines()), 'lines'
        f.close()

The use of the else clause is better than adding additional code to the try clause because it avoids accidentally catching an exception that wasn’t raised by the code being protected by the try ... except statement.

When an exception occurs, it may have an associated value, also known as the exception’s argument. The presence and type of the argument depend on the exception type.

The except clause may specify a variable after the exception name (or tuple). The variable is bound to an exception instance with the arguments stored in instance.args. For convenience, the exception instance defines __str__() so the arguments can be printed directly without having to reference .args.

One may also instantiate an exception first before raising it and add any attributes to it as desired.

>>>

>>> try:
...    raise Exception('spam', 'eggs')
... except Exception as inst:
...    print type(inst)     # the exception instance
...    print inst.args      # arguments stored in .args
...    print inst           # __str__ allows args to be printed directly
...    x, y = inst.args
...    print 'x =', x
...    print 'y =', y
...
<type 'exceptions.Exception'>
('spam', 'eggs')
('spam', 'eggs')
x = spam
y = eggs

If an exception has an argument, it is printed as the last part (‘detail’) of the message for unhandled exceptions.

Exception handlers don’t just handle exceptions if they occur immediately in the try clause, but also if they occur inside functions that are called (even indirectly) in the try clause. For example:

>>>

>>> def this_fails():
...     x = 1/0
...
>>> try:
...     this_fails()
... except ZeroDivisionError as detail:
...     print 'Handling run-time error:', detail
...
Handling run-time error: integer division or modulo by zero

8.4. Raising Exceptions

The raise statement allows the programmer to force a specified exception to occur. For example:

>>>

>>> raise NameError('HiThere')
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
NameError: HiThere

The sole argument to raise indicates the exception to be raised. This must be either an exception instance or an exception class (a class that derives from Exception).

If you need to determine whether an exception was raised but don’t intend to handle it, a simpler form of the raise statement allows you to re-raise the exception:

>>>

>>> try:
...     raise NameError('HiThere')
... except NameError:
...     print 'An exception flew by!'
...     raise
...
An exception flew by!
Traceback (most recent call last):
  File "<stdin>", line 2, in ?
NameError: HiThere

8.5. User-defined Exceptions

Programs may name their own exceptions by creating a new exception class (see Classes for more about Python classes). Exceptions should typically be derived from the Exception class, either directly or indirectly. For example:

>>>

>>> class MyError(Exception):
...     def __init__(self, value):
...         self.value = value
...     def __str__(self):
...         return repr(self.value)
...
>>> try:
...     raise MyError(2*2)
... except MyError as e:
...     print 'My exception occurred, value:', e.value
...
My exception occurred, value: 4
>>> raise MyError('oops!')
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
__main__.MyError: 'oops!'

In this example, the default __init__() of Exception has been overridden. The new behavior simply creates the value attribute. This replaces the default behavior of creating the args attribute.

Exception classes can be defined which do anything any other class can do, but are usually kept simple, often only offering a number of attributes that allow information about the error to be extracted by handlers for the exception. When creating a module that can raise several distinct errors, a common practice is to create a base class for exceptions defined by that module, and subclass that to create specific exception classes for different error conditions:

class Error(Exception):
    """Base class for exceptions in this module."""
    pass

class InputError(Error):
    """Exception raised for errors in the input.

    Attributes:
        expr -- input expression in which the error occurred
        msg  -- explanation of the error
    """

    def __init__(self, expr, msg):
        self.expr = expr
        self.msg = msg

class TransitionError(Error):
    """Raised when an operation attempts a state transition that's not
    allowed.

    Attributes:
        prev -- state at beginning of transition
        next -- attempted new state
        msg  -- explanation of why the specific transition is not allowed
    """

    def __init__(self, prev, next, msg):
        self.prev = prev
        self.next = next
        self.msg = msg

Most exceptions are defined with names that end in “Error,” similar to the naming of the standard exceptions.

Many standard modules define their own exceptions to report errors that may occur in functions they define. More information on classes is presented in chapter Classes.

8.6. Defining Clean-up Actions

The try statement has another optional clause which is intended to define clean-up actions that must be executed under all circumstances. For example:

>>>

>>> try:
...     raise KeyboardInterrupt
... finally:
...     print 'Goodbye, world!'
...
Goodbye, world!
KeyboardInterrupt
Traceback (most recent call last):
  File "<stdin>", line 2, in ?

A finally clause is always executed before leaving the try statement, whether an exception has occurred or not. When an exception has occurred in the try clause and has not been handled by an except clause (or it has occurred in a except or else clause), it is re-raised after the finally clause has been executed. The finally clause is also executed “on the way out” when any other clause of the try statement is left via a break, continue or return statement. A more complicated example (having except and finally clauses in the same try statement works as of Python 2.5):

>>>

>>> def divide(x, y):
...     try:
...         result = x / y
...     except ZeroDivisionError:
...         print "division by zero!"
...     else:
...         print "result is", result
...     finally:
...         print "executing finally clause"
...
>>> divide(2, 1)
result is 2
executing finally clause
>>> divide(2, 0)
division by zero!
executing finally clause
>>> divide("2", "1")
executing finally clause
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
  File "<stdin>", line 3, in divide
TypeError: unsupported operand type(s) for /: 'str' and 'str'

As you can see, the finally clause is executed in any event. The TypeError raised by dividing two strings is not handled by the except clause and therefore re-raised after the finally clause has been executed.

In real world applications, the finally clause is useful for releasing external resources (such as files or network connections), regardless of whether the use of the resource was successful.

8.7. Predefined Clean-up Actions

Some objects define standard clean-up actions to be undertaken when the object is no longer needed, regardless of whether or not the operation using the object succeeded or failed. Look at the following example, which tries to open a file and print its contents to the screen.

for line in open("myfile.txt"):
    print line,

The problem with this code is that it leaves the file open for an indeterminate amount of time after the code has finished executing. This is not an issue in simple scripts, but can be a problem for larger applications. The with statement allows objects like files to be used in a way that ensures they are always cleaned up promptly and correctly.

with open("myfile.txt") as f:
    for line in f:
        print line,

After the statement is executed, the file f is always closed, even if a problem was encountered while processing the lines. Other objects which provide predefined clean-up actions will indicate this in their documentation.