AQA Computer Science GCSE
Data Representation – Representing Text and Characters
Everything stored inside a computer has to ened up being represented using just binary digits. That means everything has to become a series of numbers. That includes writing (just like this paragraph that you just read...).
True/False slides – a starting point
To do this, each character on a keyboard needs to have a number associated with it. That means that each letter, punctuation mark, symbol and digit needs to be able to be represented using a number. This includes spaces and some "non-printing characters" - for example, the "character" which represents a new line.
This was first done using ASCII code - a series of 128 character codes to represent characters. But 128 characters isn't enough to deal with modern computers, so the system was extended using Unicode. You need to know how both work and why they work the way they do.
ASCII code – slides from class
Unicode – slides from class
ASCII and Unicode – double page spread
The 128 character ASCII code table – which uses 7 bit binary
So, the word "tomato" has 6 characters, each encoded using 7 bits. It needs 6 x 7 bits = 42 bits to save the word.This becomes important when you start to use data compression to reduce file sizes.
ASCII is an example of a computing standard.
Working with Characters and Strings
When you manipulate string variables in programming languages, the ASCII (or Unicode) values for each character are actually how the programming language deals with the variables. The slides below will help explain how this works.
Converting Characters to Codes:
You can convert a character to its ASCII code representation in most programming languages. This can be helpful sometimes and is something you need to know for exams.
Some pseudocode to do this is shown below:
FOR i <- 0 TO LEN(theName) - 1 #LEN gives the length of the string
This code iterates over the string letter by letter using the FOR loop. CHAR_TO_CODE then converts the next character to its ASCII code number and prints that out (so, D is 68, o is 111 etc...). The space in the name will be converted to 32 - the ASCII code value for a space character.
Some Python to do the same thing is shown below:
for i in range(0, len(theName)):
ord() is the Python equivalent of CHAR_TO_CODE. Perhaps the only time that Pseudocode is easier to use than Python!
A slightly different way of iterating over the string in Python is shown below. This is easier to use when you have to use a FOR loop.
for letter in theName:
This method means you don't have to keep track of the value of i.
Converting Codes to Characters:
You can also convert from a character code to a character using CODE_TO_CHAR:
theChar <- CODE_TO_CHAR(theCode)
This code uses USERINPUT to allow you to enter a character code. When you enter a number in Python it gets stored as a string, so when using Python we need to make sure we convert to an integer first.
theChar = chr(theCode)
This time Python uses the built in function chr() to convert from an integer to a character.
- STRING_TO_INT(aString) - converts from a string to an integer
- INT_TO_STRING(anInteger) - the opposite
- STRING_TO_REAL(aString) - converts from a string to a real number (a decimal number, e.g. 3.142)
- REAL_TO_STRING(aRealNumber) - the opposite