Programming for GCSETopic 1.3: Python
Variables and Types
Teaching London Computing
William MarshSchool of Electronic Engineering and Computer Science
Queen Mary University of London
Aims• Understand the idea of a variable• Using metaphors to explain variables• Using variables to breakdown complex expressions• Assignment statements• Input from the user
Program Variable • Name for a value
• Names are really important• Name meaning
• Value can change
Which are the following everyday values are like variables?
• Your height• Your age• The credit on your
oyster
Python Variables• Python variable appear when used:
>>> bill = 21>>> bill21>>> bill = "hello">>> bill'hello'
In Python, a variable can
change between an integer and a
string. Confusing?
Using a Variable• A variable can be used instead of a value:
• The output is:
• The variables are: ‘greeting’ and ‘planet’• Any names
greeting = "Hello"planet = "World"print(greeting, planet)
'Hello World'
Errors• A variable must be given a value before it is used:
• Notice:• The message is complex – read it carefully• Only the first error is mentioned
>>> area = length * widthTraceback (most recent call last): File "<pyshell#4>", line 1, in <module> area = length * widthNameError: name 'length' is not defined
Variable as a Memory Location• A variable is a
location in the computer’s memory
• Python takes care of ‘where’ to put the values
"CS"
4
“FUN”
Name Value
subject
number
activity
Decomposition using Variable • How to break a complex calculation down into simpler
steps?• Recurring question in programming
>>> km_mile = (1760 * 36 * 2.54) / 100 / 1000>>> km_mile1.609344
>>> inch_mile = 1760 * 36>>> cm_mile = inch_mile * 2.54>>> m_mile = cm_mile / 100>>> km_mile = m_mile / 1000>>> km_mile1.609344
Assignment• The statement to change a variable is call ‘assignment’
• … can be read as ‘10 is assigned to x’
• How do you read?
>>> x = 10
>>> x = y
Assignment II• “x is assigned the value of y” or “the value of y is assigned
to variable x”
• How are the variables used?• Variable ‘x’ changes value; the value of ‘y’ stays the same• The value of ‘y’ is used (or read); an error occurs if ‘y’ has
never been given a value.• It does not matter if ‘x’ has no value before the statement; if it
does, it is lost
>>> x = y
Quiz – I • What is the value of ‘x’ after these statements have been
executed?
• You can check the answer using the Python shell
x = 5x = x + 3
Quiz – II • What is the values of ‘x’ and ‘y’ after these statements
have been executed?• Each box is a separate problem
• You can check the answers using the Python shell
x = 3y = 2x = x + y
x = 3y = 2 + xx = x + y
x = 3y = 2x = x + yy = x – 2
Input• A make a flexible greeting program let’s ask the name
first
• The output is (if you type the letter underlined):
#Greeting anyonename = input("What's your name?")print("Hello", name)
What's your name?WilliamHello William
Input a Number• Input always reads a string• Must not confuse string and number
• Consider:
• The result is:
#This program calculates your age next year#... unfortunately it does not workage = input("How old are you? ")print("Next year you will be", age+1)
How old are you? 21Traceback (most recent call last): File “age-wrong.py", line 4, in <module> print("Next year you will be", age+1)TypeError: Can't convert 'int' object to str implicitly
Using the ‘int’ function• Use the ‘int’ function to convert a string (of digits) to a
number• Try the corrected program:
#This program calculates your age next yearage = input("How old are you? ")print("Next year you will be", int(age)+1)
Summary• A variable gives a name to a value• Choose a meaningful name
• Reading and Assignment• A variable can be read in an expression• A variable can be changed in an assignment statement
• Use variables • To replace a complex expression with several simpler ones• To hold an ‘input’ string from the user
• Use ‘int’ to convert a string to an ‘integer’
