1. Teaching London Computing
FUNCTIONS in Python
Margaret Derrington KCL Easter 2014

2. Let’s start with some revision…
What will these instructions do?
print (“Hello World”)
print (2+3)
input (“2+3 = “)
int(input (“type a number “))
len(“Margaret”)
hex(42)
bin(15)
If you type them into python shell, what will happen?
Check with your neighbour that you agree

3. These are actually all FUNCTIONS
print (“Hello World”)
print (2+3)
input (“2+3 = “)
int(input (“type a number “))
len(“Margaret”)
hex(42)
bin(15)
print, input, len, input, hex, bin ….
…………..Are all FUNCTIONS
What do they have in common…? besides being PINK!

4. Functions We CALL a function It has a name, and brackets
PASS
PARAMETER
RETURN
We CALL a function
It has a name,
and brackets
and inside the brackets are PARAMETERS
Parameters are PASSED to a function
The function does something with those parameters,
and returns an output or result

5. procedures, subroutines,
BUILT – IN FUNCTIONS
Python has lots of build in functions
We have already learnt and used several of them
But today we are going to learn to CREATE OUR OWN NEW FUNCTIONS
We will DEFINE functions by giving them a NAME, saying what the parameters will be
And what the result will be when we CALL the function and pass it the necessary parameters
Also called
procedures, subroutines,
methods
Also called DECLARING

6. Which is now one more than it was
We can call a function and use the result afterwards
or do one function inside another
Defining a function
Which is now one more than it was
def AddOne (number):
number = number+1
return number
answer= AddOne(7)
print (answer)
print (AddOne(4))
def repeat (text):
return text*2
print (repeat(“hello”))
print (repeat(“2”))
print (repeat (2))
What happens?
What happens?

7. Defining a function def NameOfFunction (parameters) :
Don’t forget the colon
def NameOfFunction (parameters) :
explain what happens
and how you work out the
return RESULT
<~~~stop indenting at end of definition
INDENT
Parameters are like variables;
names that can have lots of different values
The syntax is important and must be correct

8. More than one parameter
A function may have more than one parameter….
def AreaOfRectangle (length, width)
return length*width
This function with multiply the length by the width to give the area of a rectangle
It has TWO parameters.

9. Exercise Create some simple functions to add three to a number
To triple a number
To get a user input name and print it in capital letters
To get a username and print a string “Hello username”
Remember if the function does not include printing the result, you will have to do that in a separate function…. print.
Define a new function ShoutName which uses these functions to make a function which gets a user’s name and shouts (in capital letters) “HELLO USERNAME!!!”
Functions aren’t actually programs, but a program which defines and uses functions is much easier to follow.

10. SCOPE (of a variable)
GLOBAL and LOCAL are used to describe variables which exist throughout a program (global) or just within a small part of it (local)… like inside a function
These are important concepts, because they can cause a lot of confusion if they get mixed up
def doubler (number):
answer = number*2
return answer
Here answer is a local variable, it only exists and has a meaning inside the function
It will cause confusion if answer is used elsewhere in the program with a different meaning.

11. GCSE Syllabus USE of functions is required Creating functions is on AS
But creating them helps you undertsand how to use them
And it also helps to break problems into separate parts that can get reused.
And to use functions to fix those separate parts