Introduction to python

AN INTRODUCTION TO PYTHON

OCR GCSE COMPUTING

An Introduction to Python
Table Of Contents

Section 1 - Getting Started
• 1.1 - Meeting Python
• 1.2 - Writing Programs
• 1.3 - Arithmetic
• 1.4 - Comments

Section 2 - Data Types
• 2.1 - Strings
• 2.2 - Numbers
• 2.3 - Boolean
• 2.4 - Variables
• 2.5 - Arrays

Section 3 - Control Flow
• 3.1 - Selection
• 3.2 - Iteration
• 3.3 - Procedures

Section 4 - Additional Exercises
• 4.1 - Straightforward Exercises
• 4.2 - Challenging Exercises
• 4.3 - File Handling

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MARK CLARKSON, 2010


OCR GCSE COMPUTING

1.1 Meeting Python

1.1a - Downloading Python
Python is a freely available programming language. You will need to install the Python compiler in
order to make your own programs, although you don’t need any special software to WRITE Python
programs.

Go to the python.org website and download a standard installation of the latest version of Python
(currently 3.1). This will provide you with everything you need.

NB: Do NOT download Python 2.6.5 as some of the programs in this course will NOT work! It
must be version 3 or higher!!!

The documentation for Python is very good, and you can ﬁnd it at http://docs.python.org/py3k/

1.1b - IDLE
You can write Python code in notepad, or any other simple text editors - but it is easier if you use a program like IDLE
(which comes with the standard Python installation) because it helps with things like indentation and debugging
(things we will cover later). We will be using IDLE throughout most of the course.
When you run IDLE you will see a window like this:

NB: The (few) screenshots in this booklet are taken from Mac OS X. Windows users may see something slightly
diﬀerent, but largely similar.
You can type simple commands at the prompt (>>>).
Try typing:
2+2
17-9
16/4
3*7

Check that the answers are correct!

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1.2 Writing Programs
Quick note: you are going to be making lots of Python programs. Create a Python folder somewhere so that you can
save your files here!

1.2a - Hello World
We can put simple instructions into the IDLE window, but we can’t write and save complex programs. Open IDLE
and click ‘file >> new’ - this will give you a new window that you can use to write and save your programs!
In this new window, type the following (exactly):

print(“Hello, World”)

Save this file in your Python folder (call it “hello world.py”) and then press F5 to run it.
The main IDLE window should show you your output:

Main IDLE window

“Hello World”
program window

Congratulations! You have written your first Python program!
You can run your program on any computer with Python installed, no matter what kind of computer it is.

1.2a - Hello Dave
Lets try something a little more... interactive now.
Create a new file and type in the following:

name = input(“What is your name? ”)
print(“Hello,”,name)

What do you think will happen?
Save this file as “hello dave.py” and press F5 to run it. See if you were correct.

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OCR GCSE COMPUTING

1.3 Arithmetic

1.3a - Addition
Create a new ﬁle and type in the following code:
x = int(input(“Enter a number between 1 and 100: “))
y = int(input(“Enter another number between 1 and 100: “))
print(“X + Y =”,x+y)
Save this ﬁle as “addition.py” and press F5 to run it.

1.3b - Subtraction
Repeat the last program, but this time make it subtract the two numbers. Save it as “subtraction.py” and test it works.

1.3c - Multiplication
Repeat the last program, but this time make it multiply the two numbers. Save it as “multiplication.py” and test it works.

1.3d - Division
Repeat the last program, but this time make it divide the two numbers. Save it as “division.py” and test it works.

1.3e - Square
Repeat the last program, but this time make it calculate x2. Save it as “square.py” and test it works.

1.3f - Powers
Repeat the last program, but this time make it calculate xy. Save it as “powers.py” and test it works. (Hint: use x**y)

1.3g - Mod
Repeat the last program, but this time make it calculate the modulus (remainder) of a division. Save it as “mod.py” and
test it works. (Hint: use x%y)

1.3h - Order of Operations / BIDMAS
Try writing a program that will take a number, multiply by three and then add four.

Try writing a program that will take a number, add four and then multiply by three.

Put the number 7 into both programs and check that they work correctly.

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1.4 Comments

1.4a - Simple Comments
Comments are bits of text in the program code that aren’t used by the computer, but help to explain what is going on.

You can write a comment using a # symbol (Alt-3 on a Mac).

Try this program:
#This is a comment
print("This is not a comment.")

1.4b - Inline Comments
You can include a comment at the end of a line of code. But not the start:
#This is a comment
#This is also a comment. It won't print(“anything”)
print("This is not a comment.") #although this is...

1.4c - Block Comments
Sometimes you might want to have a large block for a comment. This is often the case at the start of programs where
programmers put useful information such as who wrote it, when and what it is supposed to do.
Try to guess what the output will look like:
#This is a comment
#This is also a comment. It won't print(anything)
print("This is not a comment.") #although this is...
"""
These
are
all
comments
"""
print("But the comments are finished now.")

Comments make it MUCH easier to see what you have done when you come back to your code several months later
and you should use them regularly.

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2.1 Strings

2.1a - Adding Strings
A string is a block of text. It could be a single word, a sentence or an entire paragraph. In Python we use the term “str” to
mean a string.

Try out the following code:
start = "Hello, "
name = input("What is your name? ")
end = ". How are you today?"

sentence = start + name + end

print(sentence)
If you add strings together, you end up with one long string.

2.1b - When is a number not a number?
Try this one:
number = str(2)

print("2 + 2 = ", number + number)
This makes the value “2” into a string - so it is a word rather than a number.

2.1c - Line Breaks / New Lines
You can also write your string over several lines by using “n”. Like this:
firstLine = "I got a feeling,nThat tonight's gonna be a good night.n"
secondLine = "That tonight's gonna be a good, good night."

print(firstLine + secondLine)

Try writing a few of your own!

2.1d - Combining strings and numbers
You can use a comma to combine diﬀerent bits of text or numbers. You will use this a lot:
start = "2 + 2 ="
number = 4

print(start,number)

print("Two plus two = ",number)

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2.2 Numbers
2.2a - Integers
There are two main types of numbers in Python. We’ll start with Integers (whole numbers).

We can force Python to make something a whole number by saying int(number). Like this:
x = 3.7
y = int(x)
print(y)
Note that this won’t round the number up, it just takes the whole number part.

2.2b - Floats
The second main type is a floating point, or just “float” for short (a decimal). We can force Python to make something a
float by saying float(number). Like this:
x = 3
print(x)
y = float(x)
print(y)

2.2c - Casting
Casting is the technical term for changing the type of data you are working with. You’ve already been casting by saying
int(), float() and str() to change the data type. But now you know what it’s called!

2.2d - Automatic Casting
Python will automatically change the data type for you a lot of the time:
x = int(22)
y = int(7)
z = x/y
print(z)
Here, the two integers have provided a floating point answer and Python has changed the data type of z for you.

2.1e - Long
Another number type you might need occasionally is long, or a long integer. Integers in Python can only hold numbers
from -2billion to +2billion (actually from =2,147,483,646 to +2,147,483,647). If your number is going to be outside that
range then you will have to use a long to keep count.

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2.3 Boolean
2.3a - True or False?
A boolean (bool for short) can store one of only two possible values - True or False. (NB: Note the capital letters!)
This is useful for logical tests (e.g. is 22/7 bigger than 3?) or to check if something has been completed (this becomes
useful in section 3).

Here is an example:
x = 22/7
bigger = False
print(bigger)

if (22/7 > 3):
bigger = True
print(bigger)

The main advantage of booleans is that they are very small and simple.

2.4 Variables
2.4a - What’s a Variable?
Variables are one of the most essential aspects of programming, and you’ve been using them all the time.
A variable is literally something that can vary. It can be a number, a character, a string, a boolean or other types we
haven’t met yet.
All variables have a name. x is a variable. So is n. In the boolean example above, bigger is a variable.

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OCR GCSE COMPUTING

2.5 Arrays
2.5a - A Ray of Sunshine?
OK, so puns aren;t my strong point.
An array is a collection of something, like integers:
x = [3,5,9,6]
print(x)
print(x[0])
print(x[1])
print(x[2])
print(x[3])

You use square brackets to write the list and then square brackets again to say which ARRAY ELEMENT you want
(remembering to start with 0).

2.5b - Editing Arrays
As well as reading individual elements we can write to individual elements too:
x = [3,5,9,6]
print(x)
x[1] = 2
print(x)

2.5c - Lists
Arrays are really good for lists. Lets say we want to store a load of people’s ages:
names = ["Paul","Phillip","Paula","Phillipa"]
ages = [12,15,11,14]

print(names[0],"is",ages[0])

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OCR GCSE COMPUTING

3.1 Selection
Selection means selecting (or choosing) what to do next. Should I cycle to school, or ask for a lift? If
it’s a sunny day I might cycle. If it’s raining, I’ll ask for a lift.

3.1a - IF ... ELSE
Create a new file and type in the following code:
x = int(input(“How many hours a day do you play computer games? “)
if x < 2:
print(“That seems a fairly healthy balance. Well done!”)
else:
print(“You’re probably good enough by now with all that practice.”)
Save this file as “games.py”, press F5 and test it.

Notice how the colon (:) is used to say what should happen in each case.
Also notice that the indentation is VERY important. Python only knows when your IF statement is finished by looking
at the indentation!
Try this program to see what I mean:

if x < 2:
else:
print(“Xbox 360s are better than PS3s”)

That last line will be printed no matter which option you chose.

3.1b - IF ... ELIF ... ELSE
Sometimes there are more than two options. I could walk OR cycle OR get the bus OR get a lift. As well as IF and
ELSE, we can stick an ‘ELSE IF’ (or ELIF) in the middle:

if x < 2:
elif x < 4:
else:
print(“Put the controller down and get some fresh air once in a while!”)

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OCR GCSE COMPUTING

3.1c - IF ... ELIF ... ELIF ... ELIF ... ELSE
You can include an unlimited number of ELIFs if you need to. Try the following:
menu = "What would you like:n
1. A complement?n
2. An insult?n
3. A proverb?n
4. An idiom?n
9. Quitn"

x = int(input(menu))

if x == 1:
print("You look lovely today!")
elif x == 2:
print("You smell funny.")
elif x == 3:
print("Two wrongs don't make a right. But three lefts do...")
elif x == 4:
print("The pen is mightier than the sword.")
elif x == 9:
print("Goodbye!!!")

There are a couple of important bits here:
• You can put a line break in your string by using “n”.

• You can continue a line of code by putting a “” at the end.

• If you are testing for equality, use a double equals (is 3x2 == 6?)

Try making your own menu system.

3.1d - Inequalities
This seems a good point to mention some of the most import inequalities used in selection:
< Less Than e.g. 3 < 5
> Greater Than e.g. 5 > 3
<= Less Than Or Equal To e.g. 3 <= 5 5 <= 5
>= Greater Than Or Equal To e.g. 5 >= 3 5 >= 5
!= Not Equal To e.g. 5 != 3 3 != 5

3.1e - SELECTION CHALLENGE
Try making a calculator.
The program will ask for two numbers and then give you a menu, so you can add, subtract, multiply, divide, square
either number or ﬁnd out the power.

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OCR GCSE COMPUTING

3.2 Iteration
Iteration is a posh way of saying “loop” (iteration literally means to do something again).

Loops are absolutely vital in programming in order to avoid having to write sequences of code out again and again. And
they come in several forms:

3.2a - FOR Loops
Try this code:
for x in range(10):
print(x)
NB: Note that the “print” instruction is indented by one tab.
The most interesting thing to note is that computers start counting at 0. This is actually better than starting at one, but
isn’t always obvious straight away.

We can also see that you have to finish the loop one number HIGHER than the last number you actually want.

You can be a bit more specific with your loop by setting a start and an end number:
for x in range(5,10):
print(x)

And you can do slightly clever things than just printing numbers:
print(x,"potato")

Loops are closed (or finished) by removing the indentation:
print(x,"potato")

print("4")

print(x,"potato")

print("more")

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You can also specify how the counter will count:
for x in range(2,9,2):
print(x)

print("Who do we appreciate?")

Or the three times table:
print(x)

3.2b - FOR Loop Challenge

1. Look closely at the three times table example above. Write a similar program to calculate the four times table.
2.Write a program that will calculate the 5 times table.
3.Try writing a program that will prompt for an integer and print the correct times table (up to 12x). Test with all of the
tables up to 12.

3.2c - While Loops
For loops create their own counter and work well if you know how many times you want them to run. While loops give
you a bit more freedom in when your loop will end. Here are some examples.
Find all of the square numbers below 100:
x = 1
y = x**2

while y < 100:
print(x,"squared =",y)
x = x + 1
y = x**2

A motorbike costs £2000 and loses 10% of its value each year. Print the bike’s value each year until it falls below £1000
x = False
y = 2000

while x == False:
print(“£”,y)
y = y * 0.9

if y < 1000:
x = True

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OCR GCSE COMPUTING

3.2d - Nested Loops
Sometimes you need to put one loop inside another loop. The logic can get pretty complicated, so you have to be careful
- but it does work!

Think about what this program will do. Decide what you think will happen. Then try it.
for y in range(1,11):
print(x,"x",y,"=",x*y)

Were you correct? Was it easy to follow the logic? How many lines of code would you need to write it with only 1 loop at
a time?

3.2e - Break Out
Sometimes you need to stop a loop before coming to the end of the original instructions. To do this you can use a
break:
for x in range(10000000000):
print(x)
if x == 20:
break

Write a program that will start at 1 and end at 1,000,000. The program should print each number followed by its square
but break once this square is bigger than 4096.

3.2f - Loop Challenge
A factorial means the number and all of the preceding numbers multiplied together.
So 4 factorial (written 4!) = 1 x 2 x 3 x 4
5! = 1 x 2 x 3 x 4 x 5
And so on...

Write a program that will prompt for an integer and then calculate the factorial of that number.

To test it works, 8! > 40,320

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OCR GCSE COMPUTING

3.3 Procedures
If the same piece of code needs to be used several times we can use a loop - but only if those times
are all together. If you need to run the same bit of code again later, you can use a procedure.
3.3a - A Simple Procedure
In this example I want to print the squares of the numbers from 1 to 20:
def output(number):
print(number,"squared =",number**number)

x = 1

while x < 21:
output(x)
x = x + 1

Here the top two lines are a procedure called output. The procedure expects to be given one variable (which it calls
number) and will happily do its thing when told to.

The main program is underneath, and the line that says output(x) will run the output procedure and give it the
variable x to work with.

3.3b - Multiple Parameters
A parameter is the variable (or variables) that get passed to the procedure. you can require more than one parameter:
def output(number1,number2):
print(number1,"+",number2,"=",number1+number2)

for x in range(20):
y = x**2
output(x,y)

You can pass as many parameters as you like to a procedure as long as you set it up correctly.
3.3c - Optional Parameters
Once you have deﬁned a procedure, you can call it in a number of ways:
def output(grade,score=50,feedback="Well done!"):
print("You scored",score,"which is a grade",grade,feedback)

output("C")
output("A",87)
output("E",12,"Rubbish!")

You can pass a many (or few) of the parameters as you like - as long as they’re in the right order. This means you have to
plan the procedure quite carefully.

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OCR GCSE COMPUTING

3.3d - Functions
So far our procedures have just printed an answer to the console. Sometimes a procedure will have to send some data
back. This is called a return. Procedures that return something are called functions. In Python that isn;t a huge
distinction, but in some programming languages it is very important.
def calculate(number):
newnumber = number / 100
return (newnumber)

y = calculate(x)
print(x,"% = ",y)

The line return(newnumber) passes the value of the newnumber variable back to the main program.

The line y = caluclate(x) shows that the main program expects a value to be returned and has somewhere to
store it.

3.3e - Modules
There are some procedures already built in to Python (print, for example). Modules (sometimes known as libraries in
other languages) are collections of extra procedures and functions that are pre-written.
Try this program ﬁrst. It won’t work, but I’m trying to prove a point (sqrt is short for square root):
x = 49
y = sqrt(49)
print(y)

Now try this one:
import math

x = 49
y = math.sqrt(49)
print(y)

You can ﬁnd a list of modules at http://docs.python.org/py3k/modindex.html

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OCR GCSE COMPUTING

4.1 Straightforward Exercises
Here are some fairly straightforward exercises. They’re not completely easy, but should be quite
manageable.

All of these programs should include the following comment block at the start (preferably completed) as well as
appropriate comments within the code to explain what is going on:
"""
Filename:
Author:
Date:
Description:
"""

4.1a - Pythagoras’ Calculator
Write a program that will do the following:
• Print a menu:
Pythagoras’ Calculator
1 - Find a hypotenuse
2 - Find another side
9 - Exit
Enter an option:
• If ‘1’ is entered, prompt for the two other sides, calculate the hypotenuse and print the answer. Reprint the menu.
•If ‘2’ is entered, prompt for the hypotenuse and the other side, calculate the third side and print the answer. Reprint the
menu.
•If ‘9’ is entered, print a goodbye message and exit (break)
•If another value is entered, print an error message and print the menu again.
NB: Remember you will need to import the math module (import math) and use the sqrt() function.

4.1b - Primary Division
• Prompt the user for two numbers.
• Calculate, and print the result of the division in the format x remainder y (e.g. 17 / 4 = 4 remainder 1).
• Ask the user if they would like to run the program again or quit.

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OCR GCSE COMPUTING

4.1c - Random Arithmetic
The random module lets you generate random numbers (actually pseudo-random, but that’s another story) using the
function random.randint(x,y) where x and y are the lower and upper boundaries.

To get you started, try this program:

import random #import the random module

for n in range(20): #repeat 20 times
print(random.randint(1,100)) #print a random integer between 1 and 100

• Generate two random numbers
• Calculate and print the results if you add, subtract, divide and multiply the two numbers verbosely (e.g. “2 + 3 = 5”)

4.1d - Code Maker (and Breaker) - Lite Version
• Print a menu:
Code Maker (and Breaker)
1 - Encode a letter
2 - Decode a letter
9 - Exit
Enter an option:
• If ‘1’ is entered, prompt for a letter and use a selection statement to convert the letter into an integer (a = 1, b = 2, etc.).
• If ‘2’ is entered, prompt for an integer, check it is valid (prompt again if invalid) and convert the integer into a letter.
• If ‘9’ is entered, print a goodbye message and exit (break)
• If another value is entered, print an error message and print the menu again.

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OCR GCSE COMPUTING

4.1e - Time Calculator
• Print a menu:
Time Calculator - Arithmetic Mode
1 - Add 2 times
2 - Find the diﬀerence between 2 times
8 - Conversion mode
9 - Exit
Enter an option:
• If ‘8’ is entered, print a menu:
Time Calculator - Conversion Mode
1 - Convert Time to Days
2 - Convert Time to Hours
3 - Convert Time to Minutes
4 - Convert Minutes to Time
5 - Convert Hours to Time
6 - Convert Days to Time
8 - Conversion mode
9 - Exit
Enter an option:
• If ‘8’ is entered, return to the ﬁrst menu.
• Complete all of the required procedures.
• Times should be stored as strings in the format DD:HH:MM.
• Days, Hours and Minutes should be stored as integers.

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4.1f - ATM - Lite Version
Northern Frock needs you to write a program for their new ATMs (or Automatic Teller Machines). Assuming the
starting balance is £67.14, Write a program that will do the following:
• Print a menu:
Welcome to Northern Frock
1 - Display balance
2 - Withdraw funds
3 - Deposit funds
9 - Return card
Enter an option:
• If ‘1’ is entered, display the current balance and also the maximum amount available for withdrawal (must be a
multiple of £10)
• If ‘2’ is entered, provide another menu with withdrawal amounts of £10, £20, £40, £60, £80, £100, Other amount,
Return to main menu & Return card.
• Check that the requested withdrawal is allowed, print a message to show that the money has been withdrawn and
calculate the new balance.
• If ‘Other amount’ is selected then prompt the user for an integer value. Check this number is a multiple of 10 and that
the withdrawal is permitted (as above).
• If ‘3’ is entered, provide another menu that will allow the user to enter an amount to deposit(does not need to be a
multiple of £10), return to main menu or return card. If funds are deposited, provide appropriate feedback and update
the balance.

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OCR GCSE COMPUTING

4.2 Advanced Exercises
Here are some more challenging exercises. These are a bit more diﬃcult, but are very good practice.

All of these programs should include the following comment block at the start (preferably completed) as well as
appropriate comments within the code to explain what is going on:
"""
Filename:
Author:
Date:
Description:
"""

4.2a - Prime Candidate
Prime numbers are numbers that are only divisible by 1 and itself. The ﬁrst few prime numbers are 2, 3, 5, 7, 11 & 13.
The easiest way to test for primes is to try dividing it by every number up to the square root of that number.
e.g. tests for 27 and 31. Divide by every number up to the square root of that number (5.196 $ 5.57)
27 / 2 = 13 r 1 31 / 2 = 15 r 1
27 / 3 = 9 r 0 (we could stop now, but we won’t) 31 / 3 = 10 r 1
27 / 4 = 6 r 3 31 / 4 = 7 r 3
27 / 5 = 5 r 2 31 / 5 = 6 r 1
No point going past 5.196 No point going past 5.57
3 & 9 are factors, so NOT prime No factors found, so IS prime

The easiest way to do this is to use the % operator (27 % 2 = 1 & 27 % 3 = 0)

• Prompt the user for a lower and upper number (limited to a maximum range of 200 and going no higher than 10,000).
• Print all of the prime numbers in that range.

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4.2b - Binary Search
A binary search is the most efficient way to find a value. It involves splitting the available values into two equal halves
and testing which half it is in, and then refining this until the correct value is found.
e.g. A number must be between 1 and 100, in this case it is 53.
Midpoint between 1 and 100 is 50 [int((1+100)/2)]. Target number is higher.
Midpoint between 50 and 100 is 75. Target number is lower.
Midpoint between 50 and 56 is 53. Target number is found.

• Prompt the user for an integer between 1 and 100 (validating and prompting if invalid)
• Use a selection statement to test the value 50 (below, equal, above) and print this guess.
• Repeat the above until the correct number is found.

Extension 1: Count and output the number of steps required to find the number.
Extension 2: Generate a random number as the target.
Extension 3: Print all of the values that require exactly 7 steps to find them.
Extension 4: Find the mean average number of steps required to find every number form 1 to 100.
Extension 5: Calculate the maximum and mean number of steps required if the range of numbers is 200, 1,000, 2,000,
10,000 & 100,000. Are you surprised by the results?

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OCR GCSE COMPUTING

4.2c - Advanced Strings
You can use the following functions to do interesting things with strings:
string = “Strings are sequences of letters”
x = len(string) #Returns the length of the string x = 32
x = string[0] #Returns the 1st character of the string x=S
x = string[3] #Returns the 4th character of the string x=i
x = string[3:5] #Returns the 4th and 5th characters of the string x = in
x = string[3:6] #Returns the 4th - 6th characters of the string x = ing
x = str.lower(string[0]) #Returns a lower case string x=s
x = str.upper(string[3]) #Returns a lower case string x=I

x = str.islower(string[3])
#Returns a bool depending on whether the character is lower case x = True

x = str.isupper(string[0])
#Returns a bool depending on whether the character is lower case x = True

x = str.isdigit(string[5])
#Returns a bool depending on whether the character is a digit x = False

x = str.isspace(string[7])
#Returns a bool depending on whether the character is a space x = True

• Prompt the user for a string
• Calculate and print:
The number of characters (with spaces)
The number of characters (without spaces)
The number of upper case characters
The number of lower case characters
An upper case version of the string
A lower case version of the string
Whether the string is a palindrome (the same backwards as forwards)
Print the string in reverse

PAGE 23 OF 31

MARK CLARKSON, 2010


OCR GCSE COMPUTING

4.2d - Code Maker (and Breaker) - Full Version
Remind yourself of the lite version of this program (4.1d). This program encoded (or decoded) one character at a time.

• Print a menu:
Code Maker (and Breaker)
1 - Encode a sentence
2 - Decode a sentence
9 - Exit
Enter an option:
• If ‘1’ is entered, prompt for a string and convert the string into a series of string of digits (a = 1, b = 2, etc.) separated by
1 space per character (space = 27). Print this string.
• If ‘2’ is entered, prompt for a string of numbers separated by 1 space per character and convert into a string. Print this
string.

PAGE 24 OF 31

MARK CLARKSON, 2010


OCR GCSE COMPUTING

4.2e - Bubble Sort
A bubble sort is a simple (but not particularly efficient) method of putting a list in order.
A bubble sort looks at the first two elements in a list (or array) to see if the first is bigger than the second. If it is, they get
swapped round. Either way, the 2nd and 3rd are checked, then the 3rd and 4th, etc...
At the end, the whole thing is done again.
Only when a complete run through happens without any swaps is the list correct.
e.g.
x = [3,9,5,2]
Compare x[0] and x[1], 3 < 9 so no swap
Compare x[1] and x[2], 9 !< 5 so swap [3,5,9,2]

Run again:

Run again:

Run again:

No swaps, so sorting is complete!

• Generate an array of 6 random integer.
• Print the list, unsorted.
• Perform a bubble sort.
• Print the sorted list.

PAGE 25 OF 31

MARK CLARKSON, 2010


OCR GCSE COMPUTING

4.3 File Handling
BE CAREFUL!!
It is possible to do some serious damage here if you’re not careful!

Before we start, an important note about directory structures:
Under Windows the default file location will be in the Python installation directory (typically C:python3.1). On a school
network you can use file locations such as “h:/temp” to refer to your home directory - note which slash is used!.
Under Mac OS X the default file location is your home directory and if you store your work in a sub-directory you could
use something like “pythonFiles/temp”.
To keep the examples simple the file details here are assumed to be in the default directory. But be careful, and ask for
help if unsure!

4.3a - Do This First
All will become clear, but before we can do anything else we need to create a data file:
f = open("temp","w")
for n in range(1,11):
m = "This is test number " + str(n) + "n"
f.write(m)

f.close()
f = open("temp","r")
print(f.read())
f.close()
You should have the following printout:

4.3b - Opening Files
To open a file, you have to make it a variable (just as a string or an integer must be a variable). We do this with the
following command:
f = open("filename","mode")
Where filename is the name of the file and the mode must be one of the following:
r read only
w write only - NOTE, this destroys any previous version of the file!
r+ read AND write
a append mode (will write only after the existing data)

Once you have finished with a file you MUST get into the habit of closing it, or problems will occur. To do so simply
write f.close().
PAGE 26 OF 31

MARK CLARKSON, 2010


OCR GCSE COMPUTING

4.3c - Reading From Files
Once your file is open you can read from it using the functions:
f.read() - read the entire file
f.readline() - read the current line

It is important to remember that Python keeps track of where you are up to.
Try the following code:
print(f.readline())
print("That was the first line")
print(f.readline())
print("That was the second line")
print(f.read())
print("That was all the rest")
print(f.readline())
print("I bet that was blank")
f.close()

You can tell Python to go back to the start by using the seek() procedure:
print(f.readline())
print("That was the first line")
f.seek(0)
print(f.readline())
print("That was the first line again")
f.close()

You can read one character at a time (or 5 characters at a time) by passing a parameter:
print(“The first letter is:”)
print(f.read(1))
print(“The second letter is:”)
print(f.read(1))
f.close()

4.3d - Reading Challenge
Write a program that will read each character and append it to a string until the first space is found, then print that
string.

PAGE 27 OF 31

MARK CLARKSON, 2010


OCR GCSE COMPUTING

4.3e - Writing
At some point you will want to write some data to a ﬁle. You can only write strings to ﬁle, so you will have to convert any
numbers before you can write them:
x = 3
y = " is the magic number"

f = open("temp","w") #remember that “w” means the old file will be overwritten
x = str(x)
f.write(x + y)
f.close()

print(f.read())
f.close()

4.3f - Writing Newlines
Don’t forget that you can use the “n” character to force a new line.
#The first part will write the "1 potato, 2 potato..." rhyme.
f.write(str(n)+" potaton")
f.write("4n")
f.write(str(n) +" potaton")
f.write("More!")
f.close()

#The second part will print it to the screen to check it looks OK.
print(f.read())
f.close

PAGE 28 OF 31

MARK CLARKSON, 2010


OCR GCSE COMPUTING

4.3g - Editing or Deleting Data
The easiest way to edit or delete data is to copy the bits you want and to rewrite the ﬁle from scratch:
#The first part will write the "1 potato, 2 potato..." rhyme.
f.write(str(n)+" potaton")
f.write("n")
f.write(str(n) +" potaton")
f.write("")
f.close()

#The second part will print it to the screen to check it looks OK.
print(f.read())
f.close

#This part creates two strings either side of the 4th line.
string1=""
for n in range(0,3): #Copies the first 3 lines into string1
string1 = string1 + f.readline()
f.readline() #This command makes Python skip the 4th (blank) line
string2=""
for n in range(0,3): #Copies the next 3 lines into string2
string2 = string2 + f.readline()
f.close()

#Next we rewrite all this data to a new file (with the same name).
f.write(string1)
f.write("4n") #writes in the missing line in the middle
f.write(string2)
f.write("More!") #writes in the missing line at the end
f.close()

#Finally we print the whoel file again to check it.
print(f.read())
f.close

PAGE 29 OF 31

MARK CLARKSON, 2010


OCR GCSE COMPUTING

4.3h - Simple Database
Your task is to use a simple database that uses 5 fields - ID, shade, red, green & blue (to represent the RGB colour
values for that particular shade, in the range 0-255) with a space between each value. The data file would typically look
like this:
1 black 0 0 0
2 white 255 255 255
3 red 255 0 0
4 green 0 255 0
5 blue 0 0 255
• Create a program with a procedure that will create a new file called “colours” and populate it with the data above.
• Create a procedure that will output the entire contents of the file.
• Call both procedures and check that they work.
• Create a function that will print a menu to the screen asking which colour is to be looked up, prompt for a value and
return it as an integer.
• Create a function that will look up a given colour (given its ID) and return 4 strings for the shade and RGB values.
• Write a main program that will populate the file, print the menu and print the RGB values for the chosen shade.

Extension 1: Write the menu procedure so that the shade will automatically be shown (rather than being hard-coded)
Extension 2: Improve the menu further so that it will produce as many menu items as there are lines in the data file (it
might be useful to use the string “EOF” to signify the end of the file.
Extension 3: Include an extra menu item to allow the user to add their own shades with associated RGB colours. This
should include suitable validation to prevent impossible or incomplete entries.

PAGE 30 OF 31

MARK CLARKSON, 2010


OCR GCSE COMPUTING

4.3i - ATM - Full Version
Northern Frock needs you to write a program for their new ATMs (or Automatic Teller Machines).
In this version you will need to use a customer database that will initially look like this (ignore the top row):
ID Title First Name Surname Balance
1057 Mr. Jeremy Clarkson £172.16
2736 Miss Suzanne Perry £15.62
4659 Mrs. Vicki Butler-Henderson £23.91
5691 Mr. Jason Plato £62.17

• Generate the above data file.
• Prompt the user for their ID number, validating against the list of known users.
• If the ID number 99999 (5x 9s) is given the machine should shut down.
• Print a menu:
Welcome to Northern Frock
1 - Display balance
2 - Withdraw funds
3 - Deposit funds
9 - Return card
Enter an option:
• If ‘1’ is entered, display the current balance and also the maximum amount available for withdrawal (must be a
multiple of £10)
• If ‘2’ is entered, provide another menu with withdrawal amounts of £10, £20, £40, £60, £80, £100, Other amount,
Return to main menu & Return card.
• Check that the requested withdrawal is allowed, print a message to show that the money has been withdrawn and
calculate the new balance.
• If ‘Other amount’ is selected then prompt the user for an integer value. Check this number is a multiple of 10 and that
the withdrawal is permitted (as above).
• If ‘3’ is entered, provide another menu that will allow the user to enter an amount to deposit(does not need to be a
multiple of £10), return to main menu or return card. If funds are deposited, provide appropriate feedback and update
the balance.
• If ‘9’ is entered, print a goodbye message and return to the initial prompt (for the next customer).

Extension 1: Add an extra field to the database for a 4 digit PIN which should be prompted for and checked following
the entry of the ID number. The user should also have the option to change their PIN.

Extension 2: Add another field to record whether the card is blocked. Three incorrect PIN attempts should
permanently lock the card. PIN attempts should only be reset by correctly entering the PIN. Simply removing the card
and trying again should not work.

Extension 3: Create an encoding algorithm that will store the PIN in a format that cannot be read by just looking t the
file. Only by running the PIN attempt through the same algorithm can the match be found.

PAGE 31 OF 31

MARK CLARKSON, 2010

Introduction to python

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