Topic 6: Programming

Algorithm vs Program - What's the Difference?

Think of it like a recipe:

• Algorithm = The recipe written on paper (the plan in plain English or pseudocode)
• Program = Actually cooking the meal following the recipe (the algorithm converted into code a computer can run)

A well-written algorithm is like a clear recipe - it's easy to follow and should work every time! A program is when you actually make the computer "cook" by running the code.

# Output - displaying to user
print("Hello, World!")
print("Your score is:", score)

# Input - getting data from user
name = input("Enter your name: ")
age = int(input("Enter your age: "))

Why Do Data Types Matter?

Think of data types like different types of containers:

• Integer = A box that only holds whole items (like marbles: 1, 2, 3...)
• Float = A measuring jug that can hold precise amounts (like 2.5 litres)
• String = A label maker that stores text (like "Hello")
• Boolean = A light switch - only ON (True) or OFF (False)

Why it matters: If you try to put water (2.5) in a marble box (integer), it becomes 2 - you lose the .5! The computer needs to know what type of data it's storing.

# String to Integer
age = int("25")

# String to Float
price = float("19.99")

# Number to String
text = str(42)

# Integer to Float
decimal = float(10)  # becomes 10.0

Type Coercion - Automatic Conversion

Sometimes Python automatically converts data types for you. This is called type coercion.

The rule: When you mix an integer with a float, Python automatically converts the result to a float. It's like mixing whole apples with apple slices - you end up with slices!

# WRONG - will crash or give unexpected results!
age = input("Enter age: ")  # age is a STRING like "15"
new_age = age + 1  # ERROR! Can't add string + number

# CORRECT - convert to integer first
age = int(input("Enter age: "))  # age is now INTEGER 15
new_age = age + 1  # Works! new_age is 16

# Creating an array/list
scores = [85, 92, 78, 95, 88]
names = ["Alice", "Bob", "Charlie"]

# Accessing elements (index starts at 0)
first = scores[0]    # 85
second = names[1]    # "Bob"

# Modifying elements
scores[2] = 80

# Array length
length = len(scores)  # 5

# Looping through an array
for score in scores:
    print(score)

# Creating a 2D array (3 rows, 4 columns)
grid = [
    [1, 2, 3, 4],
    [5, 6, 7, 8],
    [9, 10, 11, 12]
]

# Accessing elements: grid[row][column]
value = grid[1][2]  # 7 (row 1, column 2)

# Looping through 2D array
for row in grid:
    for item in row:
        print(item)

Strings Are Like a Row of Boxes!

Think of a string as a row of numbered boxes, each holding ONE character. The numbering starts at 0, not 1!

Remember: The last index is always LENGTH - 1. "Computer" has 8 letters, so the last index is 7!

# Traversal - visit each character
word = "Hello"
for letter in word:
    print(letter)

# Output: H, e, l, l, o (each on a new line)

# Traversal with index position
word = "Hello"
for i in range(len(word)):
    print("Position", i, "=", word[i])

Slicing is Like Cutting a Cake!

Use [start:end] to cut out a piece. The slice goes from start up to (but NOT including) end.

largest = numbers[0]
for num in numbers:
    if num > largest:
        largest = num
print(largest)

Selection is Just Decision-Making!

You use selection in real life all the time:

• IF it's raining THEN take an umbrella
• IF I have money THEN buy lunch ELSE bring packed lunch
• IF score >= 90 THEN "A" ELIF score >= 80 THEN "B" ELSE "C"

# Simple IF
if age >= 18:
    print("You can vote")

# IF-ELSE
if score >= 50:
    print("Pass")
else:
    print("Fail")

# IF-ELIF-ELSE
if grade >= 90:
    print("A")
elif grade >= 80:
    print("B")
elif grade >= 70:
    print("C")
else:
    print("Below C")

Two Types of Loops - Know the Difference!

Loops repeat instructions. There are two types:

# Print 1 to 5
for i in range(1, 6):
    print(i)

# Loop through array
for name in names:
    print(name)

# Count to 5
count = 1
while count <= 5:
    print(count)
    count = count + 1

# Until user quits
while answer != "quit":
    answer = input()

# Nested IF
if age >= 18:
    if hasLicense:
        print("Can drive")
    else:
        print("Need license")

# Nested loops (multiplication table)
for i in range(1, 4):
    for j in range(1, 4):
        print(i * j, end=" ")
    print()  # New line

total = 0
number = int(input("Enter number (-1 to stop): "))
while number != -1:
    total = total + number
    number = int(input("Enter number (-1 to stop): "))
print("Total:", total)

When Do You Need Random Numbers?

• Rolling dice in a game (random number 1-6)
• Shuffling a deck of cards
• Generating a random password
• Making a "guess the number" game
• Choosing a random winner from a list

# First, import the random module
import random

# Generate a random integer between 1 and 10
number = random.randint(1, 10)
print("Random number:", number)

# Generate a random integer between 1 and 6 (like a dice)
dice = random.randint(1, 6)
print("You rolled:", dice)

# Pick a random item from a list
colours = ["red", "blue", "green", "yellow"]
chosen = random.choice(colours)
print("Random colour:", chosen)

def square(n):
    return n * n

answer = square(4)  # 16

def greet(name):
    print("Hello,", name)

greet("Alice")  # Hello, Alice

Parameters vs Arguments - What's the Difference?

Think of a function like a vending machine:

• Parameters = The SLOTS in the machine (labelled "coins" and "selection")
• Arguments = The actual THINGS you put in (your 50p coin and button B3)

Parameters are defined when you CREATE the function. Arguments are the actual values you PASS when you CALL it.

# Parameters are the variables in the function definition
def add(a, b):    # a and b are PARAMETERS (the slots)
    return a + b

# Arguments are the values passed when calling
result = add(5, 3)  # 5 and 3 are ARGUMENTS (actual values)

score = 0  # Global variable

def addPoints(points):
    global score  # Access global variable
    score = score + points

def calculate():
    total = 100  # Local variable
    return total

Why Use Subprograms?

Analogy: Think of subprograms like LEGO blocks. Instead of building everything from scratch each time, you create reusable pieces that snap together. One "wheel" block can be used in many different vehicles!

Three Types of Bugs (Errors)

Think of writing code like writing a recipe. There are three ways it can go wrong:

• Syntax error: Like spelling "teaspoon" wrong - the recipe can't be read!
• Logic error: Like saying "add 10 cups of salt" instead of 1 - it runs but tastes terrible!
• Runtime error: Like saying "divide the mixture into zero portions" - impossible to do!

Syntax Errors - "The Computer Can't Read This!"

Syntax errors break Python's grammar rules. The program won't even start - Python tells you immediately!

Logic Errors - "It Runs But It's Wrong!"

The sneakiest errors! Your code runs perfectly but gives the wrong answer. Python can't detect these - you have to find them yourself.

Runtime Errors - "It Crashed!"

The program starts fine, but something impossible happens while running and it crashes.

Real World Disaster: Ariane 5 Rocket (1996)

On June 4, 1996, the Ariane 5 rocket exploded just 37 seconds after launch, destroying $370 million worth of satellites!

The cause? A software bug tried to store a number too big for its variable (like trying to fit an elephant in a shoebox). This caused a runtime error that crashed the guidance system.

Lesson: Even one small bug can have HUGE consequences. That's why testing is so important!

Why Test with Different Data?

Testing is like checking a door lock works properly. You don't just try ONE key - you test with:

• The correct key (normal data) - it should open
• Keys that are almost right (boundary data) - testing the limits
• Completely wrong keys (erroneous data) - they should NOT open!

What is a Trace Table?

A trace table is like being a detective following footprints. You track what happens to each variable at every step of the program.

Think of it as "playing computer" - you BE the computer and write down what happens in each line!

Why Use Trace Tables?

• Find logic errors: See exactly where your code goes wrong
• Understand loops: Track how many times a loop runs
• Debug code: Figure out why you're getting wrong answers
• Exam requirement: You WILL be asked to complete trace tables!

# Code to trace:
x = 1
while x < 4:
    y = x * 2
    x = x + 1

What is an IDE?

An IDE (Integrated Development Environment) is like a Swiss Army knife for programmers - it has tools to write, run, test, and debug code all in one place!

Examples: IDLE (comes with Python), Visual Studio Code, PyCharm, Thonny

Common Python Error Messages (and what they mean!)

GIGO - Garbage In, Garbage Out!

If you put bad data INTO a program, you get bad results OUT. It's like making a cake:

• Put in rotten eggs (garbage in) = get a horrible cake (garbage out)
• Put in fresh ingredients (good data) = get a delicious cake (good results)

That's why we validate! We check the input BEFORE the program uses it.

# RANGE CHECK - age must be between 0 and 120
age = int(input("Enter age: "))
while age < 0 or age > 120:
    print("Invalid! Age must be between 0 and 120")
    age = int(input("Enter age: "))

# PRESENCE CHECK - name cannot be empty
name = input("Enter your name: ")
while name == "":
    print("Name is required!")
    name = input("Enter your name: ")

# LOOK-UP CHECK - must be one of the valid options
valid_sizes = ["S", "M", "L", "XL"]
size = input("Enter size (S/M/L/XL): ").upper()
while size not in valid_sizes:
    print("Invalid size! Choose S, M, L, or XL")
    size = input("Enter size: ").upper()

# LENGTH CHECK - UK postcode must be 6-8 characters
postcode = input("Enter UK postcode: ")
while len(postcode) < 6 or len(postcode) > 8:
    print("Invalid! Postcode must be 6-8 characters")
    postcode = input("Enter UK postcode: ")

Why Do We Need Files?

Think about it: when your program ends, all your variables disappear! Files let you save data permanently.

Analogy: Variables are like writing on a whiteboard (erased when you leave). Files are like writing in a notebook (kept forever)!

# Writing to a file (creates new or OVERWRITES existing!)
file = open("data.txt", "w")
file.write("Hello World\n")
file.close()

# Reading entire file
file = open("data.txt", "r")
content = file.read()
file.close()
print(content)

# Reading line by line
file = open("data.txt", "r")
for line in file:
    print(line)
file.close()

# APPENDING - adds to end without deleting
file = open("data.txt", "a")
file.write("New line added!\n")
file.close()

# Saving an array to a file (with commas)
scores = [85, 92, 78, 95]
file = open("scores.txt", "w")
for score in scores:
    file.write(str(score) + ",")
file.close()

# Reading back into an array
file = open("scores.txt", "r")
data = file.read().split(",")  # Split by comma
file.close()

What is a Test Plan?

A test plan is like a checklist before launch. Before NASA sends a rocket into space, they don't just hope it works - they test EVERYTHING and write down the results!

A test plan documents: what you're testing, what data you'll use, what SHOULD happen, and what ACTUALLY happened.

a = 5
b = 3
while a > b:
    a = a - 1
    b = b + 1

What is Program Evaluation?

Evaluation is like being a judge on a talent show. You look at the program and ask: "Is it any good? Does it do what it's supposed to? Could it be better?"

A program can work correctly but still be poor quality (slow, hard to read, or difficult to use).

Memory Trick: "FIVER"

Fitness for purpose
Interface usability
Validation (robustness)
Efficiency
Readability

Remember: A good program should score a FIVER on all five criteria!

Why Does Readability Matter?

• You will forget! Come back to your code in a month - will you remember what it does?
• Teamwork: Other programmers need to understand your code
• Debugging: Finding errors is much easier in readable code
• Maintenance: Code often needs updating - readable code is easier to change