Python Prep

What is Decision Making?

In everyday life, you make decisions all the time: "If it's raining, I'll take an umbrella. Otherwise, I'll wear sunglasses."

Python lets your programs make decisions too, using if, elif (short for "else if"), and else statements.

The if Statement

The simplest form — do something only if a condition is true:

age = 15
if age >= 13:
    print("You are a teenager!")

The if...else Statement

When you want to do one thing if a condition is true, and something else if it's false:

marks = 35
if marks >= 40:
    print("Pass")
else:
    print("Fail")

The if...elif...else Statement

When there are multiple conditions to check — Python tests them one by one, top to bottom, and runs the first one that's true:

marks = 75
if marks >= 90:
    print("Grade: A")
elif marks >= 70:
    print("Grade: B")
elif marks >= 50:
    print("Grade: C")
else:
    print("Grade: F")

How it Works — Flowchart

Here's how Python decides which block to run:

Why Loops?

Imagine you want to print "Hello" 100 times. Writing print("Hello") 100 times would be crazy! A for loop lets you repeat code as many times as you want.

The for Loop

A for loop goes through a sequence of values one by one:

for i in [1, 2, 3]:
    print(i)

The range() Function

Instead of writing out a list of numbers, range() generates them for you. It has three forms:

Visualizing range(1, 6)

Common Patterns

total = 0
for i in range(1, 6):
    total += i
print(total)

for i in range(5, 0, -1):
    print(i)

for i in range(0, 11, 2):
    print(i, end=" ")

What are Built-in Functions?

Built-in functions are like tools in a toolbox — Python provides them ready to use, no installation needed. You've already been using print() and range()!

These are different from functions you write yourself. They're part of Python's standard library.

Quick Reference

len() — Count things

len() returns the number of items in a sequence (string, list, etc.):

word = "Python"
print(len(word))    # 6 characters

numbers = [10, 20, 30]
print(len(numbers))  # 3 items

max() and min() — Find extremes

These find the largest and smallest values:

scores = [85, 92, 78, 95, 88]
print("Highest:", max(scores))
print("Lowest:", min(scores))

You can also pass values directly: max(5, 10, 3) returns 10.

sum() — Add it all up

sum() adds all the numbers in a list (or any sequence):

prices = [10, 25, 15, 30]
print("Total:", sum(prices))

input() — Get user input

input() pauses the program and waits for the user to type something:

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

What are String Methods?

Strings (text) in Python come with built-in methods — special powers you can use to transform text. Unlike functions like len(), methods are called on the string using a dot:

# Function — called with the string inside parentheses
len("hello")       # → 5

# Method — called ON the string with a dot
"hello".upper()   # → "HELLO"

.upper() — Convert to UPPERCASE

Returns a new string with all letters converted to uppercase:

word = "Python standard library"
print(word.upper())

.lower() — Convert to lowercase

Returns a new string with all letters in lowercase:

name = "JOHN DOE"
print(name.lower())

.replace() — Swap text

Replaces all occurrences of one substring with another:

sentence = "I love cats. Cats are great."
new_sentence = sentence.replace("cats", "dogs")
print(new_sentence)

Chaining Methods

You can chain multiple methods together:

text = "Hello World"
print(text.upper().replace("WORLD", "PYTHON"))

String Transformation Flow

What are Booleans?

A Boolean is a value that is either True or False. Every time you write a condition like x > 5, Python evaluates it to a Boolean.

print(5 > 3)     # True
print(5 == 3)    # False
print(10 <= 10)   # True

Combining Conditions with and

and means both conditions must be true. Think of it like: "You can ride the roller coaster if you are tall enough AND old enough."

age = 15
height = 160

if age >= 12 and height >= 150:
    print("You can ride!")
else:
    print("Sorry, not allowed.")

Truth Table for and

Combining Conditions with or

or means at least one condition must be true. Think: "You get a discount if you are a student OR a senior citizen."

is_student = True
is_senior = False

if is_student or is_senior:
    print("You get a discount!")
else:
    print("Regular price.")

Truth Table for or

Using and / or with if

A practical exam-style example:

x = True
y = False

if x and y:
    print("Both x and y are True")
else:
    print("x is False or y is False or both x and y are False")

What is Binary?

We normally count in decimal (base 10) using digits 0–9. Computers count in binary (base 2) using only two digits: 0 and 1.

Each binary digit is called a bit. Here's how binary numbers map to decimal:

Binary Addition Rules

Binary addition is just like decimal addition, but simpler — there are only 4 rules:

Worked Example: 1011 + 1101

Let's add step by step, from right to left (just like decimal addition):

Step by step:

1. Column 1 (rightmost): 1 + 1 = 0, carry 1
2. Column 2: 1 + 0 + carry 1 = 0, carry 1
3. Column 3: 0 + 1 + carry 1 = 0, carry 1
4. Column 4: 1 + 1 + carry 1 = 1, carry 1
5. Column 5: carry 1 = 1

Result: 11000 (which is 24 in decimal: 11 + 13 = 24 ✓)

Another Example: 1010 + 0111