Foundations

Variables

Naming Values

You're tracking your monthly budget. Instead of typing 2500.00 every time you calculate savings, rent, and groceries, you store it in a variable called income. Now when your income changes, you update one number and all calculations adjust.

Store and use a value

Give a value a name, then use that name in calculations.

basic.py 
x = 
print(x + 5)
print(x * x)

x = 
print(x + 5)
print(x * x)

x = 
print(x + 5)
print(x * x)

A variable holds a value. Use it anywhere you'd use the value itself.

variable In Python, just assign a value to a name: `x = 5`. No type declaration needed.

Building expressions step by step

Break a complex calculation into named pieces. Each piece has meaning.

steps.py 
width = 
height = 
area = width * height
perimeter = 2 * (width + height)

width = 
height = 
area = width * height
perimeter = 2 * (width + height)

width = 
height = 
area = width * height
perimeter = 2 * (width + height)

width = 
height = 
area = width * height
perimeter = 2 * (width + height)

width = 
height = 
area = width * height
perimeter = 2 * (width + height)

width = 
height = 
area = width * height
perimeter = 2 * (width + height)

width = 
height = 
area = width * height
perimeter = 2 * (width + height)

width = 
height = 
area = width * height
perimeter = 2 * (width + height)

width = 
height = 
area = width * height
perimeter = 2 * (width + height)

Reading area tells you more than reading 7 * 5. Variables make code self-documenting.

int Whole numbers: -2, 0, 42, 1000. Python handles arbitrarily large integers.

Temperature conversion (Fahrenheit to Celsius)

Convert 98.6°F (body temperature) to Celsius using the formula: C = (F - 32) × 5 / 9

temperature.py 
fahrenheit = 
celsius = (fahrenheit - 32) * 5 / 9

fahrenheit = 
celsius = (fahrenheit - 32) * 5 / 9

fahrenheit = 
celsius = (fahrenheit - 32) * 5 / 9

fahrenheit = 
celsius = (fahrenheit - 32) * 5 / 9

Variables make the formula readable: fahrenheit, celsius - you know what they mean.

float Decimal numbers: 3.14, -0.5, 98.6. Use for measurements and precise values.

Exercise: temperature_ex01.py

Write the reverse conversion: Celsius to Fahrenheit using F = C × 9/5 + 32

Fibonacci sequence (without loops)

Each Fibonacci number is the sum of the two before it: 0, 1, 1, 2, 3, 5, 8...

fibonacci.py 
c0 = 0
c1 = 1
c2 = c0 + c1
c3 = c1 + c2
c4 = c2 + c3
c5 = c3 + c4
c6 = c4 + c5
c7 = c5 + c6

Watch how each variable depends on the previous ones. This pattern is called building up from base cases.

Circle calculations

Given a radius, calculate circumference and area.

circle.py 
pi = 3.14159
radius = 

circumference = 2 * pi * radius
area = pi * radius * radius

pi = 3.14159
radius = 

circumference = 2 * pi * radius
area = pi * radius * radius

pi = 3.14159
radius = 

circumference = 2 * pi * radius
area = pi * radius * radius

pi = 3.14159
radius = 

circumference = 2 * pi * radius
area = pi * radius * radius

pi and radius as named values make the formulas clear.

Exercise: circle_ex01.py

Extend to sphere: calculate volume (4/3 × π × r³) and surface area (4 × π × r²)