8. Programming fundamentals

• A variable is a named memory location whose value can change while the program runs.
• A constant is a named value that does not change while the program runs. Python has no constant keyword, so by convention constants are written in CAPITALS and never re-assigned.
• Initialise = give a starting value before it is used.

MAX_STUDENTS = 30     # constant
VAT_RATE = 0.2        # constant
total = 0             # variable initialised to 0
name = ""             # variable initialised to an empty string

Why constants? The value is changed in one place only, the name explains the value, and it cannot be changed by mistake elsewhere.

Casting converts one type to another: int(), float(), str(), bool().

age = int("16")          # str -> int: 16
price = float("2.5")     # str -> float: 2.5
label = "Age: " + str(age)
whole = int(7.9)         # float -> int: 7 (cuts off, does not round)
print(label, price, whole)

Output: Age: 16 2.5 7. A phone number or ID with a leading 0 should be a string.

input() always returns a string, so cast it when you need a number. print() outputs; separate items with commas (adds a space) or join strings with +.

name = input("Enter your name: ")
mark = int(input("Enter your mark: "))
print("Well done", name, "you scored", mark)

Sequence = statements run one after another in order. Selection = choosing a path with if / elif / else, nested ifs or match-case.

mark = int(input("Mark: "))
if mark >= 70:
    grade = "A"
elif mark >= 50:
    grade = "B"
else:
    grade = "U"
print(grade)

day = input("Day: ")
match day:
    case "Saturday" | "Sunday":
        print("Weekend")
    case "Friday":
        print("Nearly the weekend")
    case _:
        print("School day")

A nested selection is an if inside another if — used when a second question only matters after the first is true. case _ catches every other value.

• Count-controlled — for repeats a known number of times. range(1, 6) gives 1 to 5 (the stop value is not included).
• Pre-condition — while condition: tests before each pass, so it may run zero times.
• Post-condition — Python writes it as while True: with an if … break at the end: the body always runs at least once.
• Nested iteration — a loop inside a loop (e.g. rows and columns).

for count in range(1, 6):
    print(count * count)

password = ""
while password != "secret":
    password = input("Password: ")

while True:
    choice = input("Play again? (y/n) ")
    if choice == "n":
        break

Totalling adds each value to a running total; counting adds 1 each time something happens. Both must be initialised to 0 before the loop.

total = 0
count = 0
number = int(input())
while number != -1:
    total = total + number
    count = count + 1
    number = int(input())
print(total, count)

With inputs 4, 7, 2, -1 this prints 13 3 (−1 is a rogue value that stops the loop and is not counted).

The first character is at position 0.

String methods return a new string — store it: s = s.upper(). Slicing s[0:3] gives "the".

/ always gives a float (8 / 2 is 4.0). % is used to test even numbers (n % 2 == 0) and divisibility; // with % splits minutes into hours and minutes:

minutes = 135
print(minutes // 60, "h", minutes % 60, "min")

Output: 2 h 15 min. Brackets first, then *, then / // %, then + -.

They compare two values and give a Boolean (True/False):

age = 16
print(age >= 16, age != 16, age < 13)

Output: True False False.

• and — True only if both conditions are True.
• or — True if at least one is True.
• not — reverses a Boolean.

mark = 85
attendance = 92
if mark >= 80 and attendance >= 90:
    print("Merit award")
if mark < 0 or mark > 100:
    print("Invalid")
finished = False
if not finished:
    print("Keep going")

Output: Merit award then Keep going. Each side of and/or must be a complete comparison: mark < 0 or mark > 100, never mark < 0 or > 100.