6.2.1 Program structural components

An initialisation statement gives a variable its first value when it is created. An assignment statement updates a variable's value at any point.

Copy# Python
total = 0              # initialisation
total = total + 10     # assignment (update)
total = total * 2      # assignment (update again)

In Python, the same syntax (=) is used for both. In C# and VB.NET, type declarations distinguish initialisation from later reassignment.

A command sequence is a list of statements executed in order, one after another, from top to bottom. Sequence is the default flow in a program — every program begins as a sequence, with selection and repetition layered on top.

Copy# Python — a sequence of three assignments and a print
width = 8
height = 5
area = width * height
print("Area:", area)   # statements run in this exact order

Selection executes different code depending on a condition. Implemented with if / elif / else in Python.

Copy# Python
grade = int(input("Enter score: "))
if grade >= 80:
    print("A")
elif grade >= 60:
    print("B")
elif grade >= 40:
    print("C")
else:
    print("Fail")

Copy// C#
if (grade >= 80) Console.WriteLine("A");
else if (grade >= 60) Console.WriteLine("B");
else if (grade >= 40) Console.WriteLine("C");
else Console.WriteLine("Fail");

Repetition executes a block of code multiple times. Two types:

• Count-controlled (for loop): repeats a fixed number of times
• Condition-controlled (while loop): repeats while a condition is true

Iteration is repeating over every item in a data structure:

Copy# Python
# Count-controlled
for i in range(5):
    print(i)

# Condition-controlled
count = 0
while count < 5:
    print(count)
    count = count + 1

# Iteration over every item in a list
scores = [70, 85, 60, 90]
for score in scores:
    print(score)

Subprograms are named blocks of reusable code — either procedures (no return value) or functions (return a value). Parameters are the inputs a subprogram accepts.

Copy# Python
def greet(name):           # name is a parameter
    """Procedure: prints a greeting — no return value."""
    print("Hello,", name)

def square(n):             # n is a parameter
    """Function: returns the square — has a return value."""
    return n * n

greet("Alice")             # calls procedure: prints Hello, Alice
result = square(7)         # calls function: result = 49

Copy// C#
void Greet(string name) { Console.WriteLine("Hello, " + name); }
int Square(int n) { return n * n; }

Input reads data from the user; output displays results. Data structures store multiple related values.

Copy# Python — input/output
name = input("Enter your name: ")   # input
print("Hello,", name)               # output

# 1D list (array)
scores = [70, 85, 60, 90]
print(scores[0])    # 70

# 2D list
grid = [[1, 2, 3],
        [4, 5, 6],
        [7, 8, 9]]
print(grid[1][2])   # 6 (row 1, column 2)

# Record (list used as a named structure)
student = ["Alice", 17, 85]   # [name, age, score]
print(student[0], "scored", student[2])