Programming Techniques

This lesson covers the fundamental programming techniques required by the OCR A-Level Computer Science (H446) specification. These are the building blocks of all programs: sequence, selection, iteration, subroutines, and parameter passing.

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Sequence

Sequence is the most basic programming construct. Instructions are executed one after another, in the order they are written.

# Sequence: each line executes in order
name = input("Enter your name: ")
age = int(input("Enter your age: "))
print(f"Hello {name}, you are {age} years old")

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Selection

Selection allows a program to choose between different paths of execution based on a condition. The condition evaluates to either True or False.

if / elif / else

score = int(input("Enter score: "))

if score >= 90:
    grade = "A*"
elif score >= 80:
    grade = "A"
elif score >= 70:
    grade = "B"
elif score >= 60:
    grade = "C"
elif score >= 50:
    grade = "D"
else:
    grade = "U"

print(f"Grade: {grade}")

Pseudocode

IF score >= 90 THEN
    grade = "A*"
ELSEIF score >= 80 THEN
    grade = "A"
ELSEIF score >= 70 THEN
    grade = "B"
ELSE
    grade = "U"
ENDIF

Switch/Case Statements

Some languages (Java, C++, but not Python) provide switch statements for multi-way selection based on a single value:

// Pseudocode switch statement
SWITCH day:
    CASE "Monday":
        OUTPUT "Start of the week"
    CASE "Friday":
        OUTPUT "End of the week"
    DEFAULT:
        OUTPUT "Midweek"
ENDSWITCH

In Python, the equivalent is the match statement (Python 3.10+):

match day:
    case "Monday":
        print("Start of the week")
    case "Friday":
        print("End of the week")
    case _:
        print("Midweek")

Selection Type	Use Case
if/elif/else	When conditions are ranges or complex expressions
switch/case	When selecting based on a single value with multiple options
Nested if	When conditions depend on the outcome of previous conditions

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Iteration (Loops)

Iteration means repeating a block of code. There are three main types:

For Loop (Count-Controlled)

The loop runs a predetermined number of times.

# Print numbers 1 to 10
for i in range(1, 11):
    print(i)

# Iterate over a list
names = ["Alice", "Bob", "Charlie"]
for name in names:
    print(f"Hello, {name}")

Pseudocode

FOR i = 1 TO 10
    OUTPUT i
NEXT i

While Loop (Condition-Controlled -- Pre-check)

The loop repeats while a condition is True. The condition is checked before each iteration, so the body may execute zero times.

password = ""
while password != "secret123":
    password = input("Enter password: ")
print("Access granted")

Pseudocode

WHILE password != "secret123"
    password = INPUT("Enter password: ")
ENDWHILE

Do-While Loop (Condition-Controlled -- Post-check)

The loop body executes at least once because the condition is checked after each iteration. Python does not have a built-in do-while, but it can be simulated:

# Simulating do-while in Python
while True:
    number = int(input("Enter a positive number: "))
    if number > 0:
        break
print(f"You entered: {number}")

Pseudocode

DO
    number = INPUT("Enter a positive number: ")
UNTIL number > 0

Comparison of Loop Types

Loop Type	Condition Check	Minimum Iterations	Best For
for	Count-based	Predetermined	Known number of iterations
while	Before each iteration	0	Unknown iterations; may not execute
do-while	After each iteration	1	Must execute at least once (e.g., menu)

Exam Tip: The examiner may ask you to choose between loop types. Use a for loop when you know how many iterations. Use a while loop when the number of iterations is unknown. Use a do-while when the body must execute at least once.

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Subroutines: Procedures and Functions

A subroutine is a named block of code that performs a specific task. Subroutines promote code reuse, modularity, and readability.

Type	Returns a Value?	Purpose
Procedure	No	Performs an action (e.g., printing, modifying data)
Function	Yes	Computes and returns a value

Procedure Example

def display_menu():
    print("1. Play Game")
    print("2. View Scores")
    print("3. Quit")

display_menu()  # Calling the procedure

Function Example

def calculate_area(length: float, width: float) -> float:
    return length * width

area = calculate_area(5.0, 3.0)
print(f"Area: {area}")  # Area: 15.0

Pseudocode

PROCEDURE displayMenu()
    OUTPUT "1. Play Game"
    OUTPUT "2. View Scores"
    OUTPUT "3. Quit"
END PROCEDURE

FUNCTION calculateArea(length: REAL, width: REAL) RETURNS REAL
    RETURN length * width
END FUNCTION

Benefits of Subroutines