Programming Constructs (OCR A-Level Computer Science): Revision Notes

Programming Constructs

Overview

In programming, sequence, iteration, and branching are the three fundamental constructs that form the basis of all algorithms. These constructs allow programmers to control the flow of a programme, enabling it to make decisions, repeat tasks, and execute instructions in a specific order.

Understanding and effectively using these constructs is essential for developing efficient and correct programmes.

Sequence

Definition:

• Sequence is the simplest construct, where instructions are executed one after another in the order they are written. Purpose:

• Ensures that tasks are completed in a specific order, which is critical for programmes where the output of one step depends on the completion of the previous step.

INPUT name
PRINT "Hello, " + name
PRINT "Welcome to the programme!"

name = input("Enter your name: ")
print("Hello, " + name)
print("Welcome to the programme!")

Iteration

Iteration is the process of repeating a set of instructions until a certain condition is met.

Types of Iteration:

Condition-Based Iteration (While Loop):

Repeats as long as a condition is true.

WHILE count < 5
    PRINT count
    count = count + 1
ENDWHILE

count = 0
while count < 5:
    print(count)
    count += 1

Count-Controlled Iteration (For Loop):

Repeats a specific number of times.

FOR i = 1 TO 5
    PRINT i
ENDFOR

for i in range(1, 6):
    print(i)

Repeat-Until Loop (Post-Test Loop):

Executes the loop body at least once and repeats until a condition is met.

REPEAT
    INPUT value
UNTIL value > 0

Branching

Branching allows a programme to make decisions and choose between different paths based on conditions.

Types of Branching:

IF Statement:

Executes a block of code if a condition is true.

IF score >= 50 THEN
    PRINT "You passed!"
ENDIF

if score >= 50:
    print("You passed!")

IF-ELSE Statement:

Provides an alternative action if the condition is false.

IF score >= 50 THEN
    PRINT "You passed!"
ELSE
    PRINT "You failed."
ENDIF

if score >= 50:
    print("You passed!")
else:
    print("You failed.")

IF-ELSEIF-ELSE Ladder:

Allows checking multiple conditions.

IF score >= 80 THEN
    PRINT "Grade A"
ELSEIF score >= 60 THEN
    PRINT "Grade B"
ELSE
    PRINT "Grade C"
ENDIF

if score >= 80:
    print("Grade A")
elif score >= 60:
    print("Grade B")
else:
    print("Grade C")

CASE/SWITCH Statement:

Simplifies multiple condition checks by matching a value to predefined cases.

CASE day OF
    "Monday": PRINT "Start of the workweek."
    "Friday": PRINT "Almost the weekend!"
    "Saturday", "Sunday": PRINT "It's the weekend!"
ENDCASE

Note: Python does not have a direct switch statement but can use if-elif or dictionaries for similar functionality.

Combining Constructs

In real-world programmes, sequence, iteration, and branching are often combined to solve complex problems.

Example: Simple ATM System

Pseudocode:

START
BALANCE = 1000
WHILE TRUE
    PRINT "1. Check Balance"
    PRINT "2. Withdraw Money"
    PRINT "3. Exit"
    INPUT choice
    IF choice == 1 THEN
        PRINT "Balance: ", BALANCE
    ELSEIF choice == 2 THEN
        PRINT "Enter amount to withdraw:"
        INPUT amount
        IF amount <= BALANCE THEN
            BALANCE = BALANCE - amount
            PRINT "Withdrawal successful."
        ELSE
            PRINT "Insufficient funds."
        ENDIF
    ELSEIF choice == 3 THEN
        PRINT "Goodbye!"
        BREAK
    ELSE
        PRINT "Invalid choice."
    ENDIF
ENDWHILE

Benefits of Using Constructs

1. Sequence: Ensures tasks are completed in the correct order.
2. Iteration: Automates repetitive tasks, reducing redundancy.
3. Branching: Enables dynamic decision-making, allowing the programme to handle different inputs and scenarios.

Note Summary