Selection Constructs (SQA National 5 Computing Science): Revision Note

Selection constructs

What is a selection construct?

• Selection constructs (often referred to as if statements) are fundamental components of programming, allowing a computer program to make decisions based on some input

• Students are required to describe, exemplify, and implement these constructs in a high-level (textual) language

Simple conditional statements

• Simple conditional statements rely on logical comparators to compare two values

• Students must be able to use and implement the following operators

• A basic structure uses IF...THEN...ELSE...END IF to execute different code blocks depending on whether the condition is met

• Indentation is used in pseudocode to clearly show the selection structure

SEND "Enter your age:" TO DISPLAY
RECEIVE age FROM KEYBOARD

IF age < 12 THEN
    SEND "You are a child." TO DISPLAY
ELSE
    IF age < 18 THEN
        SEND "You are a teenager." TO DISPLAY
    ELSE
        IF age ≥ 18 THEN
            SEND "You are an adult." TO DISPLAY
        END IF
    END IF
END IF

SEND "Enter your favourite colour:" TO DISPLAY
RECEIVE colour FROM KEYBOARD

IF colour = "blue" THEN
    SEND "You like the colour blue!" TO DISPLAY
END IF

IF colour ≠ "blue" THEN
    SEND "You like a different colour!" TO DISPLAY
END IF

• An example written in Python would be

age = int(input("Enter your age: "))

if age < 12:
    print("You are a child.")
elif age < 18:
    print("You are a teenager.")
elif age >= 18:
    print("You are an adult.")

colour = input("Enter your favourite colour: ")

if colour == "blue":
    print("You like the colour blue!")
if colour != "blue":
    print("You like a different colour!")

• Notice Python does not use THEN and END IF

• ELIF can be used to extend a selection statement (ELSE IF)

Complex conditional statements

• Complex conditional statements combine multiple simple conditions using logical operators

• Students must understand and implement the three main logical operators: AND, OR, and NOT

Example

SEND "Enter your test score:" TO DISPLAY
RECEIVE score FROM KEYBOARD

IF score >= 70 THEN
    SEND "Grade: A" TO DISPLAY
ELSE
    IF score >= 60 AND score < 70 THEN
        SEND "Grade: B" TO DISPLAY
    ELSE
        IF score >= 50 AND score < 60 THEN
            SEND "Grade: C" TO DISPLAY
        ELSE
            IF score < 50 THEN
                SEND "Fail" TO DISPLAY
            END IF
        END IF
    END IF
END IF

SEND "Enter your resit status (yes/no):" TO DISPLAY
RECEIVE resit FROM KEYBOARD

IF resit ≠ "yes" THEN
    SEND "Good luck with your next course!" TO DISPLAY
ELSE
    SEND "You will be entered for a resit." TO DISPLAY
END IF

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

if score >= 70:
    print("Grade: A")
elif score >= 60 and score < 70:
    print("Grade: B")
elif score >= 50 and score < 60:
    print("Grade: C")
elif score < 50:
    print("Fail")

resit = input("Enter your resit status (yes/no): ")

if resit != "yes":
    print("Good luck with your next course!")
else:
    print("You will be entered for a resit.")

Efficiency and logic

Efficiency in complex conditions

• Using a single complex conditional statement is often more efficient than nesting multiple simple conditional statements

  • Less Efficient (Nested IFs): Two separate simple conditional statements nested together.

  • More Efficient (Complex IF): IF X > 4 AND Y < 6 THEN....

Efficiency in multi-way selection

• When checking multiple alternatives (e.g., assigning a grade based on a score), using nested IF constructs or equivalents like ELSE IF/ELIF statements is generally more efficient than using multiple, separate IF constructs

• When separate IF constructs are used, the program always carries out all comparisons

• Using nested structures means the program stops once a condition is met, potentially requiring fewer comparisons

Potential logic errors

• If logical operators are used incorrectly, the program will suffer a logic error, meaning the code produces an unexpected result

  • A common example is an incorrect range check, such as using AND where OR is required (e.g., checking if $number < 30$ AND $number > 50$), which can never be true, resulting in a logic error