A programmer writes a program that uses loops and selection. (a) Describe the difference between a FOR loop and a WHILE loop. [2] (b) State one situation wh...

(a) A FOR loop repeats a block of code a predetermined number of times. It uses a counter variable that is automatically initialised, incremented and checked by the loop structure itself. [1] A WHILE loop repeats a block of code as long as a specified condition remains true. The number of repetitions is not known in advance; the loop continues until the condition becomes false. [1]

(b) A FOR loop is more appropriate when the number of iterations is known in advance. For example, printing exam results for exactly 30 students is best handled by a FOR loop. [1] The loop counter automatically counts from 1 to 30, making the code simpler and avoiding the need for a separate counter variable and manual condition check. [1]

(c) Selection (IF-THEN-ELSE) allows the program to choose between different paths of execution based on whether a condition evaluates to true or false. [1] For example: IF a student's mark is 50 or above THEN display "Pass" ELSE display "Fail". The condition is evaluated, and only one of the two branches executes. [1]

(d) Nested selection is when one IF statement is placed inside another, allowing the program to test multiple conditions in a structured sequence. [1] For example, a ticketing system might first check if the customer is under 16 (child), and if so, check whether they are under 5 (free ticket) or aged 5-15 (child-rate ticket). Each subsequent condition refines the decision further. [1]

(e) A WHILE loop checks its condition before the loop body executes. If the condition is initially false, the body never executes at all. [1] A REPEAT-UNTIL loop executes the body first and then checks the condition afterwards. This guarantees the body always executes at least once, regardless of the condition. [1]

(f)

REPEAT
    INPUT Password
UNTIL Password = "abc123"

The REPEAT-UNTIL structure is ideal here because the user must be asked for input at least once before the password can be checked. [1] The loop continues prompting until the correct password is entered. [1]

(g)

INPUT Mark
IF Mark >= 70 THEN
    OUTPUT "Distinction"
ELSE
    IF Mark >= 50 THEN
        OUTPUT "Merit"
    ELSE
        IF Mark >= 40 THEN
            OUTPUT "Pass"
        ELSE
            OUTPUT "Fail"
        ENDIF
    ENDIF
ENDIF

The nested IF structure tests conditions from highest to lowest. Once a condition is met, the corresponding grade is output and no further conditions are tested. [1] [1] [1] [1]