A program uses a sentinel value to control input into an array. DECLARE Numbers : ARRAY[1:20] OF INTEGER DECLARE Count : INTEGER DECLARE Value : INTEGER Co...

(a) A sentinel value is a special value that signals the end of data input, rather than being part of the actual data. [1] In this example, -1 is the sentinel. When the user enters -1, the WHILE loop condition Value <> -1 becomes false, causing the loop to exit. The value -1 itself is never stored in the array because the check happens before the assignment. [1]

(b) If the user enters 15, 8, 42, 3, -1: [2]

Array contents: Numbers[1]=15, Numbers[2]=8, Numbers[3]=42, Numbers[4]=3 [1]

Count = 4 [1]

The sentinel value -1 terminates the loop and is not stored. Positions 5-20 remain unchanged (uninitialised).

(c) Purpose of Count < 20 in the WHILE loop: [2]

It prevents the array from overflowing by ensuring no more than 20 values can be stored. [1] Without this guard, if the user enters more than 20 values without typing -1, the program would attempt to write to Numbers[21], which is beyond the array bounds and would cause a runtime error (array index out of bounds). [1]

(d) Pseudocode to calculate and output the average after the loop: [2]

IF Count > 0
    THEN
        DECLARE Total : INTEGER
        Total <- 0
        FOR Index <- 1 TO Count
            Total <- Total + Numbers[Index]
        NEXT Index
        OUTPUT "Average: ", Total / Count
ENDIF

[1] for calculating the total using a loop that runs to Count (not to 20, which would include uninitialised values), [1] for dividing by Count and handling the edge case where Count = 0 (division by zero).

(e) Advantage of a sentinel value over pre-specifying the count: [1]

The user does not need to know in advance how many numbers they will enter. [1] They can enter as many or as few values as needed and simply type the sentinel value when finished. This is more flexible and user-friendly, especially when the number of items is not known beforehand.