A shop records the prices of items sold during a day. The shopkeeper enters prices one at a time, using -1 to indicate the end of input. (a) Write pseudocod...

(a) The algorithm uses a sentinel (-1) to end input, tracks the total, count, max, and min. The first valid item initialises both max and min trackers: [6]

DECLARE Price : REAL
DECLARE Total : REAL
DECLARE Count : INTEGER
DECLARE Average : REAL
DECLARE MaxPrice : REAL
DECLARE MinPrice : REAL

Total ← 0
Count ← 0

OUTPUT "Enter price (-1 to finish): "
INPUT Price

IF Price <> -1 THEN
    MaxPrice ← Price
    MinPrice ← Price
ENDIF

WHILE Price <> -1 DO
    Total ← Total + Price
    Count ← Count + 1
    IF Price > MaxPrice THEN
        MaxPrice ← Price
    ENDIF
    IF Price < MinPrice THEN
        MinPrice ← Price
    ENDIF
    OUTPUT "Enter price (-1 to finish): "
    INPUT Price
ENDWHILE

OUTPUT "Total sales: ", Total
OUTPUT "Items sold: ", Count
IF Count > 0 THEN
    Average ← Total / Count
    OUTPUT "Average price: ", Average
    OUTPUT "Most expensive: ", MaxPrice
    OUTPUT "Cheapest: ", MinPrice
ELSE
    OUTPUT "No items entered"
ENDIF

[1 mark for sentinel loop, 1 mark for totalling, 1 mark for counting, 1 mark for max tracking, 1 mark for min tracking with correct initialisation, 1 mark for average with zero check]

(b) The initial value for the maximum tracker should be the first valid item entered, not 0 or an arbitrary large/small number. [1] Initialising with the first actual data value ensures accurate tracking regardless of the data range. If set to 0 and all prices are positive, 0 would never be replaced as the minimum unless the code explicitly handles it. [1]

(c) If no items are entered (Count = 0), calculating the average would require dividing Total by 0. [1] This division by zero would cause a runtime error and crash the program. The check allows the program to display an informative message instead. [1]

(d) A WHILE loop tests the condition before the loop body executes, so the body may never run if the condition is initially false. [1] A REPEAT...UNTIL loop tests the condition after the loop body executes, so the body always runs at least once. [1]