A mail-order company calculates the postage cost for a parcel using its weight in grams. The FUNCTION below returns the correct postage cost, and the main program then adds together the postage for two separate parcels. FUNCTION CalculatePostage(WeightGrams : INTEGER) RETURNS RE…

Answer: ((a)) CalculatePostage(80) returns 1.50, produced by the WeightGrams <= 100 branch of the outer IF (80 is less than or equal to 100, so Cost is set to 1.50 and this is the value returned). ((b)) CalculatePostage(650) returns 4.20, produced by the innermost ELSE branch (650 is not less than or equal to 100, and is also not less than or equal to 500, so Cost is set to 4.20, and this is the value returned). ((c)) TotalCost = 1.50 + 4.20 = 5.70, so the final line outputs Total postage: 5.70. ((d)) CalculatePostage must be a FUNCTION because only a FUNCTION returns a value, using RETURN, that can be used directly inside an expression such as CalculatePostage(80) + CalculatePostage(650). A PROCEDURE does not return a value in this way, so it could not be combined with the +…. Part (a): Tracing CalculatePostage(80) WeightGrams = 80. The outer IF checks WeightGrams <= 100: 80 <= 100 is true, so the THEN branch of the outer IF runs, setting Cost ← 1.50. The inner IF is never reached. RETURN Cost then returns 1.50.…

AS & A Level Computer Science 9618: Tracing a FUNCTION with a Nested IF

FUNCTION CalculatePostage(WeightGrams : INTEGER) RETURNS REAL DECLARE Cost : REAL IF WeightGrams <= 100 THEN Cost ← 1.50 ELSE IF WeightGrams <= 500 THEN Cost ← 2.75 ELSE Cost ← 4.20 ENDIF ENDIF RETURN Cost ENDFUNCTION // Main program DECLARE TotalCost : REAL TotalCost ← CalculatePostage(80) + CalculatePostage(650) OUTPUT "Total postage: ", TotalCost

Part (a): Tracing CalculatePostage(80)

WeightGrams = 80. The outer IF checks WeightGrams <= 100: 80 <= 100 is true, so the THEN branch of the outer IF runs, setting Cost ← 1.50. The inner IF is never reached. RETURN Cost then returns 1.50. [2 marks]: [1] for the correct returned value (1.50), [1] for correctly identifying that it comes from the outer IF’s WeightGrams <= 100 branch.

Part (b): Tracing CalculatePostage(650)

WeightGrams = 650. The outer IF checks WeightGrams <= 100: 650 <= 100 is false, so the ELSE branch runs, reaching the inner IF. This checks WeightGrams <= 500: 650 <= 500 is also false, so the inner ELSE runs, setting Cost ← 4.20. RETURN Cost then returns 4.20. [2 marks]: [1] for the correct returned value (4.20), [1] for correctly identifying that it comes from the innermost ELSE branch (both WeightGrams <= 100 and WeightGrams <= 500 are false).

Part (c): Calculating TotalCost

TotalCost ← CalculatePostage(80) + CalculatePostage(650)
          ← 1.50 + 4.20
          ← 5.70

So the final line outputs:

Total postage: 5.70

[2 marks]: [1] for correctly adding the two returned values (1.50 and 4.20), [1] for the correct final total, 5.70.

Part (d): Why a FUNCTION is required

A FUNCTION executes a block of code and then uses RETURN to send a single value back to the point where it was called, so that value can be used immediately inside an expression. Exactly what happens in CalculatePostage(80) + CalculatePostage(650), where both returned values are added together. A PROCEDURE, by contrast, does not return any value to the caller at all; it can only be invoked on its own using CALL, not embedded inside an arithmetic expression. If CalculatePostage were written as a PROCEDURE, the line TotalCost ← CalculatePostage(80) + CalculatePostage(650) would not be valid, since there would be no returned values for the + operator to add together. [1 mark].

Final answers

(a) CalculatePostage(80) returns 1.50, from the outer IF’s WeightGrams <= 100 branch.
(b) CalculatePostage(650) returns 4.20, from the innermost ELSE branch.
(c) TotalCost = 5.70, output as Total postage: 5.70.
(d) A FUNCTION must be used because only a FUNCTION returns a value via RETURN that can be used inside an expression; a PROCEDURE returns no value at all.