The purpose of an algorithm

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Quick Questions

Shade in the A or B circle for each question.

1. Which is a statement of the PURPOSE of an algorithm?

A Finds the largest of ten numbers

B Sets Max to 0, then loops 10 times and compares each number

2. An exam asks: 'State the purpose of the algorithm.' Which answer is better?

A Line 1 declares Total, line 2 sets it to 0, line 3 starts a FOR loop

B Calculates the average of five test marks

3. Where is the best place to look first for clues about what an algorithm does?

A The OUTPUT statement at the end

B The comment on the first line only

4. The algorithm ends with OUTPUT "Total cost is ", TotalCost. What is the most likely purpose?

A Calculates and displays a total cost

B Counts how many times a loop runs

5. An algorithm uses the identifiers Highest, Score and Count. What do these names suggest?

A Identifier names never give any clues

B It probably finds the highest score

Match the Pairs

Draw a line connecting each item on the left to its match on the right.

Left column		Right column
1Max ← Number[1] FOR Index ← 2 TO 10 IF Number[Index] > Max THEN Max ← Number[Index] NEXT Index OUTPUT Max		AThis only names the programming constructs used. It does not say what the algorithm achieves, and it ignores the OUTPUT and identifier names.
2Total ← 0 FOR Count ← 1 TO 5 INPUT Mark Total ← Total + Mark NEXT Count OUTPUT Total / 5		BTotalCost ← TotalCost + Price
3Start from the OUTPUT: the value that is output shows what the algorithm is trying to achieve.		CThis is a line-by-line description of the steps, not the purpose. It does not say what the whole algorithm achieves.
4Clues in identifier names: meaningful names hint at the purpose of an algorithm.		DPurpose: calculates the average of five marks entered by the user.
5Purpose: Sets Total to 0, then adds each Mark to Total in a loop.		EOUTPUT PassCount
6Purpose: Uses a FOR loop and an IF statement.		FPurpose: finds the largest of ten numbers stored in an array.

Definitions

purpose

Fill in the Blanks

Complete each line of code by filling in the missing parts.

// Purpose: finds the largest number IF Number _ Largest THEN Largest ← Number

Total ← Total + Mark Average ← Total / 5 ______ "Average mark: ", Average

IF Age >= 18 THEN AdultCount ← __________ + 1 ENDIF

Process: Remainder ← Number ___ 2 Decision: Remainder = 0? Output: "Even"

FOR Count ← 1 __ 10 INPUT Number Total ← Total + Number

Spot the Error

Shade in the letter of the line that contains an error.

Question 1

A Total ← 0

B FOR Count ← 1 TO 10

C INPUT Num

D Total ← Num

E NEXT Count

F OUTPUT Total

Question 2

A Passes ← 10

B INPUT Mark

C IF Mark >= 50 THEN

D Passes ← Passes + 1

E ENDIF

F OUTPUT Passes

Question 3

A INPUT Mark1

B INPUT Mark2

C Average ← Mark1 + Mark2 / 2

D OUTPUT Average

Question 4

A INPUT Length

B INPUT Height

C Area ← Length * Width

D OUTPUT Area

Question 5

A INPUT Price

B Discount ← Price + 0.1

C NewPrice ← Price - Discount

D OUTPUT NewPrice

Answer Key

The purpose of an algorithm

Section A: Quick Questions

A — Finds the largest of ten numbers

B — Calculates the average of five test marks

A — The OUTPUT statement at the end

A — Calculates and displays a total cost

B — It probably finds the highest score

Section B: Match the Pairs

Max ← Number[1] FOR Index ← 2 TO 10 IF Number[Index] > Max THEN Max ← Number[Index] NEXT Index OUTPUT Max → Purpose: finds the largest of ten numbers stored in an array.

Total ← 0 FOR Count ← 1 TO 5 INPUT Mark Total ← Total + Mark NEXT Count OUTPUT Total / 5 → Purpose: calculates the average of five marks entered by the user.

Start from the OUTPUT: the value that is output shows what the algorithm is trying to achieve. → OUTPUT PassCount

Clues in identifier names: meaningful names hint at the purpose of an algorithm. → TotalCost ← TotalCost + Price

Purpose: Sets Total to 0, then adds each Mark to Total in a loop. → This is a line-by-line description of the steps, not the purpose. It does not say what the whole algorithm achieves.

Purpose: Uses a FOR loop and an IF statement. → This only names the programming constructs used. It does not say what the algorithm achieves, and it ignores the OUTPUT and identifier names.

Section C: Definitions

purpose — What the whole algorithm achieves, stated in one sentence, such as 'finds the largest of ten numbers'. It is not a line-by-line description. Check the OUTPUT and identifier names for clues.

Section D: Fill in the Blanks

// Purpose: finds the largest number
IF Number > Largest THEN Largest ← Number

Total ← Total + Mark
Average ← Total / 5
OUTPUT "Average mark: ", Average

IF Age >= 18 THEN
  AdultCount ← AdultCount + 1
ENDIF

Process: Remainder ← Number MOD 2
Decision: Remainder = 0?
Output: "Even"

FOR Count ← 1 TO 10
  INPUT Number
  Total ← Total + Number

Section E: Spot the Error

D — The purpose is to find the total of ten numbers. The OUTPUT is Total, so each Num must be added to Total. It should be Total ← Total + Num. Without this, Total just holds the last number.

A — The purpose is to count passes, so the counter Passes must start at 0. Starting at 10 makes the OUTPUT wrong. It should be Passes ← 0.

C — The identifier name and the OUTPUT show the purpose is to find the average of two marks. Without brackets only Mark2 is divided by 2. It should be Average ← (Mark1 + Mark2) / 2.

B — The OUTPUT is Area and the calculation uses Length and Width, so the purpose is to find the area of a rectangle. Width is never given a value because Height is input instead. It should be INPUT Width.

B — Start from the OUTPUT: NewPrice is the price after a 10% discount. Discount must be 10% of Price, so it should be Discount ← Price * 0.1, not Price + 0.1.