Objectives: Students should be able to —
- 1 Identify and correct errors in flowcharts.
- 2 Identify and correct errors in programs.
- 3 Identify and correct errors in pseudocode.
- 4 Use a trace table or dry run to locate logic errors in a given algorithm.
- 5 Suggest a correction to remove each located error.
- 6 Identify validation checks (length, double-entry) and design appropriate test data.
Identifying Errors in FOR-NEXT Loop Algorithms
10 Largest = 0
20 Sum = 0
30 FOR X = 0 TO 10
40 INPUT X
50 IF X > Largest THEN X = Largest
60 OUTPUT Largest
70 Sum = Sum + X
80 NEXT X
90 Average = Sum * 10
100 OUTPUT Average
There are four errors in this algorithm. Locate these errors and suggest a correction.
| Error | Correction |
|---|---|
| Error 1: Line 40 — INPUT X is using the same counter variable of the FOR-NEXT loop. | Method: Change the counter variable of the FOR-NEXT loop. For example — FOR Count = 1 TO 10 |
| Error 2: Line 50 — X = Largest | Method: It should be, Largest = X |
| Error 3: Line 60 — OUTPUT Largest should not be inside the loop. | Method: It should be outside the loop. For example — 85 OUTPUT Largest |
| Error 4: Line 90 — Incorrect formula for calculation of average. | Method: It should be — Average = Sum / 10 |
1 Total = 0
2 FOR Counter = 1 TO 50
3 INPUT Num
4 Total = Total + 1
5 Counter = Counter + 1
6 Average = Total / Counter
7 NEXT Counter
8 PRINT Average
| Error | Correction |
|---|---|
| Error 1: Line 4 — Total = Total + 1 | Method: It should be, Total = Total + Num |
| Error 2: Line 5 — Counter = Counter + 1 | Method: It should be removed because the counter variable's value increments by 1 automatically in a FOR-NEXT loop. |
| Error 3: Line 6 — Average = Total / Counter is inside the loop. | Method: It should be outside the loop. Swap lines 6 and 7. |
| Error 4: Line 6 — Incorrect formula for calculation of average. | Method: It should be — Average = Total / 50 |
Identifying Errors in WHILE Loop & Conditional Algorithms
INPUT Number
WHILE Numbers <> 999 DO
IF Number > 100
THEN
OUTPUT Number
ENDIF
ENDWHILE
OUTPUT Number
(a) Identify the four errors and suggest corrections:
| Error | Correction |
|---|---|
| Error 1: WHILE Numbers <> 999 DO | Method: Numbers should be Number. It should be — WHILE Number <> 999 DO |
| Error 2: IF Number > 100 | Method: It should be — IF Number >= 100 |
| Error 3: INPUT Number is missing from inside the loop. | Method: Insert INPUT Number after the ENDIF statement. |
| Error 4: The final OUTPUT Number outside the loop is not needed. | Method: Remove it. |
(b) Pseudocode statement to output all numbers between 100 and 200 inclusive:
IF Number >= 100 AND Number <= 200
This uses a compound condition with the logical operator AND.
The function RANDOM(X, Y) generates a random integer greater than X and less than or equal to Y.
For example, RANDOM(1, 4) generates 2 or 3 or 4.
01 Count ← 0
02 WHILE Counter > 50 DO
03 NumRand[Counter] ← RANDOM(1, 100)
04 Counter ← Counter - 2
05 ENDWHILE
| Error | Correction |
|---|---|
| Error 1: Line 01 — Count ← 0 | Method: It should be — Counter ← 0 |
| Error 2: Line 02 — WHILE Counter > 50 DO | Method: It should be — WHILE Counter < 50 DO |
| Error 3: Line 03 — NumRand[Counter] ← RANDOM(1, 100) | Method: It should be — NumRand[Counter] ← RANDOM(0, 100) |
| Error 4: Line 04 — Counter ← Counter - 2 | Method: It should be — Counter ← Counter + 1 |
Identifying Errors in REPEAT-UNTIL with Validation & Test Data
01 Attempt ← 0
02 REPEAT
03 PassCheck ← TRUE
04 OUTPUT "Please enter your password "
05 INPUT Password
06 IF LEN(Password) < 8
07 THEN
08 PassCheck ← TRUE
09 ELSE
10 OUTPUT "Please re-enter your password "
11 INPUT Password2
12 IF Password <> Password
13 THEN
14 PassCheck ← FALSE
15 ENDIF
16 ENDIF
17 Attempt ← Attempt + 1
18 UNTIL PassCheck OR Attempt <> 3
19 IF PassCheck
20 THEN
21 OUTPUT "Password success"
22 ELSE
23 OUTPUT "Password fail"
24 ENDIF
(a) Identify the three errors and suggest corrections:
| Error | Correction |
|---|---|
| Error 1: Line 8 — PassCheck ← TRUE | Method: It should be — PassCheck ← FALSE |
| Error 2: Line 12 — IF Password <> Password | Method: It should be — IF Password <> Password2 |
| Error 3: Line 18 — UNTIL PassCheck OR Attempt <> 3 | Method: It should be — UNTIL PassCheck OR Attempt = 3 |
(b) The algorithm includes two types of check on the data input. Identify and describe each type of check:
| Type of Check | Description |
|---|---|
| Type of check 1: Length check validation | Checks the number of characters in the password. |
| Type of check 2: Double entry verification check | Asks the user to input the password twice and compares both to check if they are the same. |
(c) Give two sets of test data for this algorithm and a reason for choosing each set:
| Test Data | Reason |
|---|---|
| Set 1: ALk#24 | Abnormal data — it should be rejected because it contains fewer than 8 characters. |
| Set 2: John@7351, John@7351 | Normal data — it should be accepted because it contains not fewer than 8 characters and both entries are the same. |
Identifying Errors in CASE Statement Algorithms
01 Continue ← 1
02 WHILE Continue = 0
03 OUTPUT "Enter 1 for +, 2 for -, 3 for *, or 4 for /"
04 INPUT Operator
05 OUTPUT "Enter the first value"
06 INPUT Value1
07 OUTPUT "Enter the second value"
08 OUTPUT Value2
09 IF Operator
10 1 : Answer ← Value1 + Value2
11 2 : Answer ← Value1 - Value2
12 3 : Answer ← Value1 * Value2
13 4 : Answer ← Value1 / Value2
14 ENDCASE
15 OUTPUT "The answer is ", Value1
16 OUTPUT "Do you wish to enter more values (Yes or No)?"
17 INPUT MoreValues
18 IF MoreValues = "No"
19 THEN
20 Continue ← 1
21 ENDIF
22 UNTIL Continue = 0
Two valid alternatives are shown below. Either set is acceptable.
Alternative 1 — Convert to a WHILE loop:
| Error | Correction |
|---|---|
| Error 1: Line 01 — Continue ← 1 | Method: It should be — Continue ← 0 |
| Error 2: Line 22 — UNTIL Continue = 0 | Method: It should be — ENDWHILE |
| Error 3: Line 08 — OUTPUT Value2 | Method: It should be — INPUT Value2 |
| Error 4: Line 09 — IF Operator | Method: It should be — CASE OF Operator |
| Error 5: Line 15 — OUTPUT "The answer is ", Value1 | Method: It should be — OUTPUT "The answer is ", Answer |
Alternative 2 — Convert to a REPEAT-UNTIL loop:
| Error | Correction |
|---|---|
| Error 1: Line 02 — WHILE Continue = 0 | Method: It should be — REPEAT |
| Error 2: Line 20 — Continue ← 1 | Method: It should be — Continue ← 0 |
| Error 3: Line 22 — UNTIL Continue = 0 | Method: It should be — UNTIL Continue = 1 |
| Error 4: Line 08 — OUTPUT Value2 | Method: It should be — INPUT Value2 |
| Error 5: Line 09 — IF Operator | Method: It should be — CASE OF Operator |
| Error 6: Line 15 — OUTPUT "The answer is ", Value1 | Method: It should be — OUTPUT "The answer is ", Answer |
Revision: Statements and Key Computing Terms
| Statement | Key Term |
|---|---|
| An error that causes an algorithm to produce incorrect results, even though it runs without crashing. | Logic error |
| An error caused by breaking the rules (grammar/spelling) of the programming language. | Syntax error |
| A table used to trace and record the value of each variable as an algorithm is dry-run. | Trace table |
| Working through an algorithm on paper, line by line, without running it on a computer. | Dry run |
| A loop where the number of iterations is known in advance (e.g., FOR X = 1 TO 10). | FOR-NEXT loop |
| A loop where the condition is tested before each iteration (may run zero times). | WHILE-ENDWHILE loop |
| A loop where the condition is tested after each iteration (always runs at least once). | REPEAT-UNTIL loop |
| A multi-way selection structure that compares a single variable against a list of values. | CASE OF ... ENDCASE |
| A variable whose value increases by 1 each time a loop runs. | Counter variable |
| A variable used to add up a running total (e.g., Sum = Sum + X). | Accumulator |
| A validation check that ensures the number of characters in an input is within an acceptable range. | Length check |
| A verification check where the user is asked to enter the same data twice and the two inputs are compared. | Double entry verification |
| Test data that should be accepted by the program because it is valid. | Normal data |
| Test data that should be rejected by the program because it is invalid (also called erroneous data). | Abnormal data |
| Test data that lies on the boundary of acceptability (e.g., 8 chars when 8 is the minimum). | Boundary / Extreme data |
| A pre-defined function that returns the number of characters in a string. | LEN(X) |
| A pre-defined function that generates a random integer greater than X and less than or equal to Y. | RANDOM(X, Y) |