← IGCSE Computer Science Paper 2

Iterative testing while coding

Testing each small part of the code as it is written, fixing any errors, then retesting before moving on. This finds errors early and repeats until that part works.

Part of: IGCSE Computer Science 0478 — 7a Program Development Life Cycle — Module 16

Understanding Iterative testing while coding in IGCSE Computer Science Paper 2

What it does: Running the loop as soon as it is written shows any error straight away, while the problem is still small and easy to find. Iterative testing means testing each small part as it is written, fixing errors and retesting. After fixing an error you must retest to check that the fix worked and nothing else broke. You repeat the test and fix cycle until that part works, then move on. Both are needed. Iterative testing happens in the coding stage, and the testing stage checks the finished program. Errors found early are quicker and cheaper to fix than errors hidden in a large amount of untested code.

Common mistakes: Iterative testing means test, fix, then retest. Moving on straight after fixing skips the retest, so the fix is never checked. It should read: // Fix errors, then retest. Testing this part gives 3 instead of the expected 6, which shows an error. Total is overwritten each time instead of being added to. The fix is Total ← Total + Count, then retest.

Class: _____________________ · Name: _____________________ · Date: _________

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This module's key terms are also in a printable crossword (Modules 1–16) and word search (Modules 1–26).

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Learning

Duo

A student writes one subroutine, runs it with some test values, fixes a mistake, then runs it again. What is this called?

Iterative testingWriting the whole program first and testing it only at the end

Matches

Write the whole program, then test it only once at the end

↓

This leaves all testing to the end, so errors are found late and are harder to locate. Iterative testing tests each small part as it is written.

Applying

Meanings

Term

Iterative testing

Meaning

Testing each small part of the code as it is written, fixing any errors, then retesting before moving on. This finds errors early and repeats until that part works.

Practice

// Part 1: add two marks

Testing

Gaps

PartWorks = TRUE

Check Answer

Spots

2// Test that part now

3// Fix errors, then move on

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Quick Questions

1. A student writes one subroutine, runs it with some test values, fixes a mistake, then runs it again. What is this called?

A. Writing the whole program first and testing it only at the end

B. Iterative testing ✓

Iterative testing means testing each small part as it is written, fixing errors and retesting.

2. What is the correct order in iterative testing?

A. Test, fix, retest ✓

B. Fix, write more code, test once at the end

After fixing an error you must retest to check that the fix worked and nothing else broke.

3. When do you move on to the next part of the program?

A. When the current part passes its tests ✓

B. As soon as you have finished typing the current part

You repeat the test and fix cycle until that part works, then move on.

4. Why does iterative testing find errors early?

A. Errors cannot happen if the code is tested at the end

B. Each small part is tested straight after it is written, so a fault is easy to locate ✓

Testing small parts means that if a test fails, the error must be in the code you just wrote.

5. A student fixes an error in a loop and then moves straight on without running it again. Is this iterative testing?

A. No, the fix has not been retested ✓

B. Yes, fixing the error is enough

Iterative testing needs a retest after every fix. The fix might not work or might cause a new error.

Match the Pairs

Iterative testing → Testing each small part of the code as it is written, fixing any errors, then retesting before moving on

Test, fix, retest → The repeating cycle used on one part of the code until that part works correctly

Iterative testing finds errors early because each new part of the code is tested straight after it is written → Write the FOR loop, run it, find the error now

Repeat until the part works: after a fix, the same part is tested again before moving on → REPEAT Test the part Fix any errors UNTIL Part works

Write the whole program, then test it only once at the end → This leaves all testing to the end, so errors are found late and are harder to locate. Iterative testing tests each small part as it is written.

Find an error, fix it, then move straight on to the next part → The fix is never retested, so the programmer cannot be sure the part now works or that the fix did not add a new error. The part must be tested again before moving on.

Key Term

Iterative testing

Testing each small part of the code as it is written, fixing any errors, then retesting before moving on. This finds errors early and repeats until that part works.

Fill in the Blanks

1. REPEAT // test the part, then fix any errors UNTIL PartWorks = TRUE

Hint: Type the keyword that ends a REPEAT loop and gives the condition for stopping.

2. WHILE PartWorks = FALSE DO // fix the error and retest ENDWHILE

Hint: Type the BOOLEAN value that means the part still has errors, so the loop keeps going.

3. IF ActualOutput = ExpectedOutput THEN PartWorks ← TRUE ENDIF

Hint: Type the keyword that follows the condition when the test result matches what was expected.

4. DECLARE Retests : INTEGER Retests ← 0 Retests ← Retests + 1

Hint: Type the arithmetic operator that adds one to the count each time the part is tested again.

5. IF ErrorsFound > 0 THEN // fix the error, then retest ENDIF

Hint: Type the relational operator that checks whether at least one error was found in the test.

Spot the Error

Question 1

A // Write one small part

B // Test that part now

C // Fix errors, then move on

D // Move on when it works

Line C — Iterative testing means test, fix, then retest. Moving on straight after fixing skips the retest, so the fix is never checked. It should read: // Fix errors, then retest.

Question 2

A Total ← 0

B FOR Count ← 1 TO 3

C Total ← Count

D NEXT Count

E OUTPUT Total

Line C — Testing this part gives 3 instead of the expected 6, which shows an error. Total is overwritten each time instead of being added to. The fix is Total ← Total + Count, then retest.

Question 3

A // Test only when all is done

B // Fix any errors found

C // Retest the part

D // Then write the next part

Line A — Iterative testing tests each small part as it is written, not only at the end. Waiting lets errors build up and makes them harder to find. It should read: // Test each part as written.

Question 4

A INPUT Age

B WHILE Age < 0 AND Age > 120 DO

C OUTPUT "Invalid age"

D INPUT Age

E ENDWHILE

Line B — Testing with -5 shows the loop never runs. No number is both below 0 and above 120, so AND is never true. The condition needs OR, then the part is retested.

Question 5

A // Test 1: Total of 5 and 7

B // Expected output: 35

C Total ← 5 + 7

D OUTPUT Total

Line B — A test needs the correct expected output, or the result can't be judged. 5 + 7 gives 12, not 35, so the expected output should be 12.

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