← ACARA Digital Technologies Years 9–10

What it means to validate an algorithm or program

To prove a program gives the correct output for its problem by running or tracing it against test cases. Running without errors is not proof; it is only as validated as its tests.

Part of: ACARA Digital Technologies Years 9–10 — Testing and Validating Programs — Module 1

Understanding What it means to validate an algorithm or program in ACARA Digital Technologies Years 9–10

What it does: The code runs without any error message but gives 6 instead of 5, so it is not valid. A single test of 0 passes even if double() is wrong, such as returning x * x, so the program is barely validated. Validating a program is about correct output for the problem it was meant to solve, not just whether it runs. Running without errors is not proof of correctness. A program can run perfectly and still output the wrong result. Compare the actual output with the expected output. 105 is not 70, so the program is not valid even though it ran.

Common mistakes: The program runs without errors, but it prints 7 instead of the expected 12. Running without errors is not proof of correctness. It should be return w * h. The actual output must come from running the program. Copying expected into actual means the check is always True and proves nothing. It should be actual = area(3, 4).

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This module's key terms are also in a printable crossword (Modules 536–590) and word search (Modules 536–622).

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Learning
Duo
Which best describes what it means to validate a program?
Prove it gives the correct output for its problem by testing it against test casesCheck that it starts and finishes without showing an error message
Matches
print('No errors, so the program is valid!')
↓
Running without errors is not proof of correctness. The output must be checked against expected results from test cases.
Applying
Meanings
Term
validate a program
Meaning
To prove a program gives the correct output for its problem by running or tracing it against test cases. Running without errors is not proof; it is only as validated as its tests.
Practice
def area(w, h):
Testing
Gaps
print(double(4) 8)
Check Answer
Spots
1def area(w, h):
2 return w + h

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Quick Questions

1. Which best describes what it means to validate a program?
A. Check that it starts and finishes without showing an error message
B. Prove it gives the correct output for its problem by testing it against test cases ✓
Validating a program is about correct output for the problem it was meant to solve, not just whether it runs.
2. A program runs with no error messages. Is it proven correct?
A. No, it could still give wrong answers (a logic error) ✓
B. Yes, no errors means the program is validated
Running without errors is not proof of correctness. A program can run perfectly and still output the wrong result.
3. A program should find the average of 3 marks. It runs and prints (a + b + c) / 2 for 60, 70, 80. Which is true?
A. It passes validation because it printed a number without crashing
B. It ran, but the output 105 is wrong. The expected average is 70, so it fails validation ✓
Compare the actual output with the expected output. 105 is not 70, so the program is not valid even though it ran.
4. Which is an example of validating a program (not validating input data)?
A. Running a grade calculator with test cases and comparing results to expected outputs ✓
B. Using a while loop to reject an age that is below 0
Validating a program checks that the whole program gives correct output. Rejecting bad user input is validating input data, which is a different idea.
5. A student tests a program with only one test case and it passes. What can they conclude?
A. Only that it works for that one case. A program is only as validated as its tests ✓
B. The whole program is fully validated and will work for all inputs
One passing test does not cover boundary, abnormal or other cases. More varied tests give more confidence.

Match the Pairs

assert average([2, 4, 6]) == 4 → Validates one case: checks the function gives the expected output of 4 for this test input
if age >= 0 and age <= 120: → Validates input data, not the program: it checks the value is sensible before it is used
A program can run with no errors and still be wrong, so you must compare its output with the expected output → def total(a, b): return a * b print(total(2, 3)) # expected 5, got 6
A program is only as validated as its tests: passing every test in a weak set proves little → tests = [(0, 0)] for x, want in tests: print(double(x) == want)
print('No errors, so the program is valid!') → Running without errors is not proof of correctness. The output must be checked against expected results from test cases.
if int(age) > 0: # validates my program → This only checks the input data. It does not prove the program produces the correct output for the problem.

Key Term

validate a program
To prove a program gives the correct output for its problem by running or tracing it against test cases. Running without errors is not proof; it is only as validated as its tests.

Fill in the Blanks

1. def double(n): return n * 2 print(double(4) == 8)
Hint: Type the operator that checks whether the actual output equals the expected output.
2. if actual == expected: print("PASS") else: print("FAIL")
Hint: Type the keyword that runs when the actual output does not match the expected output.
3. def double(n): return n * 2 assert double(3) == 6
Hint: Type the keyword that stops the program with an error if a test check is false.
4. tests = [(2, 4), (5, 10)] for n, want in tests: print(double(n) == want)
Hint: Type the keyword that repeats the check once for every test case in the list.
5. tests = [(2, 4), (5, 10), (0, 0)] print(len(tests), "test cases")
Hint: Type the built-in function that counts how many test cases are in the list.

Spot the Error

Question 1
A def area(w, h):
B return w + h
C print(area(3, 4))
D # expected output: 12
Line B — The program runs without errors, but it prints 7 instead of the expected 12. Running without errors is not proof of correctness. It should be return w * h.
Question 2
A expected = 12
B actual = expected
C print(actual == expected)
Line B — The actual output must come from running the program. Copying expected into actual means the check is always True and proves nothing. It should be actual = area(3, 4).
Question 3
A def is_even(n):
B return n % 2 == 1
C print(is_even(6))
D print(is_even(7))
Line B — This returns True for odd numbers, so it gives the wrong output even though it runs fine. Testing against 6 and 7 would expose it. It should be n % 2 == 0.
Question 4
A tests = [(2, 4), (3, 6)]
B for x, want in tests:
C print(double(x) == x)
Line C — The output must be compared with the expected output (want), not with the input x. It should be double(x) == want.
Question 5
A tests = [(0, 0), (5, 10)]
B for n, want in tests:
C print(square(n) == want)
Line A — The expected output is wrong: 5 squared is 25, not 10. A correct program would fail this test. Expected values must be worked out correctly first.

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