You typed 41, not 14, for your age. Why did the form accept it?

What validation does

• Validation is an automatic check that input is sensible and follows rules before the program uses it.
• Invalid data is rejected, and the user is told why and asked again.
• It stops bad data causing crashes or wrong results later.
• Validation cannot tell whether data is correct: a mistyped value that still follows the rules gets through.

Types of validation check

Writing a validation routine

• Ask for input, test it, and keep asking until it passes.
• A loop does this: the user cannot move on with invalid data.
• This WHILE loop is a range check for 1 to 10.

score = int(input("Score 1 to 10: "))
while score < 1 OR score > 10
  score = int(input("Must be 1 to 10: "))
endwhile

Authentication

• Authentication confirms the identity of a user before they get access, e.g. with a username and password.
• The program compares what is typed with the stored details.
• Anticipating misuse: plan for every input a user might give, by mistake or on purpose, such as blank entries or repeated wrong passwords.

username = input("Username: ")
password = input("Password: ")
if username == "sam01" AND password == "Tr33house" then
  print("Access granted")
else
  print("Access denied")
endif

It passed every test. So why did it fail for an 18-year-old?

Why test?

• Testing checks that a program works as intended and meets its requirements.
• It finds errors so they can be fixed before users meet them.
• A program can run without crashing and still give wrong results.
• Passing a few easy tests proves little: choose test data that could expose errors.
• Iterative testing tests each module while it is being written; final (terminal) testing tests the whole program at the end.

Normal, edge and wrong data

For a program that accepts whole numbers from 1 to 10:

• Normal data: sensible values that should be accepted, e.g. 5.
• Boundary data: the correct type, on the very edge of being valid: 1 and 10.
• Invalid data: the correct type but should be rejected, e.g. 15. Erroneous data: the wrong data type, e.g. ten.

Types of error

• A syntax error breaks the rules of the language, e.g. a missing bracket. The program cannot be translated or run.
• A logic error lets the program run, but it gives the wrong or unexpected result, e.g. > where ≥ was needed.
• Testing with good test data is how you find logic errors.

Test plans

• A test plan lists each test before it is run: what it tests, the test data, its type and the expected result.
• After running, record the actual result. If they differ, fix the code and test again.

• Test 2 failed, so the range check needs fixing.

Code that is easy to maintain

• Other people (and you, later) must be able to read and change code. This is maintainability.
• Naming conventions: totalScore, not t.
• Comments explain what each part of the code does.
• Indentation shows which lines belong inside loops, selection and subprograms.
• Sub programs break code into named, reusable parts.

Big ideas

• Validation automatically checks input is sensible, e.g. range, length and presence checks, and rejects data that breaks the rules; it cannot check data is correct.
• Authentication confirms who a user is, e.g. by matching a username and password.
• Choose test data of every kind: normal, at the edges of the range, and data that should be rejected (normal, boundary, invalid, erroneous).
• Syntax errors stop a program running; logic errors give wrong results.
• Meaningful names, comments, indentation and subprograms make code easy to maintain.