Validation & verification

Why validate input

• Validation checks that input is sensible before the program uses it.
• "Garbage in, garbage out" — bad data leads to wrong answers.
• The program decides to accept or reject the value.

Range check

• A range check makes sure a number is between a low and a high limit.
• Example: a month must be from 1 to 12.

month = 13
if 1 <= month <= 12:
    print("Accepted")
else:
    print("Rejected")

Length check

• A length check makes sure text is not too short or too long.
• len(text) gives the number of characters.

password = "abc"
if len(password) >= 8:
    print("Accepted")
else:
    print("Too short")

Presence check

• A presence check makes sure something was actually entered.
• An empty string "" means the user typed nothing.

Verification: checking the copy

• Verification checks that data was copied or typed correctly.
• A double-entry check asks for the value twice and compares them.
• A visual check means a human reads it back on screen.

In the exam: types of check

• Know these validation checks: range, length, type, presence, format, check digit.

IF Age >= 0 AND Age <= 120
  THEN
    OUTPUT "Valid"
  ELSE
    OUTPUT "Invalid"
ENDIF

Common mistakes

• Validation checks data is sensible (range, length, type); verification checks it was entered correctly.
• A range check needs both a lower and an upper limit.

Now you try

• Each function returns True to accept or False to reject.
• Press Check answer to test your code.