ITPEC FE Morning April 2021 Question 23

Detecting that the value of an entered code is wrong — a check digit is calculated from the other digits of the code, so a mistyped value no longer matches it.

When a code is issued, an extra digit is computed from its digits by a fixed rule (a weighted sum and a remainder, for example) and appended. At input, the system recomputes the digit from what was typed and compares. A wrong digit or two swapped digits give a different result, so the entry is rejected even though it has the right length and contains only numbers. ISBN and bank account numbers work this way.

Answer (c)

Why not others:

• •(a) — letters or symbols in a numeric field are caught by a type (numeric) check, which needs no calculation
• •(b) — a wrong number of digits is caught by a length (digit count) check
• •(d) — a value outside an allowed interval is caught by a range (limit) check; a check digit says nothing about magnitude

Key rule: Each input check targets one kind of error: type, length, range, or, with a check digit, whether the digits themselves are the ones that were issued.

Basic Theory · AI-written, may contain errors