Implementing validation in an algorithm

Overview

Benefit of writing a validation check as a separate function, defensive programming and how a drop-down list reduces the need for validation code.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   Write pseudo-code for a loop that keeps asking for an age until it lies between 0 and 120.

   Model answer

   The loop is REPEAT INPUT age UNTIL age ≥ 0 AND age ≤ 120. The program cannot continue until an acceptable value has been entered.

2. 2

   Why is a post-conditioned loop a natural structure for validating input?

   Model answer

   A post-conditioned loop always asks for the value once before testing it. The test then decides whether the question has to be repeated.

3. 3

   Explain the benefit of writing a validation check as a separate function.

   Model answer

   A function that returns TRUE or FALSE can be called from anywhere in the program. The same rule is then applied identically wherever that field is entered.

4. 4

   Write pseudo-code that rejects an empty name entry.

   Model answer

   The test is IF name = '' THEN OUTPUT 'Name required' ELSE store the record. An empty string is the value a presence check looks for.

5. 5

   How can a program stop a validation loop running forever?

   Model answer

   A counter can limit the number of retries allowed before the program gives up. The user is then returned to the menu instead of being trapped.

6. 6

   What is defensive programming?

   Model answer

   Defensive programming means writing code that expects invalid or hostile input at every entry point. Checks are placed on data before it is used rather than afterwards.

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