Worksheet

F6.1 Defensive design and validation

Robust programs · GCSE · OCR J277 2.3.1, AQA 8525 3.2.11, Edexcel 1CP2 3.2.3 · about 15 min

What this lesson is about

Anticipating misuse; presence, type, range, length, format and look-up checks.

• What goes wrong without it
• The validation checks
• Checking in order
• Asking until it is valid
• Length, format and look-up
• Anticipating misuse

Questions 6 marks in all

1. [1 mark]Which validation check makes sure something was entered at all?

   1. APresence check
   2. BRange check
   3. CLength check
   4. DFormat check

2. [1 mark]A distance must be from 1 to 50. Which check is that?

   1. ARange check
   2. BPresence check
   3. CLook-up check
   4. DType check

3. [1 mark]What does this program print?

   for answer in ["30", "", "lots", "-5"]:
       print(answer.isdigit())

4. [1 mark]Why check the type of an answer before its range?

   1. AConverting text that is not a number to int would crash
   2. BRange checks are slower
   3. CType checks are required first by Python
   4. DIt makes no difference

5. [1 mark]A user types 30 when they meant 20, and the range is 1 to 50. What does validation do?

   1. AAccepts it: validation checks data is sensible, not correct
   2. BRejects it
   3. CChanges it to 20
   4. DAsks them to type it twice

6. [1 mark]Checking a colour is one of red, green or blue is which check?

   1. ALook-up check
   2. BLength check
   3. CRange check
   4. DPresence check

The task: a validated drive

Write a program that asks How far, 1 to 50 cm? until the answer is valid, then drives that far forward. For an empty answer print you must type something; for one that is not a whole number print that is not a whole number; for a number out of range print it must be between 1 and 50. The task types an empty answer, then fifty, then 500, then 30.

# the two lines every program starts with: the commands, then the robot
from bugbot import *
connect()

far = int(input("How far, 1 to 50 cm? "))
forward(60, distance=far)

Plan your program here, then type it in and press Run.

Do it on the robot
www.bugbotlab.com/learn/f6-1-defensive-design-and-validation/
The simulator checks it and tells you when it passes. Nothing to install, no account.

Challenges

1. Accept distances with a decimal point, such as 12.5, while still rejecting twelve.
2. Add a check that refuses to drive if distance() shows a wall closer than the distance asked for, plus 5 cm.
3. Validate a class code: exactly six characters, only capital letters and digits.