1. Lesson Outcomes
2.1.3 Be able to identify differentiate between types of error in programs
2.1.4 Be able to interpret error messages and identify, locate and fix errors in a program.
2.1.7 Be able to make effective use of tools offered in an integrated development environment [watcher, break points, single step, step throughs]
What causes error messages
All of should be able to recall different error types
All of you should be able to recall error messages in python
All of you should be able to use a trace table
All of you should be able to use a debugger software

2. Quick recap – Error handling keywords
Syntax Run time Logic

3. Testing for logic errors
Trace Table
A technique used to test algorithms to see if any logic errors occur when the algorithm is processed.
Testing for logic errors

4. Are you happy with this function?
Example 1
Are you happy with this function?
for next in range(1,10,2):
number1 = next
number2 = next * next
number3 = next / 2
next
number1
number2
number3

5. Example 1 for next in range(1,10,2): number1 = next
number2 = next * next
number3 = next / 2
next
number1
number2
number3 1
0.5 3 9
1.5 5 25
2.5 7 49
3.5 81
4.5

6. Example 2 C A B
A = 3 B = 3 For C = 1 to 4 B = B*C A = A+C Next C

7. Example 2 C A B 1 4 3 2 6 9 18 13 72
(Psueudocode) A = 3 B = 3 For C = 1 to 4 B = B*C A = A+C Next C

8. Example 3 Pseudocode A=4 B=7 1 Input A 2 Input B 3 If A=B Then 4 B = 25
Else 6
While B>A 7
B=B-A 8
End While 9
A = B + 5 10
End IF
Variable
Line A B
Comment 1 2 3 4 5 6 7 8 9 10

9. Example 3 Pseudocode A=4 B=7 1 Input A 2 Input B 3 If A=B Then 4 B = 25
Else 6
While B>A 7
B=B-A 8
End While 9
A = B + 5 10
End IF
Variable
Line A B
Comment 1 4 2 7 3
False: goto 6 5 6
True: continue 8 9 10

10. Break.... Which is the odd one out? Type Error Runtime NameError
Variableinterupt
ZeroDivisionError
KeyBoardInterrupt
Typeinputerror

11. Object Orientated Programming
Bugs can be hard to find...
Constant
Assignment
Data
System
Information
Array / list
Variable
Algorithm
Boolean
Data structure
Pseudocode
Find it...
Object Orientated Programming
Integer
Binary
Identifier
Real / Float

12. “Debug tools” or “Debugger”
Single step
(where the program is executed one statement at a time – performs a ‘step through’
Setbreak points
(Positions in the program when its halted and variables can be checked to locate logic errors)
Watchers
(Allow a programmer to watch for certain events such as variables changing)

13. Debugger in Python

14. The best way to learn about it...is to use it. Your task: Over to you...
Take some of your programs and use the debugger (rock, paper, scissors game?)
Then take an old program you have made (sabotage it and look at how it might pick it up)

15. Homework
For next lesson (Tuesday)