1. HKOI Programming HKOI Training Team (Intermediate) 2005-01-15 Alan, Tam Siu Lung Unu, Tse Chi Yung

2. What you should learn from this lesson How to solve a problem How to code faster How to test your program How to score more

3. Today’s flow Competition Rules Problem Solving Coding Testing Tricks

4. Competition Rules The winner is determined by – Fastest program? – Amount of time used in coding? – Number of tasks solved? – Highest score

5. Scoring Black-box Testing Marks will be given if a program passes a certain test case A test case is passed if the output – matches the expected one, or – satisfies certain criteria No referral to source code

6. The “OI” Programming Process Choosing a problem Reading the problem Thinking Coding Testing Finalizing the program

7. Reading the problem Usually, a task consists of – Title – Problem Description – Constraints – Input/Output Specification – Sample Input/Output – Scoring

8. Reading the problem Constraints – Range of variables – Execution time NEVER make assumptions yourself – Ask whenever you are confused Read EVERY word Make sure you understand before going on

9. Thinking Classify the problem – Graph? Mathematics? Data Processing? … Compare with some past problems – Any similarity? For complex problems, divide the problem into smaller sub-problems

10. Thinking Draw diagrams Consider special cases Is the problem too simple? – Be suspicious, you may have overlooked something Still no idea? Give it up… Try again later

11. Designing the solution Some points to consider – Execution Time (Time Complexity) – Amount of memory used (Space Complexity) – How to store data? (Data Structure) – Difficulty in coding

12. Coding Coding is just a small part in the competition Less coding time means more time for thinking (which is more important) FYI, usually Unu completes a program in 15 minutes How to perform problem reduction faster?

13. Characteristics of OI programs Simple Input/Output Assumption: Data input format always matches specification Short programs (usually < 100 lines)

14. Common practices in OI-coding No comments needed Short variable names (usually 1-2 chars) Less procedures / functions Use of break, continue and goto Hardcoding (Not recommended)

15. Common practices in OI-coding Use a slightly larger array than needed Pascal users: use longint instead of integer Avoid real numbers (sometimes not possible) Avoid long and complex expressions Save and Compile frequently

16. Testing Sample Input/Output “A problem has sample output for two reasons: 1. To make you understand what the correct output format is 2. To make you believe that your incorrect solution has solved the problem correctly ”

17. Testing Create some simple input yourself (by hand) Boundary cases “Large” input – Test for execution time and integer overflow Tricky cases

18. Debugging Print values of important variables to screen and/or files Print messages to screen and/or files Use debugger – FreePascal IDE debugger – GDB

19. Finalizing Check I/O filename Check output format – Any trailing spaces? Correct source/executable name? Is the executable updated?

20. Tricks Quick and dirty ways to get marks Usually, 10-20% of total marks are allocated to simple test cases Write programs that handles these cases ONLY Use only when you have totally no idea on a task or time is running out

21. Tricks “No solution” Sample Input/Output Special cases Hardcoding Stop the program before execution time runs out

22. Demonstration (of sloppy skills) Define the value of a character be: ‘A’=1, ‘B’=2, …, ‘Z’=26 Given N strings of capital letters, sort them according to the average value of all characters. If average value of 2 strings are equal, the one which comes earlier in the input should be ranked earlier. Input: (“TOM”, “JANET”, “JOHN”) Output: (JOHN”, “TOM”, “JANET”)