1. CS1010 Discussion Group 11
Week 3 - Computational Thinking/Algorithms

2. HELLO! I am Yan Hwa Computer Science Year 3 leeyanhwa@u.nus.edu

3. Complete tutorial questions before class so we can discuss
Expectations
Complete tutorial questions before class so we can discuss
Ask any questions during class or post on forum
Attempt tutorial questions before expecting an answer
Also learning from you
Make friends!
Don’t wait till recess week to catch up

4. Programming background? (if any) Your expectations? Your concerns?
About you
Programming background? (if any)
Your expectations?
Your concerns?

5. Can arrive earlier to ask any questions
Discussion sessions
Can arrive earlier to ask any questions
Attendance will be taken; 5% of final grade
Go through some questions in Discussion Sheet
QnA
After a few weeks, may split session into two sub- sessions
First session: Complusory
Second session: Experienced students may choose to leave
Arrive earlier
Please participate~
Don’t wait till recess week to catch up
Going through some questions  group discussion and/or presentation if time permits

6. Reminders Check IVLE regularly for announcements/forum postings
Participate actively in IVLE forums
Post in forums instead of sending s
Have you tested out CodeCrunch by submitting the Lab0?
Lab1 has been released. Please take note the deadline is next Wednesday (6th September) 5pm.
Fill in “My Progress Chart” in the CS1010 Student Handbook regularly. I may spot-check!
Post in forums instead of sending s

7. Victory Road Nature of CS1010 Practice make perfect!
This module can be very easy or very tough
Don’t leave it till the last minute
Not just a course on C programming
Practice make perfect!
It is not one module where you can leave it till the last minute. Rather, if you have done it right then just before the exams you should be feeling very relaxed.

8. Google is your best friend
Let me google that for you

9. Lecture Summary
Decomposition

10. Problem Solving and Incremental Coding
Lecture Summary
Define Problem!
Problem Solving and Incremental Coding
Inputs? Outputs? Constraints? Relationship?
Make Plan!
Code, Test, Code, Test
Define problem.
Capture all relevant information
What are the inputs (data)?
What are the outputs (unknowns)?
What are the constraints?
Connection/relationship between inputs and outputs?
Make a Plan
Step by Step, code and test / debug / do logging

11. Tips on Labs Do Follow Instructions
Give meaningful names for variables
Appropriate commenting
Must do
Double/triple check your indentation (gg=G in vim will auto-indent your program nicely!)
Do not
Forget to initialise variables whenever necessary
Output format does not conform to requirement
Commenting = Think, will you understand this piece of code very quickly by just reading it? A few days later? Years later?
Never name your classes and functions ambiguously.
Always use inline comments on code blocks that are complicated or may appear unclear.
Always use descriptive variable names.
Always write comments describing the intent or reason why a piece of code exists.
Always keep comments up to date when editing commented code.

12. Ask early if unclear (Forums, Email)
Tips on Labs
Ask early if unclear (Forums, )
Do NOT copy from friends! (changing variable names, spacing, shifting codes around is not going to work!)
Last-minute uploading could be a traumatic experience
It is not one module where you can leave it till the last minute. Rather, if you have done it right then just before the exams you should be feeling very relaxed.

13. Tutorial Exercise
CodeCrunch Lab #0
Have you submitted?

14. Lab #1 Three Simple Exercises
Tutorial Exercise
Lab #1 Three Simple Exercises
Read Ex3 Packing and fill in the worksheet

15. Given a list of N integers, find out how many of them are negative.
Tutorial 1 Q4
Given a list of N integers, find out how many of them are negative.
Let the list of integers be a1, a2, …, aN.
for k from 1 to N
if (a[k] < 0)
countNeg  countNeg + 1;
print countNeg
Are you all okay with this qn? Where is the condition to change?

16. countNeg  0 Tutorial 1 Q4a Why must we initialise? for k from 1 to N
if (a[k] < 0)
countNeg  countNeg + 1;
print countNeg
Why must we initialise?
However until the variable is initialised, that piece of memory could contain anything. This is why using it is unpredictable.
Other languages may assist you in this area by initialising variables automatically when you assign them, but as a C programmer you are working with a fairly low-level language that makes no assumptions about what you want to do with your program. You as the programmer must explicitly tell the program to do everything.

17. How to use a for loop instead?
Tutorial 1 Q4d
sum  0; k  1;
while ( k ≤ N ) {
sum  sum + a[k];
k  k + 2; }
How to use a for loop instead?
Pattern is not very obvious. Need to use strong pattern recognition
sum  0;
for k from 1 to N step 2
sum  sum + ak;

18. Find the number of swaps required to move all white balls to the left of black balls
Only swapping of neighbours allowed
Tutorial 1 Q5
How many swaps needed? Not allowed!!

19. Find the number of swaps required to move all white balls to the left of black balls
Only swapping of neighbours allowed
Tutorial 1 Q5
2 swaps?

20. Find the number of swaps required to move all white balls to the left of black balls
Only swapping of neighbours allowed
Tutorial 1 Q5
2 swaps!

21. Warm-up exercise : Find the white balls! Similar to Q4?
Tutorial 1 Q5
Warm-up exercise : Find the white balls! Similar to Q4?
let’s call the balls Bk, 1 ≤ k ≤ N where N is the number of balls
countWhite  0;
for k from 1 to N
if (Bk is white)
countWhite  countWhite + 1;
print countWhite
Pattern recognition!

22. How to find out total number of swaps needed
Decompose Problem
Plan your solution
Find Patterns
We need to know: Number of swaps needed for each white ball then add them up!

23. Tutorial 1 Q5 Algorithm 1
Number of swaps needed for each white ball = Position of white ball (k) – desired pos
countSwaps  0;
desiredPos  1;
for k from 1 to N
if (Bk is white)
numSwaps  k – desiredPos;
countSwaps  countSwaps + numSwaps;
desiredPos  desiredPos + 1;
Print countSwaps
Pattern recognition!

24. Tutorial 1 Q5 Algorithm 2
Number of swaps needed for each white ball = number of black balls to the left of the white ball
countSwaps  0;
blackToLeft  0;
For k from 1 to N
if (Bk is black)
blackToLeft  blackToLeft + 1;
else
/*it is a white ball */
countSwaps  countSwaps + blackToLeft;
Print countSwaps
Pattern recognition!

25. Tips for Vim
Navigate around using arrow keys or hjkl
Google for Vim Shortcuts and cheatsheets
Use Vim Tutor!

26. Tips for Vim

27. Summary
Computational thinking and Problem Solving
Plan before programming anything
Start doing your lab! Learn Vim! Practise!

28. Sources of Practice
Reference book
Practice exercises we put up on CodeCrunch ( tml)
Google for more questions and cheatsheets sections/1/Cheatsheet-c.pdf
Books definitive-c-book-guide-and-list

29. Sources of Practice
comments-tell-you-why/