CS1010 AY20/21 S1 Lecture 1

class: middle, center

# CS1010
## Programming Methodology 
### (At Home Edition)
---
class: middle, center

# Lecture 1

### 10 August 2020

Admin Matters 
Unit 1: **What is a Program?** 
Unit 2: **Computational Problems and Algorithms**

---
class: middle, center

# Ooi Wei Tsang

### ooiwt@comp.nus.edu.sg

### Office Hour : Tuesday 4 - 5 PM

---
class: middle, center

# Daren Ler

### dler@comp.nus.edu.sg

### Office Hour : Friday 4 - 5 PM

---
class: middle, center

## What will you learn?

---
class: middle

- Comfortable with reading and developing small programs to solve a given problem

- Become proficient with C and associated programming tools (editors, debuggers)

---
class: middle

## What do you need to know before CS1010?

---
class: middle

- Nothing about programming

- Comfortable with using a computer

- Comfortable with secondary school mathematics and notations

---
class: middle,center

## Weekly Activities

---
class: middle

- **Monday**: Attend Lecture

- **Monday-Thursday**: Review, Problem Set, and Self-Diagnostic Quiz

- **Thursday**: Attend Lab; Assignment Released

- **Thursday-Tuesday**: Solve the Assignment

---
class: middle, center

## Most Learning Will Happen in the Lab

---
- problem set and programming assignment will be discussed

- 10 students per group

- lead by a student tutor

- online

- starting Week 3

---
class: middle

### For lab registration issue, please contact

- Ng Tzer Bin ([`dcsnt@nus.edu.sg`](mailto:dcsnt@nus.edu.sg))

- Sunimal Rathnayake ([`dcssrrm@nus.edu.sg`](mailto:dcssrrm@nus.edu.sg))

Daren and I have no access to the system and are powerless on this issue.

---
class: middle

## "Catch-Up" Sessions

- About every three weeks

- Saturday 10 AM - 12 PM

- Q&A format

---
class: middle, center

## Important Resources

---
class: middle, center

## Module Homepage

[`https://nus-cs1010.github.io/2021-s1/`](https://nus-cs1010.github.io/2021-s1/)

lecture notes 
assignments 
guides and manuals 
readings 
etc.

---
class: middle, center

## Piazza

[`https://piazza.com/class/kdgunoizhic105`](https://piazza.com/class/kdgunoizhic105)

online forum 
announcements

---
class: middle, center

## Github

assignment submissions 
assignment grading

---
class: middle, center

## Luminus

quizzes 
grades 
survey 
etc.

---
class: middle, center

# Assessments

(or, how you will be graded)

---
class:middle,center

---
class:middle,center

.smaller[
Assessment         | Date                       | Time
:------------------|:-------------------------- |:-------------
Midterm | Mon, 28 Sep| 12 - 2 PM
Final | Tue, 24 Nov| 5 - 7 PM
Practical Exam 1   | Sat, 3 Oct| 9 AM - 12 PM
Practical Exam 2   | Sat, 7 Nov| 9 AM - 12 PM
]

---
class:middle,center

## Open Book Exam

### Bring any materials you want

### Nothing to memorize

---
class:middle

## Not "bell-curved"

- Your grade is based solely on your marks
- Not relative to others in the class.

---
class: middle

## Plagiarism and Cheating

- No mercy policy in dealing with cases
- Not worth it to cheat/copy

---
class: middle

## Things To Do

- Read about other policies
- Get your accounts ready

---
class: center,middle

# Words of Advice

---
class: center,middle

## Work hard
## Work very very hard

---
class: center,middle

## Expected workload is 10 hours/week

But your mileage may vary

---
class: center,middle

## Learning in NUS SoC is different from your experience

---
class: center,middle

### Less rote learning
### More open-ended problems
### A lot more self-learning

---
class: center,middle

### You cannot just "study" programming or problem solving

### You have to invest lots of time actually doing it

---
class: center,middle

## Do not optimize for CAP and grades

---
class: center,middle

## Optimize for skills, knowledge, and experience

---
class: center,middle

## Invest time to master the tools

(e.g., `bash`, `vim`, `git`, etc)

---
class: center,middle

## Think long term gain, not just to get things done now

---
class: center,middle

## Don't be shy interacting with your instructors and tutors

---
class: center,middle

## This is a new start.

### Put down your baggage about where you are from and what you have done to get here.

---
class: middle, center

# Lecture 1

### 10 August 2020

Admin Matters 
Unit 1: **What is a Program?** 
Unit 2: **Computational Problems and Algorithms**

---
class: middle,center

## What is a program?
---
class: middle,center

### A sequence of step-by-step instructions given to computers to process data in order to perform a certain task.
---
class: middle,center

### How CPU Executes a Program

.fit[![simplified](../figures/simplfied-cpu-mem/simplfied-cpu-mem.001.png)]

.tiny[(A highly simplfied view)]

---
## Machine Code

- Instructions that can be interpreted by a CPU

- Different CPU architecture may use different instruction sets

`1001010001011110100010101010101000101`

---
## Assembly Language

- Use mnemonic to represent the instructions such as `incr`, `decr`, `store`, etc. in a more human-readable way.

- Still difficult to write: operation that is conceptually simple still requires multiple instructions to be issued.

---
## Higher-Level Language

- Allow programmers to express an operation closer to their intention

- E.g., $x = x + y$ if $y < 0$

---
## Higher-Level Languages

- FORTRAN (first popular programming language from 1950s)

- C (1972, still influential today)

- Many many others

---
## Compilation

A compiler _compiles_ a program written in high-level language to machine code.

.fit[![simplified](../figures/c-compile-run/c-compile-run.001.png)]

---
class: middle,center

# What you will learn?

---
class: middle,center

# How a C program behaves

---
class: middle,center

# How to write C programs
---
class: middle,center

## Tools and techniques to write good and correct C programs
---
class: middle,center

## How to use C to solve computational problems
---
class: middle

### Learning to write a program that does what you want is the easy part.

### _Knowing what you want your program to do is the real challenge_!

---
class: middle,center

## What is a computational problem?

---
class: middle

## Computational problems are

- problems that can be solved _step-by-step_ by a computer

- have _well-defined_ inputs, outputs, and constraints

---
class: middle,center

# Is 1933091 prime?

---
class: middle,center

# Is there a route from a Point A to Point B?

---
class: middle,center

# Find a route from Point A to Point B.

(search problem)

---
class: middle,center

## How many different routes are there from Point A to Point B?

(counting problem)
---
class: middle,center

# Find the shortest route from Point A to Point B.

(optimization problem)
---
class: middle,center

# What is the meaning of life?

---
class: middle,center

# Do I look good in this outfit?

---
class: middle

## Computational problem are

- problems that can be solved _step-by-step_ by a computer

- have _well-defined_ inputs, outputs, and constraints

---
class: middle,center

### Find the maximum among these numbers

```
5  9  8
1  3  2
```

---
class: middle,center

### Find the maximum among these numbers

.tiny[
```
12  20  11  20  14  2   24  36  43  16  27  6   11  7 
10  15  38  10  22  7   16  26  32  30  11  9   30  6 
20  6   27  19  26  10  27  6   19  34  5   9   3   22
10  4   13  1   10  22  43  36  39  29  41  12  13  25
17  8   30  31  29  38  2   42  7   45  24  33  9   40
34  29  37  2   26  17  19  34  6   7   34  22  21  41
38  5   15  13  9   1   42  39  5   29  38  4   22  29
41  10  26  32  30  26  16  16  18  22  32  34  14  10
5   17  25  16  19  6   31  16  3   13  8   42  41  0 
13  30  44  1   41  14  5   39  40  38  6   37  38  9 
.

```
]

---
class: middle

## How to Solve It?

- Scan the integers one-by-one

- Keep track of the "max so far"

- When we are done scanning, the "max so far" will be the answer.

---
class: middle

## Problem Formulation

- Input: $L$ = $\langle l\_0,...,l\_{k-1\} \rangle$

- Output: $m$

- Constraint $m \in L$ and $m \ge l_i$ for all $i$.

---
class: middle

## Scan One-By-One

- $l_i$ : The number being scanned

- $i = 0, 1, \ldots k-1$

---
class: middle

## "Maximum So Far"

- $m$ : max so far

- $m = l_0$ initially

---
class: middle

## Keeping track

- Compare $m$ with $l\_i$

- if $m < l\_i$, then update $m$ with the value of $l\_i$.

---
class: middle

## Done scanning?

- When we have scanned $l\_{k-1}$

---
class: middle

## The steps to solve a computational problem form the _algorithm_

---
class: middle

## Flowchart

.fit[![flowchart](../figures/max-flowchart/max-flowchart.001.png)]

---
class:center

![:scale 75%](../figures/max-flowchart/max-flowchart.001.png)

.smaller.center[
$k$ | $i$ | $l\_i$ | $m$ 
:---|:----|--------|-----
    |     |        |  
    |     |        |  
    |     |        |  
    |     |        |    
]

---
## Variables

- $i$, $m$, $k$ are _variables_
- They have corresponding location in the memory and hold a _value_
- You can _read_ a value from a variable
- You can _write_ a value to a variable

---
## Assignment

- We can assign the value of a variable to a constant (e.g., set $i$ to 1)
- or to the value of another variable (e.g., set $m$ to $l_i$).

---
## Comparison

- We can compare the values of two variables
- E.g., $i$ equals $k$?  $l\_i > m$?

---
## Arithmetic Operations

- We can perform arithmetic operations (`+`, `-`, etc)
- E.g., we set $i$ to $i + 1$

---
class: middle

### Does this algorithm always correctly find the maximum value?

.fit[![flowchart](../figures/max-flowchart/max-flowchart.001.png)]

---
class: middle

## Correctness

- If an algorithm produces the correct output for one instance of input, it is not necessary correct

- To be correct, it has to produce the correct output _for all possible_ inputs!

---
class: middle

## How to check for correctness?

- Carefully choose a set of test cases to try
- Argue formally about the properties of the algorithms
---
class: middle

## Homework

- Post-Lecture Diagnostic Quiz (1%)
  - Due Thursday 10 AM of Week 3
  - You can have three attempts
- Problem Set 1.1 to 1.3 
  - Not graded.  To be discussed Week 3.