CS2030S AY24/25 S2 Lecture 1

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## CS2030s

### Programming Methodology II
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## Lecture 1

### 13 January 2025
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## Academic Staff

Boyd Anderson 
Ashish Dandekar 
Ooi Wei Tsang

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## Recitation Instructors

Enzio Kam 
Tan Yugin 
BrianCheong 
Ooi Wei Tsang 
Ashish Dandekar 
Boyd Anderson

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## A big teaching team!

65 UG Tutors!

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## Course Administrative Manager

Enzio Kam

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## Administrative Matters

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CS2030S Canvas $\blacktriangleright$

[Course Information and Policies](https://canvas.nus.edu.sg/courses/69902/pages/important-course-information-and-policies)

[Course Schedule](https://canvas.nus.edu.sg/courses/69902/pages/course-schedule)

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## Course Resources

Our [course handbook and notes:](https://nus-cs2030s.github.io/2425-s2/)

[Ed Discussion forum](https://edstem.org/us/courses/72783/discussion) for discussing course concepts.

[The CS2030S Ticketing System™](https://mysoc.nus.edu.sg/app/cs2030s)

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## If you must email:

`cs2030s@comp.nus.edu.sg`

Prefix your email subject with [CS2030s]

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### Recitations and Lab Sessions

- Start in Week 2
- Face-to-face lessons
- Registration starts this week: manage via CourseReg

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### Lab and Recitation Registration

First, Don't Panic!

Everyone will get a lab and a recitation.

Academic staff are not in charge of this process so use CourseReg.

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### Vim and Unix Boot Camp

- Saturday, 10 to 12 on Zoom.
- Link on Canvas
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## Course Information

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.smaller[*Note: The 1% for PE0 is awarded on the condition that students do not have full marks for PE1 + PE2.]
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### Labs Exercises

The most important and useful component of CS2030S.

We have many programming questions. Practice and learn.

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# Plagiarism Policy

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# Use of AI

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### Important Dates

.smaller[
Assessment         | Date                       | Time
:------------------|:-------------------------- |:-------------
Midterm | Mon, 10 Mar | 12 - 2 PM
Practical Exam 1   | Week 8 | Time: TBC
Practical Exam 2   | Week 12 | Time: TBC
Final | Wed, 30 Apr | 1 - 3 PM
]

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### "Catch-Up" Sessions

- About every three or so weeks
- Saturday 10 AM - 12 PM, online
- Q&A format
- No new material

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### Not "bell-curved"

- Your grade is based solely on your marks

- Not relative to others in the class.

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### Take care of yourself

Mental wellness is important.

If you can't cope or is falling behind, talk to us.

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## Why CS2030s?

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Real-world software is complex, evolves continuously, and is a product of team work.

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CS2030S teaches you how to write better code, where "better" means:

- more human-friendly
- easier to change and maintain
- fewer crashes

through programming paradigms, idioms, and language design.

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CS2030S is not a course about

- algorithms or efficiency
- software design 
- Java language and libraries

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## Running Our Programs

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The compiler is our friend

- it helps us catch errors early
- but can only be based on the code
- not by running it

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### A well-designed programming language should help us catch errors during compilation phase as soon as possible.

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### Variable: Abstraction over Data

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### Recall Typed Source from CS1101S

```javascript
let x : number = 1;
```

```javascript
function f(x : number): number { 
	return x;
}
```

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### The _type_ of a variable decides the computation that can be performed on a variable.

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### _Type system_: A set of rules about types of variables, expression, functions, and their interactions.

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### Static vs. Dynamic Types

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### Dynamically Typed

```javascript
let x = 4;
x = "5"; // Ok
```

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### Statically Typed

```Java
int x;
x = 4;
x = "5"; // Error
```

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### When a dynamically type language is annoying:

```Python
i = 0
while i < 100:
 i = something_slow()
print("i is " + i)
```

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### Java is a _statically typed_ language.

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### The type assigned to variable during declaration is called the _compile-time type_.

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## Strong vs. Weak Typing

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### in C (weak typing)

```C
int x;
x = 4;
x = (int)"5"; // okay, fine..
```

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### in Java (strong typing)

```C
int x;
x = 4;
x = (int)"5"; // error
```

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### Primitive Types in Java

- `byte`, `short`,  `int`, `long` 
- `float`, `double`
- `char`
- `boolean`

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### Primitive types in Java do not share their values

```Java
int i = 10;
int j = i;
i = i + 1;
```

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### Subtype and Supertype

A type $T$ is a _subtype_ of $S$ ($T <: S$) if:

- a piece of code written for variables of type $S$ can also safely be used on variables of type $T$.

$S$ is a _supertype_ of $T$.

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.small[
| Data Type | Size (in bits) | Minimum Value                  | Maximum Value                  |
|-----------|----------------|--------------------------------|--------------------------------|
| `byte`    | 8 bits         | -128                           | 127                            |
| `long`    | 64 bits        | -9,223,372,036,854,775,808     | 9,223,372,036,854,775,807      |
]
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### `byte` $<:$ `long`

```Java
byte b = 10;
long l = 9_223_372_036_854_775_807L;
b = l; // type error!
l = b; // OK!
```

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### Subtyping Among Java Primitive Types

- `byte` $<:$ `short` $<:$ `int` $<:$ `long`
- `long` $<:$ `float` $<:$ `double`
- `char` $<:$ `int`

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### Subtyping is Reflexive and Transitive

- $T <: T$
- if $S <: T$ and $T <: U$, then $S <: U$.

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### Java Widening Type Conversion

Java allows putting a value of type $T$ into a variable of type $S$ iff $T <: S$.

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### Function: Abstraction over Computation

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.small[
```javascript
let i = 10;
let fac10 = 1;
while (i >= 0) {
  fac10 = fac10 * i;
  i = i - 1;
}
i = 5;
let fac5 = 1;
while (i >= 0) {
  fac5 = fac5 * i;
  i = i - 1;
}
```
]

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.small[
```javascript
function factorial(i) {
	let fac = 1;
	while (i >= 0) {
        fac = fac * i;
        i = i - 1;
	}
	return fac;
}
const fac10 = factorial(10);
const fac5 = factorial(5);
```
]

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### Function as an abstraction barrier

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### Abstraction: Composite Data Type

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## What defines a circle?

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```javascript
function circle(x, y, r) {
    function dispatch(q) {
        if (q === "x") {
            return x;
        } else if (q === "y") {
            return y;
        } else if (q === "r") {
            return r;
        }
    }
    return dispatch;
}
```
]

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```javascript
const c = circle(1, 1, 5);
c("x"); // returns 1
c("y"); // returns 1
c("r"); // returns 5
const area = circle_get_area(c);
```

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```javascript
function circle_get_area(c) {
    return math_PI * c("r") * c("r");
}
```

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```C
typedef struct {
	double x;
	double y;
	double r;
} circle;
```

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```C
circle c;
c.x = 1.0;
c.y = 1.0;
c.r = 5.0;
double area = circle_get_area(c);
```

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```C
double circle_get_area(circle c) {
	return 3.1415926 * c.r * c.r;
}
```

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### Encapsulation: bundle related variables and functions together to create a _class_

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In Java, the keyword `class` allows us to define a new class named `Circle`.

```Java
class Circle {
    double x;
    double y;
    double r;
}
```

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```Java
class Circle {
    double x;
    double y;
    double r;
    
    double getArea() {
      return 3.1415926 * r * r;
    }
}
```

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In Java classes:

- Functions are known as _methods_
 
 - The bundled data in the class are known as _fields_ (or members, or states, or attributes)

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Objects are instances of a class.

In Java, the keyword `new` creates (or _instantiates_) an object of a given class.
```Java
Circle c = new Circle();
```

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Unlike before, we can now call the method `getArea` on the `Circle` object rather than passing it to the method.
```Java
Circle c = new Circle();
c.r = 10;
double area = c.getArea();
```

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# Object-Oriented Programming (OOP)

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A program runs by instantiating objects of different classes.

These objects interact with each other by calling each others' methods.

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### An Example: Modelling a Bank

Classes: Nouns?

Fields: Attributes of our Nouns?

Methods: Verbs in our system?

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### An Example: Modelling a Bank

Classes: `BankAccount`, `Customer`, `Teller`, ...

Fields: `balance`, `creditRating`, `accountNumber`, ...

Methods: `withdraw`, `deposit`, `takeOutLoan`, ...

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### Java Reference Types

Everything else that is not a primitive type. Our new `Circle` class is an example.

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A reference variable stores only the reference to the value not the value itself.
```Java
Circle c1 = new Circle();
Circle c2 = c1;
System.out.println(c2.r); // print 0
c1.r = 10.0;
System.out.println(c2.r); // print 10.0
```

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What about sub/supertype relationships among Java Reference Types?

How do Reference types fit into the Type System?

We will get there don't worry!

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What happens if we don't initialise our reference variable?

```Java
Circle c1;
c1.r = 10.0;  // NullPointerException
```

All non-initialised variables will have the special reference value `null`.

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## Reminders:

- Submit your Github username
- Join the Ed forum
- Read Units 0 to 4

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## Reminders:

- Look out for emails about the Unix and Vim clinics
- Read the notes on PE environment, Unix, Vim, and GitHub setup
- Take the end of lecture _Diagnostic Quiz_

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Version: v1.0

Last Updated: Sun Jan 12 15:21:33 +08 2025
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