CS1010 AY24/25 S1 Lecture 3

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# Lecture 3

### 26 August 2024

Admin Matters<br>
Unit 5: **First C Program**<br>
Unit 6: **CS1010 I/O Library**<br>
Unit 7: **Arithmetic Operations**<br>

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### Exercises

- Read the [exercise guide](https://nus-cs1010.github.io/2425-s1/guides/exercises.html)

- Read policy about:
    - [late submission](https://nus-cs1010.github.io/2425-s1/about.html#latemissed-submission-policy)
	- [collaboration](https://nus-cs1010.github.io/2425-s1/about.html#collaboration-on-programming-exercises)
	- [use of AI](https://nus-cs1010.github.io/2425-s1/about.html#use-of-ai-tools)

- Read about [how to ask for help](https://nus-cs1010.github.io/2425-s1/about.html#asking-questions-and-getting-help)

---
- Exercises are meant to help you learn.

- Submitting a perfect solution is not the point.

- Do not copy verbatim from AI, others, or the Internet.  You will not learn anything.

- Making mistakes is a critical part of the learning process.

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### Collaboration

- Help each other learn by _teaching each other_ the concepts and thought process behind.

- Copy-pasting of code does _NOT_ help each other learn.

- Give credits to those that help you.

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### Catch-Up Session I

- __Date__: Saturday, 31 August, 2024

- __Time__: 10 AM to 12 PM

- Via [Zoom](https://nus-sg.zoom.us/j/7308691994?omn=86463549431)

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.small[
### Catch-Up Session I

- Optional session

- Q&A format

- Ask via [PigeonHole](https://pigeonhole.at/FUNCTIONSEVERYWHERE) (code: `FUNCTIONSEVERYWHERE`)

- No new materials will be covered

- Will be recorded
]

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.small[
To follow along today's lecture, copy the following three files from the home directory of `cs1010` to your working directory:

- `Makefile`

- `compile_flags.txt`

- `.clang-tidy`

**Note**: Every weekly programming exercise comes with three files with the same name.  Don't mix them up.

**Note**: My `vim` environment is already [set up according to here](https://nus-cs1010.github.io/2425-s1/guides/vim-setup.html).  Yours might behave differently.
]

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### Mathematical functions

- Function declaration: $f : \mathbb{Z} \rightarrow \mathbb{Z}$

- Function definition: $f(x) = x^2$

- Referring to a function: $g(x) = f(x) + x$

- Evaluating a function: $f(4)$ evaluates to 16

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.w20[![square](figures/lec03/functions-0.png)]

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## Let's code

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.fit[![simplified](../notes/figures/function-anatomy/function-anatomy.001.png)]

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### Compilation

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

.fit[![simplified](../notes/figures/c-compile-run/c-compile-run-pdf-0.png)]

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### The `main` function

- Entry point of a C program

- Always return an `int`

- Returns 0 if the program runs successfully.  Non-zero otherwise.

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.w20[![square](figures/lec03/functions-1.png)]

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.fit[![meme](figures/meme/bad-good-global.jpg)]

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.tiny.center[
type  | min bits | num of bits on PE nodes
-----:|---------:|-----------------------:
`char` | 8 | 8
`short` | 16 | 16
`int` | 16 | 32
`long` | 32 | 64
`long long` | 64 | 64
]
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`int` is not sufficient for CS1010 purposes.

We will only use `long` in CS1010.

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.tiny.center[
type  | num of bits
-----:|:--------
`float` | 32
`double` | 64
]

We will only use `double` in CS1010.

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.fit[![meme](figures/meme/bad-good-int-long.jpg)]

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### Basic operators

`+` `-` `*` `/` `%`

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### Pitfall: Order of operations can be confusing

`8 / 2 * (2 + 2)`

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background-image: url("figures/ny-times-math-eq-stumps-internet.png")

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### CS1010: Use parentheses everywhere

`8 / (2 * (2 + 2))`

`(8 / 2) * (2 + 2)`

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### CS1010: No need to worry about

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.fit[![meme](figures/meme/bad-good-arithmetic.jpg)]

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### Pitfall: Integer Overflow

```C
unsigned char c = 255;
c = c + 1;
```

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### Pitfall: Integer Division

```C
long three = 3;
long two = 2;
double one_half = three/two;
```

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### Pitfall: The % Operator

`x % n` is equivalent to `x - ((x / n) * n)`

- What is `-9 % 4`?

- What is `9 % -4`?

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### Compound Operator

```C
x <op>= y;
```

is equivalent to

```C
x = x <op> y;
```

E.g., `+=`, `-=`, `*=`, `/=`, `%=`, etc.
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### Increment/Decrement Operator

Postfix operator

```C 
x++; 
x--; 
```

Use `x` first, then add/subtract one from `x`.

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### Increment/Decrement Operator

Prefix operator

```C 
++x; 
--x; 
```

Add/subtract one from `x`, use `x`.

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### Increment/Decrement Operator

```C 
long x = 1;
y = x++; 
// y becomes 1, x becomes 2
```

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### Increment/Decrement Operator

```C 
long x = 1;
y = ++x; 
// y becomes 2, x becomes 2
```
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### Undefined Behavior

```C
x = x++;
x = x++ * ++x;
```

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.fit[![meme](figures/meme/bad-good-plusplus.jpg)]

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### Programming cannot be learned by reading

"Play" with code, ask "what if..", "why..", "why not..", etc

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### Homework

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- Problem Set in [Unit 5](https://nus-cs1010.github.io/2425-s1/05-first-c.html#problem-set-5)

- [Diagnostic Quiz for Lecture 3](https://canvas.nus.edu.sg/courses/62224/quizzes/40049)

- [Exercise 0](https://nus-cs1010.github.io/2425-s1/exercises/ex00.html)
]

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.tiny[
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