Finch/docs/en/for-beginners.md

# Finch for Beginners

**Never written a program before? This guide is for you.**
No experience needed, and no math beyond what you learned in primary school.
Take it slowly, one section at a time, and type every example yourself: that is how it sticks.

> 🇵🇱 Polska wersja: [dla-poczatkujacych.md](../pl/dla-poczatkujacych.md)

---

## Contents

1. [What is a program?](#1-what-is-a-program)
2. [Getting Finch ready](#2-getting-finch-ready)
3. [Your first program](#3-your-first-program)
4. [Showing things on the screen](#4-showing-things-on-the-screen)
5. [Boxes with names: variables](#5-boxes-with-names-variables)
6. [Kinds of values](#6-kinds-of-values)
7. [Doing math](#7-doing-math)
8. [Making decisions: if](#8-making-decisions-if)
9. [Repeating things: loops](#9-repeating-things-loops)
10. [Your own commands: functions](#10-your-own-commands-functions)
11. [Lists: many values in one box](#11-lists-many-values-in-one-box)
12. [Playing with text](#12-playing-with-text)
13. [Asking the user](#13-asking-the-user)
14. [Your own kinds of values: structs](#14-your-own-kinds-of-values-structs)
15. [Saving to a file](#15-saving-to-a-file)
16. [When something goes wrong](#16-when-something-goes-wrong)
17. [Small projects to try](#17-small-projects-to-try)
18. [Cheat sheet](#18-cheat-sheet)
19. [Words you will hear](#19-words-you-will-hear)

---

## 1. What is a program?

A computer is very fast, but it does not think. It only does exactly what it is told,
one step at a time.

A **program** is a list of those steps, written down. Like a recipe:

> 1. Take two eggs.
> 2. Break them into a bowl.
> 3. Stir.

A **programming language** is the way we write that recipe so the computer understands it.
**Finch** is one of those languages. It was made to be small and simple, so you
can learn it quickly.

The computer reads your program from top to bottom, line by line.

---

## 2. Getting Finch ready

Finch runs on **Linux** computers. If a teacher, parent or friend has already set it up
for you, skip to the next section.

Otherwise, open the **Terminal** program (a window where you type commands) and type
these lines one by one, pressing **Enter** after each:

```sh
git clone https://github.com/Nexoniarz/Finch.git
cd Finch
nix-shell
bootstrap/build.sh
```

If the last line finishes without the word `error`, Finch is ready.
(If your computer does not have `nix-shell`, ask someone to follow the
[guide for technicians](for-technicians.md#2-installing). It explains the other way.)

To check it works, type:

```sh
./build/finch version
```

You should see something like `Finch 2.5.0`.

---

## 3. Your first program

1. Open any text editor (for example *Text Editor*, *Kate*, *gedit* or *VS Code*).
2. Type exactly this:

```c
fn main() {
    print("Hello!")
}
```

3. Save it in the `Finch` folder as **`hello.fch`**. The `.fch` at the end tells everyone it is a Finch program.
4. In the Terminal, type:

```sh
./build/finch run hello.fch
```

You will see:

```
Hello!
```

🎉 **You have just written and run a program.**

### What did we write?

```text
fn main() {            ← "here starts the main part of my program"
    print("Hello!")    ← "show the text Hello! on the screen"
}                      ← "the main part ends here"
```

- `fn main()` is where every Finch program **starts**. Each program has exactly one `main`.
- The curly brackets `{` and `}` hold everything that belongs to `main`, like the two covers of a book.
- `print(...)` shows something on the screen.
- Text always goes inside double quotes: `"Hello!"`.

The spaces at the start of `print` are not required, but they make it easy to see
what is inside the brackets. Always add them. It is a good habit.

---

## 4. Showing things on the screen

`print` can show text and numbers. Put a comma between things, and Finch adds a space for you:

```c
fn main() {
    print("I am", 10, "years old")
    print("2 + 2 =", 2 + 2)
    print()
    print("The line above is empty")
}
```

Shows:

```
I am 10 years old
2 + 2 = 4

The line above is empty
```

Every `print` starts a new line. `print()` with nothing inside makes an empty line.

### Notes for yourself: comments

Anything after `//` is ignored by the computer. Use it to leave notes:

```c
// This program says hi
fn main() {
    print("Hi")   // this shows Hi
}
```

---

## 5. Boxes with names: variables

Imagine a box with a label on it. You can put something in the box and later look
at what is inside by reading the label. In programming that box is called a **variable**.

```c
fn main() {
    age := 10
    print(age)
}
```

`age := 10` means: *"make a new box, label it `age`, and put 10 in it."*

You can change what is in the box later, using just `=`:

```c
fn main() {
    score := 0
    print("Score:", score)

    score = 5
    print("Score:", score)

    score = score + 1
    print("Score:", score)
}
```

```
Score: 0
Score: 5
Score: 6
```

`score = score + 1` reads as: *"take what is in `score`, add 1, and put the result back into `score`."*
There is a shorter way to write the same thing: `score += 1`.

**Remember:**

- `:=` makes a **new** box. You use it only once per name.
- `=` puts a new value into a box **that already exists**.

### Naming your boxes

Names can use the English letters `a`–`z` and `A`–`Z`, digits and `_`, but cannot start with a digit:
`age`, `total_points`, `player2` are fine. `2player` is not.
Letters like `é`, `ü` or `ż` cannot be used in names, only inside text in quotes: `print("Zürich")` is fine.
Pick names that say what is inside: `price` is better than `p`.

---

## 6. Kinds of values

Finch keeps track of what *kind* of thing is in each box. The kind is called a **type**.
These are the ones you will use most:

| Type    | What it holds                | Examples                 |
|---------|------------------------------|--------------------------|
| `int`   | whole numbers                | `0`, `42`, `-7`          |
| `float` | numbers with a decimal point | `3.14`, `0.5`, `-2.0`    |
| `str`   | text                         | `"Hello"`, `"Ola"`       |
| `bool`  | yes or no                    | `true`, `false`          |
| `char`  | a single letter or sign      | `'A'`, `'z'`, `'?'`      |

Finch figures out the type by itself when you use `:=`:

```c
name := "Ola"       // str
age := 10           // int
height := 1.42      // float
likes_cats := true  // bool
```

You can also write the type yourself, in front of the name:

```c
int age = 10
str name = "Ola"
```

Both ways do the same thing. Use the one you like.

**A box keeps its type.** If `age` holds whole numbers, you cannot put text in it later.
Finch will stop and tell you, which helps you catch mistakes early.

---

## 7. Doing math

| Sign | Meaning                    | Example  | Result |
|------|----------------------------|----------|--------|
| `+`  | add                        | `7 + 2`  | `9`    |
| `-`  | subtract                   | `7 - 2`  | `5`    |
| `*`  | multiply                   | `7 * 2`  | `14`   |
| `/`  | divide                     | `7 / 2`  | `3`    |
| `%`  | what is left after dividing | `7 % 2` | `1`    |

**Careful with dividing whole numbers:** `7 / 2` gives `3`, not `3.5`, because both are
whole numbers, so the answer is a whole number too (the part after the dot is dropped).
If you want `3.5`, use a decimal number: `7.0 / 2`.

Multiplication and division happen before adding and subtracting, just like at school.
Use brackets to change the order:

```c
print(2 + 3 * 4)     // 14
print((2 + 3) * 4)   // 20
```

`%` is handy to check if a number is even: an even number divided by 2 leaves nothing,
so `n % 2` is `0`.

You **cannot divide by zero**. If your program tries, it stops and tells you which line did it.

---

## 8. Making decisions: if

Programs can choose what to do. `if` means *"only do this when something is true"*.

```c
fn main() {
    temperature := 25

    if temperature > 20 {
        print("It's warm, wear a t-shirt")
    }
}
```

Add `else` for *"otherwise, do this"*:

```c
if temperature > 20 {
    print("It's warm")
} else {
    print("Take a jacket")
}
```

And `else if` for more choices. Finch checks them from top to bottom and does only
the **first** one that is true:

```c
if temperature > 25 {
    print("Hot!")
} else if temperature > 15 {
    print("Nice")
} else {
    print("Cold")
}
```

### Comparing things

| Sign | Means                       |
|------|-----------------------------|
| `==` | is equal to                 |
| `!=` | is not equal to             |
| `<`  | is smaller than             |
| `>`  | is bigger than              |
| `<=` | is smaller than or equal to |
| `>=` | is bigger than or equal to  |

⚠️ **One `=` puts a value in a box. Two `==` ask "are these equal?"**

You can compare text too: `if name == "Ola" { ... }`.

### Joining questions

- `&&` means **and**: both must be true. `if age > 6 && age < 12 { ... }`
- `||` means **or**: at least one must be true. `if day == "Sat" || day == "Sun" { ... }`
- `!` means **not**: it flips true and false. `if !raining { ... }`

---

## 9. Repeating things: loops

Computers never get bored. A **loop** repeats the same steps many times.

### Counting with `for`

```c
fn main() {
    for i in 1..6 {
        print("Number", i)
    }
}
```

```
Number 1
Number 2
Number 3
Number 4
Number 5
```

`for i in 1..6` means: *"count with `i` from 1, up to but **not including** 6"*.
So `0..10` gives you ten numbers: 0, 1, 2 … 9.

### Repeating while something is true: `while`

```c
fn main() {
    count := 3
    while count > 0 {
        print(count)
        count -= 1
    }
    print("Go!")
}
```

```
3
2
1
Go!
```

⚠️ Make sure the loop can end. If `count` never went down, the loop would run forever.
If that happens, press **Ctrl + C** in the Terminal to stop it.

### Stopping early and skipping

- `break` leaves the loop right away.
- `continue` skips the rest of this round and goes to the next one.

```c
for i in 1..100 {
    if i == 5 {
        break         // stop when we reach 5
    }
    print(i)          // prints 1 2 3 4
}
```

---

## 10. Your own commands: functions

When you do the same thing in many places, give it a name. That named block is a **function**.
You already know one: `main`.

```c
fn say_hello(str name) {
    print("Hello,", name)
}

fn main() {
    say_hello("Ola")
    say_hello("Tom")
}
```

```
Hello, Ola
Hello, Tom
```

`str name` in brackets means: *"this function needs one piece of text, and inside it
we call that text `name`."* The thing you put in brackets when using it is passed in.

### Functions that give back an answer

Write `->` and the type of the answer, and use `return` to give it back:

```c
fn add(int a, int b) -> int {
    return a + b
}

fn main() {
    result := add(2, 3)
    print(result)          // 5
}
```

It does not matter where in the file you put your functions, above or below `main`.

---

## 11. Lists: many values in one box

Sometimes one box is not enough. A shopping list has many things on it.
A **list** (programmers also say **array**) keeps many values in order:

```c
fn main() {
    fruits := ["apple", "banana", "cherry"]
    print(fruits)
    print("I have", fruits.len, "fruits")
    print("The first one is", fruits[0])
}
```

```
["apple", "banana", "cherry"]
I have 3 fruits
The first one is apple
```

- Square brackets `[ ]` make a list. Commas separate the values.
- `.len` tells you how many things are in the list.
- `fruits[0]` is the **first** thing. Counting starts at **0**, so `fruits[1]` is the second, and `fruits[2]` the third.

If you ask for a place that doesn't exist, like `fruits[10]`, the program stops and tells you:
`index 10 is out of range (the length is 3)`.

### Adding and taking away

```c
fn main() {
    []int scores          // an empty list of whole numbers
    scores.push(10)       // put 10 at the end
    scores.push(25)
    scores.push(7)
    print(scores)         // [10, 25, 7]

    last := scores.pop()  // take the last one out
    print(last, scores)   // 7 [10, 25]

    scores.sort()
    print(scores)
}
```

### Going through a list

`for ... in` visits every value, one by one:

```c
fn main() {
    total := 0
    for n in [3, 5, 2] {
        total += n
    }
    print("Total:", total)    // Total: 10
}
```

If you also want to know *where* you are in the list, give `for` two names: the first one counts
0, 1, 2…, the second one is the value:

```c
fn main() {
    for i, fruit in ["apple", "banana", "cherry"] {
        print(i + 1, fruit)
    }
}
```

```
1 apple
2 banana
3 cherry
```

Lists can do more: `contains`, `find`, `insert`, `remove`, `sort`, `reverse`, `join`.
The [guide for technicians](for-technicians.md#10-arrays-and-maps) lists them all.

### Looking things up by name: maps

A list finds things by their place: `fruits[0]`. Sometimes you want to find things by a **name**
instead, like looking up a word in a dictionary, or a phone number in a phone book.
That's a **map**: each **key** (the name) has a **value**:

```c
fn main() {
    phone := ["Ola": "555-1234", "Jan": "555-9876"]
    print(phone["Ola"])          // 555-1234

    phone["Ewa"] = "555-0000"    // add a new name
    print(phone.len)             // 3

    if phone.has("Tom") {
        print("Tom is here")
    } else {
        print("no Tom")
    }

    for name, number in phone {  // every name with its number
        print(name, number)
    }
}
```

- `["key": value, ...]` makes a map. Keys are often text, but numbers work too.
- `phone["Ola"]` gets the value for a key. Asking for a key that isn't there stops the program,
  so check first with `phone.has("Tom")`, or use `phone.get("Tom", "unknown")`.
- `for name, number in phone` visits every key with its value, in the order you added them.

Maps are great for **counting**. A new key starts at 0, so `+= 1` just works:

```c
fn main() {
    map[str]int votes            // an empty map: text keys, whole-number values
    for v in ["cat", "dog", "cat", "cat", "fish"] {
        votes[v] += 1
    }
    print(votes)                 // {"cat": 3, "dog": 1, "fish": 1}
}
```

---

## 12. Playing with text

Text can be glued together with `+`:

```c
fn main() {
    name := "Ola"
    greeting := "Hello, " + name + "!"
    print(greeting)
}
```

To glue a number to text, turn the number into text first with `str(...)`:

```c
age := 10
print("Age: " + str(age))
```

Text has handy tools too:

```c
fn main() {
    word := "Finch"
    print(word.len)            // 5 letters
    print(word[0])             // F  (the first letter)
    print(word.upper())        // FINCH
    print(word.lower())        // finch
    print(word.contains("in")) // true
    for letter in word {
        print(letter)          // F, i, n, c, h, one per line
    }
}
```

---

## 13. Asking the user

`input` shows a question and waits until the person types an answer and presses Enter:

```c
fn main() {
    name := input("What's your name? ")
    print("Nice to meet you,", name)
}
```

The answer is always **text**. If you need a number, turn it into one with `int(...)`:

```c
fn main() {
    answer := input("How old are you? ")
    age := int(answer)
    print("Next year you will be", age + 1)
}
```

If the person types something that isn't a number, the program stops with a message
like `can't turn "abc" into int`.

---

## 14. Your own kinds of values: structs

Imagine a card in a library: each card has a title, an author and a year.
A **struct** lets you make your own kind of value with named parts:

```c
struct Book {
    str title
    str author
    int year
}

fn main() {
    b := Book(title: "The Hobbit", author: "Tolkien", year: 1937)
    print(b.title, "by", b.author)
    b.year = 1938
    print(b)
}
```

```
The Hobbit by Tolkien
Book(title: "The Hobbit", author: "Tolkien", year: 1938)
```

- `struct Book { ... }` describes what every book has. Put it **outside** `main`.
- `Book(title: ..., author: ..., year: ...)` makes one book.
- `b.title` reads one part. `b.year = 1938` changes it.

You can give parts a starting value. Then you don't have to fill them in:

```c
struct Player {
    str name
    int lives = 3
}

fn main() {
    p := Player(name: "Ola")
    print(p.lives)     // 3
}
```

And you can keep many of them in a list: `[]Book shelf`, then `shelf.push(b)`.

### Commands for your struct: methods

A struct can have its own commands. Write the struct's name, a dot, and the command's name.
Inside, `self` is the struct the command was used on:

```c
struct Player {
    str name
    int lives = 3
}

fn Player.hit() {
    self.lives -= 1
    print(self.name, "has", self.lives, "lives left")
}

fn Player.is_alive() -> bool {
    return self.lives > 0
}

fn main() {
    p := Player(name: "Ola")
    p.hit()                  // Ola has 2 lives left
    p.hit()                  // Ola has 1 lives left
    print(p.is_alive())      // true
}
```

`p.hit()` changes `p` itself, so after two hits `p.lives` really is 1.

---

## 15. Saving to a file

Programs forget everything when they end. To remember things, save them to a file:

```c
fn main() {
    write_file("note.txt", "Buy milk")
    print(read_file("note.txt"))
}
```

- `write_file(name, text)` saves the text in a file (and replaces what was there).
- `read_file(name)` gives you the file's text back.
- `file_exists(name)` tells you if the file is there (`true` or `false`).

The file appears in the folder where you ran the program.

---

## 16. When something goes wrong

Everyone makes mistakes when programming, even experts, every day.
Finch tries hard to explain what went wrong in plain words. Example:

```c
fn main() {
    age := 10
    print(agee)
}
```

```
hello.fch:3:11: error: there is no variable named 'agee'
    3 |     print(agee)
      |           ^
```

How to read it:

- `hello.fch` is the file,
- `3` is the **line** number,
- `11` is how far into the line (the **column**),
- then the explanation, and the line itself with a `^` pointing at the problem.

Here we simply misspelled `age`.

### Common mistakes

| Finch says | What it means | Fix |
|---|---|---|
| `there is no variable named 'x'` | The name is misspelled, or you never made that box | Check spelling. Create it with `x := ...` |
| `a variable named 'x' already exists here` | You used `:=` twice for the same name | Use `=` the second time |
| `'x' must be int, but this is str` | Wrong kind of value for that box | Put the right kind of value in |
| `put each statement on its own line` | Two commands on one line | Press Enter between them |
| `this '{' is never closed` | A `}` is missing | Add the `}` |
| `a condition must be bool` | `if` needs a yes/no question | Compare something: `if x > 0` |
| `runtime error: division by zero` | You divided by 0 while the program ran | Check the number before dividing |
| `index 5 is out of range (the length is 3)` | You asked a list for a place it doesn't have | Remember counting starts at 0; check `.len` |
| `can't turn "abc" into int` | `int(...)` got text that isn't a number | Check what the person typed |
| `can't use '+' on str and int` | Text plus a number | Use `str(...)` on the number |
| `'f' can fail, so say what happens then` | You used a command that can fail without saying what to do | Add `or ...` (see below) |

### Things that can fail: `or` and `try`

Some things can go wrong *while the program runs*: the person types `abc` where you wanted a number,
or a file isn't there. Instead of stopping the program, you can say what to do then, with **`or`**:

```c
fn main() {
    age := int(input("Your age? ")) or 0        // not a number? use 0
    print("Next year you'll be", age + 1)

    note := read_file("note.txt") or "(no note yet)"
    print(note)
}
```

`or` can also be followed by a block. Inside it, `err` is the message that says what went wrong:

```c
fn main() {
    while true {
        n := int(input("Pick a number: ")) or {
            print("That's not a number, try again")
            continue
        }
        print("You picked", n)
        break
    }
}
```

Your own functions can fail too. Put `!` after the result type, and use `return error("...")`:

```c
fn safe_divide(int a, int b) -> int! {
    if b == 0 {
        return error("can't divide by zero")
    }
    return a / b
}

fn main() {
    print(safe_divide(10, 2) or -1)     // 5
    print(safe_divide(10, 0) or -1)     // -1
}
```

Finch won't let you forget: calling `safe_divide(10, 0)` without `or` is an error that tells you
to handle it. (You can also pass the problem on with `try`; the
[guide for technicians](for-technicians.md#errors-as-values) explains it.)

---

## 17. Small projects to try

Try writing each one yourself first. Look at the solution only if you get stuck.

### Project 1: Times table

Show the 7 times table, from 7 × 1 to 7 × 10.

<details><summary>Solution</summary>

```c
fn main() {
    for i in 1..11 {
        print("7 x", i, "=", 7 * i)
    }
}
```
</details>

### Project 2: Even or odd

For every number from 1 to 10, show whether it is even or odd.

<details><summary>Solution</summary>

```c
fn main() {
    for n in 1..11 {
        if n % 2 == 0 {
            print(n, "is even")
        } else {
            print(n, "is odd")
        }
    }
}
```
</details>

### Project 3: Celsius to Fahrenheit

Show a table from 0 °C to 100 °C, every 10 degrees. Formula: `F = C * 9 / 5 + 32`.

<details><summary>Solution</summary>

```c
fn to_fahrenheit(float c) -> float {
    return c * 9 / 5 + 32
}

fn main() {
    for c in 0..11 {
        celsius := float(c * 10)
        print(celsius, "C =", to_fahrenheit(celsius), "F")
    }
}
```

`float(...)` turns a whole number into a decimal number.
</details>

### Project 4: Fizz Buzz

Count from 1 to 20. For numbers divisible by 3 say `Fizz`, by 5 say `Buzz`,
by both say `FizzBuzz`, otherwise say the number.

<details><summary>Solution</summary>

```c
fn main() {
    for n in 1..21 {
        if n % 15 == 0 {
            print("FizzBuzz")
        } else if n % 3 == 0 {
            print("Fizz")
        } else if n % 5 == 0 {
            print("Buzz")
        } else {
            print(n)
        }
    }
}
```
</details>

### Project 5: Biggest of three

Write a function `biggest(int a, int b, int c) -> int` that gives back the largest number.

<details><summary>Solution</summary>

```c
fn biggest(int a, int b, int c) -> int {
    if a >= b && a >= c {
        return a
    }
    if b >= c {
        return b
    }
    return c
}

fn main() {
    print(biggest(4, 9, 2))   // 9
}
```
</details>

### Project 6: Guess the number

The computer picks a secret number. The player guesses until they find it,
and the computer says "higher" or "lower" each time.

<details><summary>Solution</summary>

```c
fn main() {
    secret := 42
    tries := 0
    while true {
        guess := int(input("Your guess: "))
        tries += 1
        if guess < secret {
            print("Higher!")
        } else if guess > secret {
            print("Lower!")
        } else {
            print("Yes! You needed", tries, "tries")
            return
        }
    }
}
```

`return` inside `main` ends the program. (A version with a random number is in `examples/guess.fch`.)
</details>

### Project 7: Class average

Ask for 3 grades, keep them in a list, and show the list and the average.

<details><summary>Solution</summary>

```c
fn main() {
    []int grades
    for i in 0..3 {
        grades.push(int(input("Grade: ")))
    }
    sum := 0
    for g in grades {
        sum += g
    }
    print("Grades:", grades)
    print("Average:", float(sum) / grades.len)
}
```
</details>

---

## 18. Cheat sheet

```c
// a comment

fn main() {                       // the program starts here
    print("text", 42)             // show things

    x := 5                        // new box (type guessed)
    int y = 10                    // new box (type written)
    x = 7                         // change a box
    x += 1                        // add 1 to x

    if x > 5 {                    // decide
        print("big")
    } else if x > 0 {
        print("small")
    } else {
        print("zero or less")
    }

    for i in 0..10 {              // count 0 to 9
        print(i)
    }

    while x > 0 {                 // repeat while true
        x -= 1
    }
}

fn add(int a, int b) -> int {     // your own command
    return a + b
}

struct Pet {                      // your own kind of value
    str name
    int age = 1
}

// lists:  nums := [1, 2, 3]   nums.push(4)   nums[0]   nums.len   for n in nums { }   for i, n in nums { }
// maps:   ages := ["Ola": 9]   ages["Jan"] = 10   ages["Ola"]   ages.has("Tom")   for name, age in ages { }
// text:   "a" + "b"   str(42)   int("42")   s.len   s.upper()   s.contains("x")
// ask:    name := input("Name? ")
// files:  write_file("f.txt", text)   read_file("f.txt")
// methods: fn Pet.grow() { self.age += 1 }    p.grow()
// failing: n := int(text) or 0      x := read_file("f.txt") or { print(err) ... }
```

---

## 19. Words you will hear

| Word | Meaning |
|---|---|
| **program** | A list of steps for the computer |
| **code** | The text of a program |
| **run** | Make the computer do the steps |
| **compile** | Translate your code into something the computer can run. `finch run` does this for you |
| **variable** | A named box holding a value |
| **type** | The kind of value: number, text, yes/no… |
| **function** | A named group of steps you can use again |
| **loop** | Steps that repeat |
| **condition** | A yes/no question, like `x > 5` |
| **list / array** | Many values kept in order, like `[1, 2, 3]` |
| **index** | The position of a value in a list, starting at 0 |
| **struct** | Your own kind of value made of named parts |
| **method** | A command that belongs to a struct, like `p.hit()` |
| **map** | Values you find by a key (a name), like a dictionary |
| **input** | What the person types into the program |
| **error** | A message saying something is wrong, and where |
| **bug** | A mistake in a program |

---

**What next?** When this all feels easy, move on to the
[guide for technicians](for-technicians.md). It covers every part of Finch.