7. Collections

Vectors

fn main() {
    let a = [1, 2, 3, 4, 5];
    let v: Vec<i32> = Vec::new();
}

vector can grow or shrink in size

fn main() {
    let a = [1, 2, 3, 4, 5];
    let mut v: Vec<i32> = Vec::new();
    v.push(6);
    v.push(7);
    v.push(8);
 
    let v2 = vec![1, 2, 3, 4, 5];
}

vec! macro creates a new vector with initial values of the inffered type

use std::vec;
 
fn main() {
    let a = [1, 2, 3, 4, 5];
    let mut v: Vec<i32> = Vec::new();
    v.push(6);
    v.push(7);
    v.push(8);
    {
        let v2 = vec![1, 2, 3, 4, 5];
    }
}

Refferecing elements in a vector

fn main() {
        let v2 = vec![1, 2, 3, 4, 5];
        let third: &i32 = &v2[2];
        println!("The third element is {}", third);
}

fn main () {
    let v2 = vec![1, 2, 3, 4, 5];
    let third: &i32 = &v2[20];
    println!("The third element is {}", third);
}

⛔

Runtime Error:
thread main panicked at 'index out of bounds: the len is 5 but the index is 20'

vector are stored in the heap, so size of a vector is not known at compile time
array are stored on the stack, so size of an array is known at compile time

fn main () {
    let v2 = vec![1, 2, 3, 4, 5];
    let elem: Option<&i32> = v2.get(2);
 
    match elem {
        Some(value) => println!("The element is {}", value),
        None => println!("There is no element"),
    }
}

get method returns Option<&T>, which is Some(&element) or None

fn main() {
    let mut v = vec![1, 2, 3, 4, 5];
 
    let elem = &v[2];
    v.push(6);
 
    println!("The third element is {}", elem);
}

⛔

Error:
cannot borrow v as mutable because it is also borrowed as immutable

when we use an immutable refference to a vector, we expect to get the orginal vector back, which means that the orginal vector has to be stored somewhere else in memory
which cannot be done because refferenced to the modified vector

Iterating over the values in a vector

fn main() {
    let v = vec![1, 2, 3, 4, 5];
    for i in &v {
        println!("{}", i);
    }
}

&v creates an immutable refference to the vector

fn main() {
    let mut v = vec![1, 2, 3, 4, 5];
    for i in &mut v {
        *i += 50;
    }
 
    for i in &v {
        print!("{} ", i);
    }
}

&mut v creates a mutable refference to the vector
*i dereferences i to get the value in the vector

$ cargo run
 
51 52 53 54 55

Storing Enums in a Vector

fn main() {
    enum SpreadsheetCell {
        Int(i32),
        Float(f64),
        Text(String),
    }
 
    let row = vec![
        SpreadsheetCell::Int(3),
        SpreadsheetCell::Float(10.12),
        SpreadsheetCell::Text(String::from("blue")),
    ];
 
    match &row[0] {
        SpreadsheetCell::Int(value) => println!("The value is {}", value),
        _ => println!("Not an integer"),
    }
}

Strings

Strings are stored as a collection of UTF-8 encoded bytes
ASCII characters are encoded in 1 byte, while other characters are encoded in 2 bytes
Unicode scalar values are stored in 4 bytes
UTF-8 encoding allows for a string to be valid UTF-8, but not valid Unicode, size of a string is not known at compile time

fn main() {
    let hello = String::from("السلام عليكم");
    let hello = String::from("Dobrý den");
    let hello = String::from("Hello");
    let hello = String::from("שלום");
    let hello = String::from("नमस्ते");
    let hello = String::from("こんにちは");
    let hello = String::from("안녕하세요");
    let hello = String::from("你好");
    let hello = String::from("Olá");
    let hello = String::from("Здравствуйте");
    let hello = String::from("Hola");
}

Appending to a String

fn main() {
    let mut s = String::from("Hello");
    s.push_str(", World");
    s.push('!');
    println!("{}", s);
}

push_str appends a string slice to a string
push appends a single character to a string

Concatenation with the `+` Operator

fn main() {
    let s1 = String::from("Hello");
    let s2 = String::from(", World");
    let s3 = s1 + &s2;
    println!("{}", s3);
}

+ operator uses the add method, which takes ownership of s1 and borrows s2
fn add(self, s: &str) -> String, takes in an refference to a string slice &str and returns a new string
s3 is the result of the concatenation, so it takes ownership of s1 and s1 is no longer valid

Formatting Strings

fn main() {
    let s1 = String::from("tic");
    let s2 = String::from("tac");
    let s3 = String::from("toe");
 
    let s = format!("{}-{}-{}", s1, s2, s3);
    println!("{}", s);
}

format! macro creates a string without taking ownership of any of its parameters

Indexing into Strings

fn main() {
    let s1 = String::from("hello");
    let h = s1[0];
}

⛔

| let h = s1[0];
| ------------^ string indices are ranges of usize
|
= help: the trait SliceIndex<str> is not implemented for {integer}, which is required by String: Index<_>

use unicode_segmentation::UnicodeSegmentation;
 
fn main() {
    let s1 = String::from("नमस्ते");
 
    for b in s1.bytes() {
        print!("{} ", b);
    }
 
    for c in s1.chars() {
        print!("{} ", c);
    }
 
    for g in s1.graphemes(true) {
        print!("{} ", g);
    }
}

String is a wrapper over a Vec<u8>, so indexing into a string would return a byte, which is not a character

Bytes

bytes() method returns an iterator over the bytes of a string

[224, 164, 168, 224, 164, 174, 224, 164, 184, 224, 165, 141, 224, 164, 164, 224, 165, 135]

Scalar Values

chars() method returns an iterator over the scalar values of a string

['न', 'म', 'स', '्', 'त', 'े']

Grapheme Clusters

graphemes() in 📦 unicode-segmentation crate method returns an iterator over the grapheme clusters of a string

["न", "म", "स्", "ते"]

⚠️

Using &str[0] is not a good idea because it may not return the expected character

Hash Maps

A hash map stores a collection of key-value pairs

use std::collections::HashMap;
 
fn main() {
    let mut scores = HashMap::new();
 
    scores.insert(String::from("Blue"), 10);
    scores.insert(String::from("Yellow"), 50);
 
    let team_name = String::from("Blue");
    let score = scores.get(&team_name);
 
    match score {
        Some(value) => println!("The score is {}", value),
        None => println!("There is no score"),
    }
}

HashMap::new() creates a new hash map
insert() method inserts a key-value pair into the hash map
get() method returns an Option<&V> where V is the value type

use std::collections::HashMap;
 
fn main() {
    let mut scores = HashMap::new();
 
    scores.insert(String::from("Blue"), 10);
    scores.insert(String::from("Blue"), 50);
    //The score is 50
 
    scores.entry(String::from("Yellow")).or_insert(10);
    scores.entry(String::from("Yellow")).or_insert(50);
    // The score is 10
 
}

entry() method returns an Entry enum
insert() method inserts a value, overwrites the existing value
or_insert() method inserts a value if the key does not exist

use std::collections::HashMap;
fn main() {
    let text = "hello world wonderful world";
 
    let mut map = HashMap::new();
 
    for word in text.split_whitespace() {
        let count = map.entry(word).or_insert(0);
        *count += 1;
    }
 
    println!("{:?}", map);
}

or_insert() method returns a mutable reference to the value &mut V
*count += 1 dereferences count to get the value in the hash map

6. Modules 8. Error Handling