3章　一般的なプログラミングの概念#

$ cargo new variables

fn main() {
    let x = 5;
    println!("The value of x is: {x}");     // xの値は{x}です
    x = 6;
    println!("The value of x is: {x}");
}

$ cargo run
   Compiling variables v0.1.0 (file:///projects/variables)
error[E0384]: cannot assign twice to immutable variable `x`
 --> src/main.rs:4:5
  |
2 |     let x = 5;
  |         - first assignment to `x`
3 |     println!("The value of x is: {x}");
4 |     x = 6;
  |     ^^^^^ cannot assign twice to immutable variable
  |
help: consider making this binding mutable
  |
2 |     let mut x = 5;
  |         +++

For more information about this error, try `rustc --explain E0384`.
error: could not compile `variables` (bin "variables") due to 1 previous error

fn main() {
    let mut x = 5;
    println!("The value of x is: {x}");
    x = 6;
    println!("The value of x is: {x}");
}

$ cargo run
   Compiling variables v0.1.0 (file:///projects/variables)
    Finished dev [unoptimized + debuginfo] target(s) in 0.30s
     Running `target/debug/variables`
The value of x is: 5
The value of x is: 6

const THREE_HOURS_IN_SECONDS: u32 = 60 * 60 * 3;

fn main() {
    let x = 5;

    let x = x + 1;

    {
        let x = x * 2;
        println!("The value of x in the inner scope is: {x}");
    }

    println!("The value of x is: {x}");
}

$ cargo run
   Compiling variables v0.1.0 (file:///projects/variables)
    Finished dev [unoptimized + debuginfo] target(s) in 0.31s
     Running `target/debug/variables`
The value of x in the inner scope is: 12
The value of x is: 6

$ cargo run
   Compiling variables v0.1.0 (/home/icn/project/variables)
error[E0384]: cannot assign twice to immutable variable `x`
 --> src/main.rs:3:5
  |
2 |     let x = 5;
  |         - first assignment to `x`
3 |     x = x + 1;
  |     ^^^^^^^^^ cannot assign twice to immutable variable
  |
help: consider making this binding mutable
  |
2 |     let mut x = 5;
  |         +++

For more information about this error, try `rustc --explain E0384`.
error: could not compile `variables` (bin "variables") due to 1 previous error

let spaces = "   ";
let spaces = spaces.len();

let mut spaces = "   ";
spaces = spaces.len();

let guess u32 = "42".parse().expect("Not a number!");

fn main() {
    let x = 2.0; // f64

    let y: f32 = 3.0; // f32
}

fn main() {
    // addition
    // 足し算
    let sum = 5 + 10;

    // subtraction
    // 引き算
    let difference = 95.5 - 4.3;

    // multiplication
    // 掛け算
    let product = 4 * 30;

    // division
    // 割り算
    let quotient = 56.7 / 32.2;
    let truncated = -5 / 3; // Results in -1
                            // 結果は-1

    // remainder
    // 余り
    let remainder = 43 % 5;
}

fn main() {
    let t = true;

    let f: bool = false; // with explicit type annotation
                         // 明示的型注釈付きで
}

fn main() {
    let c = 'z';
    let z: char = 'ℤ'; // with explicit type annotation
                       // 明示的型注釈付きで
    let heart_eyed_cat = '😻';    //ハート目の猫
}

fn main() {
    let tup: (i32, f64, u8) = (500, 6.4, 1);
}

fn main() {
    let tup = (500, 6.4, 1);

    let (x, y, z) = tup;

    println!("The value of y is: {y}");
}

fn main() {
    let x: (i32, f64, u8) = (500, 6.4, 1);

    let five_hundred = x.0;

    let six_point_four = x.1;

    let one = x.2;
}

fn main() {
    let a = [1, 2, 3, 4, 5];

    let months = ["January", "February", "March", "April", "May", "June", "July",
              "August", "September", "October", "November", "December"];

    let a: [i32; 5] = [1, 2, 3, 4, 5];

    let a = [3; 5];     // [3,3,3,3,3]

    let first = a[0];   // firstは1
    let second = a[1];  // secondは2
}

fn main() {
    println!("Hello, world!");

    another_function();
}

fn another_function() {
    println!("Another function.");  // 別の関数
}

fn main() {
    another_function(5);
}

fn another_function(x: i32) {
    println!("The value of x is: {x}");   // xの値は{x}です
}

$ cargo run
   Compiling functions v0.1.0 (file:///projects/functions)
    Finished dev [unoptimized + debuginfo] target(s) in 1.21s
     Running `target/debug/functions`
The value of x is: 5

fn main() {
    print_labeled_measurement(5, 'h');
}

fn print_labeled_measurement(value: i32, unit_label: char) {
    println!("The measurement is: {value}{unit_label}");
}

fn main() {
    let y = 6;
}

fn main() {
    let x = (let y = 6);
}

$ cargo run
   Compiling functions v0.1.0 (file:///projects/functions)
error: expected expression, found `let` statement
 --> src/main.rs:2:14
  |
2 |     let x = (let y = 6);
  |              ^^^
  |
  = note: only supported directly in conditions of `if` and `while` expressions

warning: unnecessary parentheses around assigned value
 --> src/main.rs:2:13
  |
2 |     let x = (let y = 6);
  |             ^         ^
  |
  = note: `#[warn(unused_parens)]` on by default
help: remove these parentheses
  |
2 -     let x = (let y = 6);
2 +     let x = let y = 6;
  |

warning: `functions` (bin "functions") generated 1 warning
error: could not compile `functions` (bin "functions") due to 1 previous error; 1 warning emitted

fn main() {
    let y = {
        let x = 3;
        x + 1
    };    // このスコープは4に評価される

    println!("The value of y is: {y}");
}

icn@zenigame:~/project/variables$ cargo run
   Compiling variables v0.1.0 (/home/icn/project/variables)
error[E0277]: `()` doesn't implement `std::fmt::Display`
 --> src/main.rs:7:34
  |
7 |     println!("The value of y is: {y}");
  |                                  ^^^ `()` cannot be formatted with the default formatter
  |
  = help: the trait `std::fmt::Display` is not implemented for `()`
  = note: in format strings you may be able to use `{:?}` (or {:#?} for pretty-print) instead
  = note: this error originates in the macro `$crate::format_args_nl` which comes from the expansion of the macro `println` (in Nightly builds, run with -Z macro-backtrace for more info)

For more information about this error, try `rustc --explain E0277`.
error: could not compile `variables` (bin "variables") due to 1 previous error

fn five() -> i32 {
    5
}

fn main() {
    let x = five();

    println!("The value of x is: {x}");
}

fn main() {
    let number = 3;

    if number < 5 {
        println!("condition was true"); 
    } else {
        println!("condition was false");
    }
}

fn main() {
    let condition = true;
    let number = if condition { 5 } else { 6 };

    println!("The value of number is: {number}");
}

fn main() {
    let mut counter = 0;

    let result = loop {
        counter += 1;

        if counter == 10 {
            break counter * 2;
        }
    };

    println!("The result is {result}");
}

fn main() {
    let mut count = 0;
    'counting_up: loop {
        println!("count = {count}");
        let mut remaining = 10;

        loop {
            println!("remaining = {remaining}");
            if remaining == 9 {
                break;
            }
            if count == 2 {
                break 'counting_up;
            }
            remaining -= 1;
        }

        count += 1;
    }
    println!("End count = {count}");
}

fn main() {
    let mut number = 3;

    while number != 0 {
        println!("{number}!");

        number -= 1;
    }

    // 発射！
    println!("LIFTOFF!!!");
}

fn main() {
    let a = [10, 20, 30, 40, 50];

    for element in a {
        println!("the value is: {element}");
    }
}

fn main() {
    for number in (1..4).rev() {
        println!("{number}!");
    }
    println!("LIFTOFF!!!");
}