2. Variables
About 6 min
2. Variables κ΄λ ¨
Rust By Practice
Rust By Practice
Binding and mutability
1. π A variable can be used only if it has been initialized.
πProblem
// Fix the error below with least amount of modification to the code
fn main() {
let x: i32; // Uninitialized but used, ERROR !
let y: i32; // Uninitialized but also unused, only a Warning !
assert_eq!(x, 5);
println!("Success!");
}
//
//
// Compiling playground v0.0.1 (/playground)
// warning: unused variable: `y`
// --> src/main.rs:5:9
// |
// 5 | let y: i32; // Uninitialized but also unused, only a Warning !
// | ^ help: if this is intentional, prefix it with an underscore: `_y`
// |
// = note: `#[warn(unused_variables)]` on by default
//
// error[E0381]: used binding `x` isn't initialized
// --> src/main.rs:7:5
// |
// 4 | let x: i32; // Uninitialized but used, ERROR !
// | - binding declared here but left uninitialized
// ...
// 7 | assert_eq!(x, 5);
// | ^^^^^^^^^^^^^^^^ `x` used here but it isn't initialized
// |
// = note: this error originates in the macro `assert_eq` (in Nightly builds, run with -Z macro-backtrace for more info)
// help: consider assigning a value
// |
// 4 | let x: i32 = 0; // Uninitialized but used, ERROR !
// | +++
//
// For more information about this error, try `rustc --explain E0381`.
// warning: `playground` (bin "playground") generated 1 warning
// error: could not compile `playground` (bin "playground") due to previous error; 1 warning emitted
Solution
// Fix the error below with least amount of modification to the code
fn main() {
let x: i32 = 5; // uninitialized but using, ERROR !
let y: i32; // uninitialized but also unusing, only warning
println!("{} is equal to 5", x);
}
//
//
// Success!
2. π Use mut to mark a variable as mutable.
πProblem
// Fill the blanks in the code to make it compile
fn main() {
let __ __ = 1;
__ += 2;
assert_eq!(x, 3);
println!("Success!");
}
//
//
// Compiling playground v0.0.1 (/playground)
// error: expected one of `:`, `;`, `=`, `@`, or `|`, found `__`
// --> src/main.rs:4:12
// |
// 4 | let __ __ = 1;
// | ^^ expected one of `:`, `;`, `=`, `@`, or `|`
//
// error: could not compile `playground` (bin "playground") due to previous error
Solution
// Fill the blanks in the code to make it compile
fn main() {
let mut x = 1;
x += 2;
assert_eq!(x, 3);
println!("Success!");
}
//
//
// Success!:
Scope
A scope is the range within the program for which the item is valid.
3. π
πProblem
// Fix the error below with least amount of modification
fn main() {
let x: i32 = 10;
{
let y: i32 = 5;
println!("The value of x is {} and value of y is {}", x, y);
}
println!("The value of x is {} and value of y is {}", x, y);
}
//
//
// Compiling playground v0.0.1 (/playground)
// error[E0425]: cannot find value `y` in this scope
// --> src/main.rs:9:62
// |
// 9 | println!("The value of x is {} and value of y is {}", x, y);
// | ^ help: a local variable with a similar name exists: `x`
//
// For more information about this error, try `rustc --explain E0425`.
// error: could not compile `playground` (bin "playground") due to previous error
Solution
// Fix the error below with least amount of modification
fn main() {
let x: i32 = 10;
let y: i32 = 20;
{
let y: i32 = 5;
println!("The value of x is {} and value of y is {}", x, y);
}
println!("The value of x is {} and value of y is {}", x, y);
}
//
//
// The value of x is 10 and value of y is 5
// The value of x is 10 and value of y is 20
4. ππ
πProblem
// Fix the error with the use of define_x
fn main() {
println!("{}, world", x);
}
fn define_x() {
let x = "hello";
}
// Compiling playground v0.0.1 (/playground)
// error[E0425]: cannot find value `x` in this scope
// --> src/main.rs:4:27
// |
// 4 | println!("{}, world", x);
// | ^ not found in this scope
//
// For more information about this error, try `rustc --explain E0425`.
// error: could not compile `playground` (bin "playground") due to previous error
Solution π
// Fix the error with the use of define_x
fn main() {
let x = define_x();
println!("{}, world", x);
}
fn define_x() -> String {
let x = "hello".to_string();
x
}
//
//
// hello, world
Solution ππ
// Fix the error below with least amount of modification
fn main() {
let x = define_x();
println!("{:?}, world", x);
}
fn define_x() -> &'static str {
let x = "hello";
x
}
//
//
// "hello", world
Shadowing
You can declare a new variable with the same name as a previous variable, here we can say **the first one is shadowed by the second one.
5. ππ
πProblem
// Only modify `assert_eq!` to make the `println!` work(print `42` in terminal)
fn main() {
let x: i32 = 5;
{
let x = 12;
assert_eq!(x, 5);
}
assert_eq!(x, 12);
let x = 42;
println!("{}", x); // Prints "42".
}
//
//
// Compiling playground v0.0.1 (/playground)
// Finished dev [unoptimized + debuginfo] target(s) in 0.55s
// Running `target/debug/playground`
// thread 'main' panicked at 'assertion failed: `(left == right)`
// left: `12`,
// right: `5`', src/main.rs:7:9
// note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
Solution
// Only modify `assert_eq!` to make the `println!` work(print `42` in terminal)
fn main() {
let x: i32 = 5;
{
let x = 12;
assert_eq!(x, 12);
}
assert_eq!(x, 5);
let x = 42;
println!("{}", x); // Prints "42".
}
//
//
// 42
6. ππ
πProblem
// Remove a line in the code to make it compile
fn main() {
let mut x: i32 = 1;
x = 7;
// Shadowing and re-binding
let x = x;
x += 3;
let y = 4;
// Shadowing
let y = "I can also be bound to text!";
println!("Success!");
}
//
//
// Compiling playground v0.0.1 (/playground)
// warning: value assigned to `x` is never read
// --> src/main.rs:4:13
// |
// 4 | let mut x: i32 = 1;
// | ^
// |
// = help: maybe it is overwritten before being read?
// = note: `#[warn(unused_assignments)]` on by default
//
// warning: variable `x` is assigned to, but never used
// --> src/main.rs:7:9
// |
// 7 | let x = x;
// | ^
// |
// = note: consider using `_x` instead
// = note: `#[warn(unused_variables)]` on by default
//
// warning: value assigned to `x` is never read
// --> src/main.rs:8:5
// |
// 8 | x += 3;
// | ^
// |
// = help: maybe it is overwritten before being read?
//
// warning: unused variable: `y`
// --> src/main.rs:11:9
// |
// 11 | let y = 4;
// | ^ help: if this is intentional, prefix it with an underscore: `_y`
//
// warning: unused variable: `y`
// --> src/main.rs:13:9
// |
// 13 | let y = "I can also be bound to text!";
// | ^ help: if this is intentional, prefix it with an underscore: `_y`
//
// error[E0384]: cannot assign twice to immutable variable `x`
// --> src/main.rs:8:5
// |
// 7 | let x = x;
// | -
// | |
// | first assignment to `x`
// | help: consider making this binding mutable: `mut x`
// 8 | x += 3;
// | ^^^^^^ cannot assign twice to immutable variable
//
// For more information about this error, try `rustc --explain E0384`.
// warning: `playground` (bin "playground") generated 5 warnings
// error: could not compile `playground` (bin "playground") due to previous error; 5 warnings emitted
Solution
// Remove a line in the code to make it compile
fn main() {
let mut x: i32 = 1;
x = 7;
// Shadowing and re-binding
let x = x;
let y = 4;
// Shadowing
let y = "I can also be bound to text!";
println!("Success!");
}
//
//
// Success!
Unused variables
7. Fix the warning below with :
- π Only one solution
- ππ Two distinct solutions
Note
none of the solutions is to remove the line let x = 1
πProblem
fn main() {
let x = 1;
}
// Warning: unused variable: `x`
//
// No output
Solution π
fn main() {
let _x = 1;
}
Solution ππ
#[allow(unused_variables)]
fn main() {
let x = 1;
}
Destructuring
8. ππ We can use a pattern with let to destructure a tuple to separate variables.
Tips
you can use Shadowing or Mutability
πProblem
// Fix the error below with least amount of modification
fn main() {
let (x, y) = (1, 2);
x += 2;
assert_eq!(x, 3);
assert_eq!(y, 2);
println!("Success!");
}
//
//
// Compiling playground v0.0.1 (/playground)
// error[E0384]: cannot assign twice to immutable variable `x`
// --> src/main.rs:5:5
// |
// 4 | let (x, y) = (1, 2);
// | -
// | |
// | first assignment to `x`
// | help: consider making this binding mutable: `mut x`
// 5 | x += 2;
// | ^^^^^^ cannot assign twice to immutable variable
//
// For more information about this error, try `rustc --explain E0384`.
// error: could not compile `playground` (bin "playground") due to previous error
Solution π
// Fix the error below with least amount of modification
fn main() {
let (mut x, y) = (1, 2);
x += 2;
assert_eq!(x, 3);
assert_eq!(y, 2);
}
Solution ππ
// Fix the error below with least amount of modification
fn main() {
let (x, y) = (1, 2);
let x = 3;
assert_eq!(x, 3);
assert_eq!(y, 2);
}
Destructuring assignments
Introduced in Rust 1.59: You can now use tuple, slice, and struct patterns as the left-hand side of an assignment.
9. ππ
Note
the feature Destructuring assignments need 1.59 or higher Rust version
πProblem
fn main() {
let (x, y);
(x,..) = (3, 4);
[.., y] = [1, 2];
// Fill the blank to make the code work
assert_eq!([x,y], __);
println!("Success!");
}
//
//
// Compiling playground v0.0.1 (/playground)
// error[E0425]: cannot find value `__` in this scope
// --> src/main.rs:7:23
// |
// 7 | assert_eq!([x,y], __);
// | ^^ not found in this scope
//
// For more information about this error, try `rustc --explain E0425`.
// error: could not compile `playground` (bin "playground") due to previous error
Solution
fn main() {
let (x, y);
(x, ..) = (3, 4);
[.., y] = [1, 2];
// fill the blank to make the code work
assert_eq!([x, y], [3, 2]);
}
Note
You can find the solutions here (sunface/rust-by-practice) (under the solutions path), but only use it when you need it