Lesson 06-control and loop

한국어(Korean) Page

Generally, every language has control statements like if and switch, and loops like for and while.

Rust also has these commands, which we will briefly explore.

The characteristics of Rust’s control statements and loops are that the conditions are not included in (), and the for statement uses for..in...

Since these are basic commands, we will focus on examples.

if

if number < 5 {
} else if {
} else {
}

• You cannot use an int type as a bool type.

let number = 1;
if number {
} else {
}
// error

You should write it in the following way:

fn main() {
    let number = 3;

    if number != 0 {
        println!("number was something other than zero");
    }
}

• You can put the result into a variable.

let condition = if number > 0 {
    5
} else {
    6
};

• At this time, the values returned from if, else if, and else must be of the same type.

let condition = if number < 0 {
    5
} else if number == 0 {
    false
} else {
    "Positive"
};
// Each are int, bool, and string types, so it doesn't work.

loop: Unlimited loop without condition

loop {
}

break

• You can put a formula into break and use it as a return value.

let mut counter = 0;
let result = loop {
    counter += 1;

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

while: loop with condition

let mut index = 0;
while index < 5 {
  println!("the value is: {}", a[index]);

  index += 1;
}

for

Only for...in from javascript is possible. If you specify a range after in, it can be stored in a variable declared before in and used in the for statement.

let a = [10, 20, 30, 40, 50];
for element in a.iter() {
    println!("the value is: {}", element);
}

const MAX = 10;
for number in (1..MAX) {
  println!("{}!", number);
}

for number in (1..4).rev() {
  println!("{}!", number);
}

i32 in fn five() -> i32 means the return type.

Attribute

• Conditional Execution

If you use #[cfg(condition)], the function will only be executed when the condition is allowed. The example below is code that is executed only when it is windows, and an error occurs otherwise.

fn main() {
    on_windows();
}

#[cfg(target_os = "windows")]
fn on_windows() {
    println!("This OS is windows.")
}

// Result - When the OS is windows,
// This OS is windows.

// Result - When the OS is linux,
//error[E0425]: cannot find function `on_windows` in this scope
// --> src/main.rs:2:5
//   |
// 2 |     on_windows();
//   |     ^^^^^^^^^^ not found in this scope

Testing

It is used as #[test] and tests whether the function can be performed. You can test with rustc or cargo as follows.

rustc --test [file name]

Or

cargo test

We will cover the #[...] syntax in more detail later.