Rust Programming Language/

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Option and Enum

Option and Enum

This lesson will teach about a built-in enum called option.

We'll cover the following...

What Is Option?

Option is a built-in enum in the Rust standard library. It has two variants Some and None.

Variants:

  • Some(T), returns Some value T
  • None, returns no value

When to Use Option?

Options is a good choice when:

  • The return value is none

    Rust avoids including nulls in the language, unlike other languages. For instance, the function that returns a value may actually return nothing. So, here the Option variant None comes in handy.

  • The value of the variable is optional

    The value of any variable can be set to some value or set to none.

  • Out of bound exception is to be displayed

    This is useful in the case of an array, string or a vector when an invalid index number tries to access it.

Example 1: Return Value Is None

The following example shows that if the else construct has no value then it can simply return None.

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Rust 1.40.0
fn main() {

   println!("{:?}", learn_lang("Rust"));

   println!("{:?}", learn_lang("Python"));

}

fn learn_lang(my_lang:&str)-> Option<bool> {

   if my_lang == "Rust" {

      Some(true)

   } else {

      None

   }

}

Explanation

  • From line 1 to 4, main function is defined.

    • On line 2, the function learn_lang is invoked by passing “Rust” within the function parameter.
    • On line 3, the function learn_lang is invoked by passing “Python” within the function parameter.

  • From line 5 to 11, learn_lang function is defined. The function learn_lang takes a parameter my_lang and return an Option<bool>.

    • On line 6, an if condition checks if the value of my_lang is equal to Rust then it returns Some(true). Note that the return type of Option is bool so true is passed within the Some.

    • On line 7, else is executed if the if condition evaluates to be false and returns None.

    Note: None does not take a parameter unlike Some.

Example 2: Optional Variable Value

The following example makes level variable of the struct Course as Option of type String. That means that it’s optional to set any value to it. It can be set to some value or it can be set to none.

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Rust 1.40.0
//declare a struct
struct Course {
   code:i32,
   name:String,
   level: Option<String>, 
}
fn main() {
   //initialize
   let course1 = Course  {
      name:String::from("Rust"),
      level:Some(String::from("beginner")),
      code:130
   };
   let course2 = Course  {
      name:String::from("Javascript"),
      level:None,
      code:122
   };
   //access
   println!("Name:{}, Level:{} ,code: {}", course1.name, course1.level.unwrap_or("Level".to_string()), course1.code);
   println!("Name:{}, Level:{} ,code: {}", course2.name, course2.level.unwrap_or("No level defined!".to_string()), course2.code);
}

Explanation

  • Line 2-5, a struct Course has three items code,name,level of type i32,String, and Option<String> respectively.
  • From line 7 to line 22, main function is defined.
    • From line 9 to line 13, a variable course1 instantiates the Course. On line 11, it initializes the level to Some value. Here the value is to set to a String object, i.e., “beginner”. Note the Option has a type String so a value of type String can only be set to it.
    • From line 14 to line 17, a variable course2 instantiates the Course. On line 16, it initializes the level to None value.
    • On line 20, the items of struct instance course1 is printed using the member access operator (.). To print the level item of the course1 instance .unwrap_or() built-in method is used because the level is of type Option, its values are accessed using .unwrap_or() with a string parameter passed to it. In this case, since the level is initialized to Some(String::from("beginner")) , it prints the level. But if it is set to None, it prints the string within the .unwrap_or() method.
    • On line 21, the items of struct instance course2 is printed using the member access operator (.). To print the level item of the course2 instance .unwrap_or() built-in method is used because the level is of type Option, and it is initialized with the value None.

Example 3: Index Out of Bound Exception

The example below uses a match statement that takes an index of string using match.str.chars().nth(index_no) and executes the Some block if index_no is in range and None block otherwise.

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Rust 1.40.0
fn main() {

  // define a variable

  let str = String :: from("Educative");

  // define the index value to be found

  let index = 12;

  lookup(str, index);

}

fn lookup(str: String, index: usize) {

  let matched_index = match str.chars().nth(index){

    // execute if match found print the value at specified index 

     Some(c)=>c.to_string(),

     // execute if value not found

     None=>"No character at given index".to_string()

     };  

  println!("{}", matched_index);

}

Explanation

  • From line 1 to 7, main function is defined.

    • On line 3, a variable str is initialized with value Educative of type String.
    • On line 5, a variable index is initialized with the value 12.
    • On line 6, function lookup is invoked which takes str and index as parameters to the function.

  • From line 8 to line16, function loopup is defined.

    • On line 9, a variable matched_index saves the value of match statement that takes str.chars().nth(index_no) as a condition. Here str and index are the values passed as an argument to the function. .chars().nth(index) finds the character at given index.
    • On line 11, Some(c) checks if the character is found at the given index, then it returns the character. Else,
    • On line 13, None returns a string saying that the character is not found.

is_some(), is_none() Functions

Rust provides is_some() and is_none() to identify the return type of variable of type Option, i.e., whether the value of type Option is set to Some or None.

Example 1

The following example checks whether the variable value of type Option is set to Some or None.

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Rust 1.40.0
fn main() {
    let my_val: Option<&str> = Some("Rust Programming!");
    print(my_val); // invoke the function
   
}
fn print(my_val: Option<&str>){
     if my_val.is_some(){ // check if the value is equal to some value
        println!("my_val is equal to some value");
    }
    else{
        println!("my_val is equal to none");
    }
}

Explanation

  • The main function is defined from line 1 to line 5.

    • On line 2, a variable my_var is declared and its value is set to Some("Rust Programming").
    • On line 3, a function print is invoked which takes the variable my_var as an argument to the function.

  • On line 6-13, function print is defined.

    • The if construct checks if the variable my_var is initialized to some value using the built-in method .is_some(). If it is, it prints that the variable is set to some value.
    • Else, it prints it is set to none.

We need to do is to ensure that these functions return true or false. That’s where assert_eq and assert_ne functions come in handy.

Assert Macros

  • assert_eq!(left, right) - evaluates to true if left value is equal to that of right
  • assert_ne!(left, right) - evaluates to true if left value is not equal to that of right

Output of assert expression?

If the assertion passes no output is displayed, and if doesn’t the code gives an error saying that the assertion failed.

Example 2

The following example uses the assert_eq! macro to check whether the variable value of type Option is set to Some or None.

Note: The assertion passes since the expression evaluates to true.

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Rust 1.40.0
fn main() {
    let my_val: Option<&str> = Some("Rust Programming!");
    // pass since my_val is set to some value so left is true, and right is also true
    assert_eq!(my_val.is_some(), true); 
    // pass since my_val is set to some value so left is false, and right is also false
    assert_eq!(my_val.is_none(), false);
}

Explanation

  • On line 2, declares a variable my_var and sets it to Some("Rust Programming").
  • On line 4, an assert_eq! takes var.is_some() and checks if it’s equal to true. Since my_val is set to some value so left my_val.is_some() is true and right is also set to true. The assertion passes.
  • On line 6, an assert_eq! takes the expression var.is_none() and checks if it’s equal to false.Since my_val is set to some value so left my_val.is_none() is false and right is also set to false. So the assertion passes.

There is often a situation when you have to display messages like “File Not found” and “Ok” on failures and successes respectively. Let’s see how the built-in Result enum can help you do this in the next lesson.

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