What Are Constant Variables? link

• Constant variables are ones that are declared constant throughout the program scope, meaning, their value cannot be modified. They can be defined in global and local scope.

Syntax link

• They are declared using the const keyword followed by the name of the variable, colon (:), and then the data type of the variable.

• Naming Convention: By convention, you write a constant variable name in a SCREAMING_SNAKE_CASE, i.e.,
  • All letters should be UPPER case.
  • All words should be separated using an underscore ( _ ).

Example link

The following example defines two const variables:

• ID_1 in global scope
• ID_2 in local scope

    const ID_1: i32 = 4; // define a global constant variable
fn main() {
   const ID_2: u32 = 3; // define a local constant variable
   println!("ID:{}", ID_1); // print the global constant variable
   println!("ID:{}", ID_2); // print the local constant variable
}
  
  

output:

  ID:4
ID:3
  

Difference Between const and let Variables link

There are many differences between const and let variables.

• Declaration

  • Constant variables are declared using the const keyword unlike let variables.

• Scope

  • const variables are declared in global and local scope unlike let variables that are declared only in the local scope.

• Mutability

  • const variable cannot be mutable unlike let which can be made mutable using mut keyword.

• Data Type

  • Unlike let variables, it is mandatory to define the data type of const variables.

• Set Value at Run-time

  • The value of const variable can only be set before running the program whereas the let variable can store the result at runtime.

• Shadowing

  • Unlike let variables, const variables cannot be shadowed.