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What Are Constant Variables?

  • 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

  • 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

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

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.