Rust Tutorial : Constants

Constant Naming Convention

the naming convention for Constants are similar to that of variables
all characters in a constant name are usually in uppercase
the let keyword is not used to declare a constant
syntax

const VARIABLE_NAME:dataType = value;

usage

fn main() {
   const USER_LIMIT:i32 = 100;    // Declare a integer constant
   const PI:f32 = 3.14;           //Declare a float constant

   println!("user limit is {}",USER_LIMIT);  //Display value of the constant
   println!("pi value is {}",PI);            //Display value of the constant
}

Constants vs Variables

constants are declared using the const keyword while variables are declared using the let keyword
a variable declaration can optionally have a data type whereas a constant declaration must specify the data type
a variable declared using the let keyword is by default immutable
there is an option to mutate a variable by using the mut keyword
constants are immutable
constants can be set only to a constant expression
not to the result of a function call or any other value that will be computed at runtime
constants can be declared in any scope
includes the global scope

Shadowing of Variables and Constants

Rust allows programmers to declare variables with the same name
the new variable overrides the previous variable
essentially changes the value of an immutable to a new immutable with a different value

fn main() {
   let salary = 100.00;
   let salary = 1.50 ; 
   // reads first salary
   println!("The value of salary is :{}",salary);
}

above code declares two variables by the name salary
the second variable shadows or hides the first variable while displaying output

index