To declare a variable in Rust, use the let
keyword.
let num = 5;
By default, variables are immutable, you can't change them.
If you wish to mutate it, you have to explicitly state that by adding mut
to that declaration.
let mut num = 5;
Now, the number stored in the variable named num
may now be changed.
num = 6;
Another piece of data that's immutable, (so can't change) is a constant.
Declared with the const
keyword.
You might already be familiar with this concept, and wonder:
If variables are immutable by default, what's the difference between variables and constants?
— you (maybe)
You aren't allowed to use
mut
on constants. No mutating, ever.The type of a constant must be declared, whereas the type of a variable may be declared.
Constants can only be set to a constant expression, not to the result of a function call or anything that could only be determined at runtime.
As a result, constants are always fixed in size, and known at compile time.
Another, less obvious result, is that constants may not be of a type that requires allocation on the heap, since they're not known at compile time.
- The naming convention for
const
isSCREAMING_SNAKE_CASE
. - The naming convention for
let
issnake_case
.
const SPEED_OF_LIGHT: u32 = 299792458;
Constants can be declared in any scope, including the global scope.
They are valid for the entire runtime of the program inside that scope.
Translation: You can put constants outside of the main
function and it will work.
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