Help me with the concept of Variable shadowing

[Ebin]

i write Fibonacci numbers series using the rust Variable shadowing:

fn main() {
    let a = 0;
    let b = 1;
    println!("{}",a);
    println!("{}",b);
    loop
    {
        let c = a + b;
        let a =b;
        let b =c;
        println!("{}",c);
    }
}

but the value of variable c is not getting update and it continuously printing 1
can any one explain me why this happen .

[TomP]

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[john01dav]

Inside your loop, you're doing let a = … and let b = … instead of a = … and b = …. When you use let, you're creating a new variable, but this new variable only exists for that particular invocation of the loop. If you remove the let in front of a and b in the loop, your program will work correctly.

[L.F]

Before you

remove the let in front of a and b in the loop

also mark a and b as mut, since variables in Rust are immutable by default.

fn main() {
    let mut a = 0;
    let mut b = 1;
    // --snip--
}

[Ebin]

i did the Fibonacci numbers using mut , i wand to know the reason why the value of c is not get update.

[L.F]

As @john01dav said before, the let a =b; inside your loop creates a new variable a and shadows (i.e., hides) the previous variable a. These two variables are not connected to each other. The inner variable a is dropped at the end of the scope in which it is introduced, and let c = a + b; always looks up the outer variable a.

In other words, your code is equivalent to:

fn main() {
    let a = 0;
    let b = 1;
    println!("{}", a);
    println!("{}", b);
    loop {
        let c = a + b;
        let d = b;
        let e = c;
        println!("{}", c);
        // d and e are dropped here
    }
}