How to access rust variables outside the scope? - scope

Question

My code look like this
fn main() {
// some other codes goes here
let int = 1;
if int == 1 {
let x = "yes";
} else {
let x = "no";
}
if x == "yes" {
// some other codes goes here
println!("yes");
} else if x == "no" {
// some other codes goes here
println!("no")
}
}
When I run this I get this
error[E0425]: cannot find value `x` in this scope
--> src/main.rs:9:8
|
9 | if x == "yes" {
| ^ not found in this scope
error[E0425]: cannot find value `x` in this scope
--> src/main.rs:12:15
|
12 | } else if x == "no" {
| ^ not found in this scope
While searching for a solution I came across this post How do I access a variable outside of an `if let` expression? but could not able to understand the reason or its solution?

Answer 1

Easiest is by far to code it such that it is in scope in the first place.
You can assign variable with the result of a statement in a single assignment.
If you can make it as a one-liner, it also makes it arguably more readable.
If the actual processing is too long, nothing prevents you from just... making it a proper function.
let x = if int == 1 { "yes" } else { "no" };
// rest of the code accessing x.
Alternatively, the compiler will let you declare unassigned variables if you properly assign them later, with all the compile-time safety checks in place. Read the docs on RAII (Resource Acquisition Is Initialization) RAII Docs to see more how it works. In practice, it's just as simple as this:
let x;
if i == 1 {
x = "yes";
}
else {
x = "no";
}
// keep doing what you love
The compiler will error if there's a logic path where x doesn't get initialized, or if it gets initialized as a different type.
Notice you also do not need to declare it as mut, as the first value it gets will stay immutable. Unless you do want to change it afterwards, obviously.

Answer 2

You can't access a variable which is out of scope. But you use a workaround and set the variable in the same scope.
fn main(){
let int = 1;
let x = if int == 1 {
"yes"
} else {
"no"
};
if x == "yes" {
println!("yes");
} else if x == "no" {
println!("no");
}
}

Answer 3

In Rust each variable has a scope that starts where the variable in initialized. In you problem you try to use the variable x which is created inside of the if int == 1 and the if x == "yes", since if statements have a separate scope from the function main you cannot create a variable inside of your if statement and expect it not to be cleared when you leave scope. The simplest solution is to initialize the variable x where you want to have it used in if x == "yes", so let's say that we want the scope of x to start in main by putting let x; in main. In Rust you may have variable from the larger scope be visible to the scopes that are within that larger scope where the variable in initialized, so assigning the variable from the scope of an if statement is perfectly valid.
Please take a look at https://doc.rust-lang.org/rust-by-example/variable_bindings/scope.html for more information.
fn main() {
let x;
// some other codes goes here
let int = 1;
if int == 1 {
x = "yes";
} else {
x = "no";
}
if x == "yes" {
// some other codes goes here
println!("yes");
} else if x == "no" {
// some other codes goes here
println!("no")
}
}
But you could get rid of the two if statements and just use match:
fn main() {
let myint = 1;
match myint {
1 => {println!("yes")}
_ => {println!("no")}
}
}

Answer 4

The question
I believe you are asking what does this error mean?
To answer that, one must first answer, what is scope?
The answer
Scope, in lay terms, is the section of code where a variable exists.
So when the error says not found in this scope, it means the variable does not exist here.
An example
fn main() {
let a_bool = true;
let main_scope_x = 0;
if a_bool == true {
let if_scope_x = 1;
} // if_scope_x stops existing here!
println!("main x has the value {}", main_scope_x);
println!("if x has the value {}", if_scope_x); // this will cause an error, if_scope_x does not exist outside the if expression.
}
Further info
https://doc.rust-lang.org/stable/book/ch04-01-what-is-ownership.html
(Read the book! It's very good!)