v0.0.1 chap3.1 variables and mutability

variables/.gitignore

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+/target

variables/Cargo.lock

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+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+version = 4
+
+[[package]]
+name = "variables"
+version = "0.1.0"

variables/Cargo.toml

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+[package]
+name = "variables"
+version = "0.1.0"
+edition = "2024"
+
+[dependencies]

variables/src/main.rs

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+const THREE_DAYS_IN_SECONDS : u32 = 3 * 24 * 60 * 60;
+
+fn main() {
+    println!("
+1/ A constant can be defined outside main,
+ and be called from main:
+
+---RUNNING THE CODE");
+    println!("const THREE_DAYS_IN_SECONDS: {THREE_DAYS_IN_SECONDS}
+---END OF EXECUTION");
+
+    println!("
+2/ A variable is mutable only if you use mut. Ex:
+let mut x = 5;
+
+---RUNNING THE CODE");
+
+    let mut x = 5;
+    println!("x (defined by 'let mut') is {x}");
+    x = 6;
+    println!("x is now {x}
+---END OF EXECUTION");
+
+
+    println!("
+3/ Instead of allowing mutability, 
+a variable can be 'shadowed'.
+It means we redefine a new variable 
+on the same namespace. Ex:
+let shadow = 7
+let shadow = shadow + 3
+
+---RUNNING THE CODE");
+    let shadow = 7;
+    println!("shadow is {shadow}");
+    
+    let shadow = shadow +3;
+    println!("shadow is {shadow}");
+
+    {
+        let shadow = shadow + 10;
+        println!("shadow in scope is {shadow}");
+    }
+        println!("shadow back outside scope is {shadow}
+---END OF EXECUTION");
+
+println!("
+4/ shadowing even allows to 
+change the type of the variable!
+
+---RUNNING THE CODE");
+    let secret = "xxxx";
+    println!("secret is {secret}");
+
+    let secret = secret.len();
+    println!("secret - now defined with secret.len() - 
+is now: {secret}
+---END OF EXECUTION");
+}
+