Variable shadowing in Rust - Step-by-Step Execution

Concept Flow - Variable shadowing

Declare variable x = 5

↓

Shadow x with new x = x + 1

↓

Shadow x again with new x = x * 2

↓

Use x (final shadowed value)

↓

End of scope

Variable shadowing means reusing the same name for a new variable that hides the old one temporarily.

Execution Sample

Rust

fn main() {
    let x = 5;
    let x = x + 1;
    let x = x * 2;
    println!("{}", x);
}

This code shows how the variable x is shadowed twice, changing its value each time.

Execution Table

Step	Code Line	Variable x Value	Action	Output
1	let x = 5;	5	Declare x with value 5
2	let x = x + 1;	6	Shadow x with 5 + 1
3	let x = x * 2;	12	Shadow x with 6 * 2
4	println!("{}", x);	12	Print final x value	12

💡 Program ends after printing the final shadowed value of x, which is 12.

Variable Tracker

Variable	Start	After Shadow 1	After Shadow 2	Final
x	5	6	12	12

Key Moments - 3 Insights

Why does the value of x change even though we never used a different variable name?

Because each 'let x = ...' creates a new variable that hides the old one temporarily, as shown in execution_table steps 2 and 3.

Is the original x (with value 5) lost after shadowing?

Yes, after shadowing, the old x is hidden and replaced by the new x, but only inside the current scope, as seen in variable_tracker.

Can we use the old x after shadowing?

No, once shadowed, the old x is inaccessible until the scope ends, so the code always uses the latest x value.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of x after the first shadowing (step 2)?

A 5

B 6

C 12

D 1

💡 Hint

Check the 'Variable x Value' column at step 2 in the execution_table.

At which step does the variable x get its final value before printing?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at the 'Variable x Value' column and the 'Action' description in the execution_table.

If we removed the second shadowing (step 3), what would be printed?

A 6

B 5

C 12

D Error

💡 Hint

Refer to the variable_tracker and imagine skipping the last shadowing step.

Concept Snapshot

Variable shadowing in Rust:
- Use 'let' with the same name to create a new variable.
- New variable hides old one temporarily.
- Allows changing value without mut.
- Old variable inaccessible until scope ends.
- Useful for transformations and cleaner code.

Full Transcript

This example shows variable shadowing in Rust. First, x is declared as 5. Then, x is shadowed by a new x equal to 5 plus 1, so 6. Next, x is shadowed again by a new x equal to 6 times 2, so 12. Finally, the program prints 12. Each shadowing creates a new variable with the same name that hides the previous one. This lets us change the value without using mut. The old x values are hidden and cannot be accessed after shadowing. This is useful for step-by-step value changes in a clean way.

Practice

1. What does variable shadowing in Rust allow you to do?

easy

A. Prevent variables from being changed after initialization

B. Create a global variable accessible everywhere

C. Reuse the same variable name to store a new value or type

D. Automatically convert variable types without explicit casting

Solution

1. Step 1: Understand variable shadowing concept

   Variable shadowing means declaring a new variable with the same name as an existing one, which replaces the old variable in the current scope.

2. Step 2: Identify what shadowing allows

   This lets you reuse the same name to update the value or even change the type without needing a new variable name.

3. Final Answer:

   Reuse the same variable name to store a new value or type -> Option C

4. Quick Check:

   Shadowing = reuse variable name [OK]

Hint: Shadowing means reusing variable names for new values [OK]

Common Mistakes:

• Thinking shadowing creates global variables
• Confusing shadowing with immutability
• Assuming shadowing changes variable type automatically

2. Which of the following Rust code snippets correctly demonstrates variable shadowing?

easy

A. let x = 5; let x = x + 1; println!("{}", x);

B. let x = 5; x = x + 1; println!("{}", x);

C. let mut x = 5; x = x + 1; println!("{}", x);

D. let x = 5; mut x = 6; println!("{}", x);

Solution

1. Step 1: Check for correct shadowing syntax

   Shadowing uses let again with the same variable name to create a new variable that hides the old one.

2. Step 2: Analyze each option

   let x = 5; let x = x + 1; println!("{}", x);: let x = 5; let x = x + 1; println!("{}", x); valid shadowing using previous value. let x = 5; x = x + 1; println!("{}", x);: let x = 5; x = x + 1; ... reassignment to immutable, compile error. let mut x = 5; x = x + 1; println!("{}", x);: let mut x = 5; x = x + 1; ... valid mutation but no shadowing (missing second let). let x = 5; mut x = 6; println!("{}", x);: let x = 5; mut x = 6; ... invalid syntax, mut requires let.

3. Final Answer:

   let x = 5; let x = x + 1; println!("{}", x); -> Option A

4. Quick Check:

   Shadowing requires let again [OK]

Hint: Shadowing always uses 'let' again with the same name [OK]

Common Mistakes:

• Trying to reassign immutable variables without 'mut'
• Confusing shadowing with mutable reassignment
• Using 'mut' incorrectly with shadowing

3. What will be the output of this Rust code?

let x = 10;
let x = x * 2;
let x = "Value: ".to_string() + &x.to_string();
println!("{}", x);

medium

A. 20

B. Value: x

C. Compilation error

D. Value: 20

Solution

1. Step 1: Follow variable shadowing and type changes

   First, x is 10 (integer). Then shadowed to 20 (integer). Then shadowed again to a String "Value: 20" by concatenating strings.

2. Step 2: Understand the final print output

   The last x is a String "Value: 20", so printing it outputs exactly that.

3. Final Answer:

   Value: 20 -> Option D

4. Quick Check:

   Shadowing can change type; final value is string [OK]

Hint: Shadowing can change variable type and value [OK]

Common Mistakes:

• Assuming variable type cannot change with shadowing
• Expecting numeric output instead of string
• Confusing variable name with literal output

4. Identify the error in this Rust code using variable shadowing:

let x = 5;
let x = x + 1;
let mut x = x * 2;
x = 10;
println!("{}", x);

medium

A. Cannot shadow a variable with 'mut' after immutable shadowing

B. No error; code runs and prints 10

C. Shadowing requires the same type, but types differ here

D. Cannot assign to 'x' because it is not mutable

Solution

1. Step 1: Analyze shadowing and mutability

   First x is immutable 5, then shadowed to 6 (immutable), then shadowed to mutable 12.

2. Step 2: Check assignment and print

   Since the last x is mutable, assigning x = 10; is valid. Printing outputs 10.

3. Final Answer:

   No error; code runs and prints 10 -> Option B

4. Quick Check:

   Shadowing can add mutability; assignment allowed [OK]

Hint: Shadowing can change mutability; last variable controls it [OK]

Common Mistakes:

• Thinking mutability cannot be added by shadowing
• Confusing shadowing with reassignment rules
• Assuming type must stay the same when shadowing

5. Given this Rust code, what is the final value of result?

let x = 3;
let x = x + 2;
{
    let x = x * 2;
    let x = "Number: ".to_string() + &x.to_string();
    println!("{}", x);
}
let result = x + 1;
println!("{}", result);

hard

A. 6

B. 7

C. Number: 10

D. Compilation error due to type mismatch

Solution

1. Step 1: Track shadowing in outer and inner scopes

   Outside the block, x is first 3, then shadowed to 5. Inside the block, x shadows to 10 (5*2), then to a String "Number: 10". This inner shadowing does not affect outer x.

2. Step 2: Calculate final result outside the block

   Outside, x remains 5, so result = x + 1 = 6. The print inside the block shows "Number: 10" but does not change outer x.

3. Final Answer:

   6 -> Option A

4. Quick Check:

   Inner shadowing does not affect outer variable [OK]

Hint: Inner scope shadowing doesn't change outer variable [OK]

Common Mistakes:

• Assuming inner shadowing changes outer variable
• Confusing printed output with variable value
• Expecting type change to persist outside inner scope