Assignment operators in Rust

Introduction

Assignment operators help you store or update values in variables easily.

When you want to save a value in a variable.

When you want to add or subtract a number from a variable's current value.

When you want to multiply or divide a variable by a number and save the result.

When you want to combine updating and assigning a value in one step.

Syntax

Rust

let mut variable = value;
variable = new_value; // simple assignment
variable += value; // add and assign
variable -= value; // subtract and assign
variable *= value; // multiply and assign
variable /= value; // divide and assign
variable %= value; // remainder and assign

Variables must be declared with mut to allow changing their value.

Assignment operators combine an operation and assignment in one step.

Examples

Assign a new value 10 to variable x.

Rust

let mut x = 5;
x = 10;

Add 3 to x and save the result back to x.

Rust

let mut x = 5;
x += 3; // x is now 8

Multiply x by 2 and save the result.

Rust

let mut x = 10;
x *= 2; // x is now 20

Assign the remainder of x divided by 3 back to x.

Rust

let mut x = 10;
x %= 3; // x is now 1

Sample Program

This program shows how to use different assignment operators to update the variable score step by step.

Rust

fn main() {
    let mut score = 10;
    println!("Initial score: {}", score);
    score += 5;
    println!("After adding 5: {}", score);
    score *= 2;
    println!("After doubling: {}", score);
    score -= 4;
    println!("After subtracting 4: {}", score);
    score /= 3;
    println!("After dividing by 3: {}", score);
    score %= 3;
    println!("After remainder by 3: {}", score);
}

Important Notes

Remember to declare variables with mut if you want to change their values.

Assignment operators make code shorter and easier to read when updating variables.

Using assignment operators is like doing math and saving the answer back to the same box.

Summary

Assignment operators store or update values in variables.

Use +=, -=, *=, /=, and %= to combine math and assignment.

Variables must be mutable (mut) to use assignment operators.

Practice

1. What does the Rust assignment operator += do?

easy

A. Subtracts the right value from the left variable and stores the result in the variable

B. Adds the right value to the left variable and stores the result in the variable

C. Multiplies the right value with the left variable and stores the result in the variable

D. Divides the left variable by the right value and stores the result in the variable

Solution

1. Step 1: Understand the += operator

   The += operator adds the right-hand side value to the left-hand side variable.

2. Step 2: Effect on the variable

   The result of the addition is stored back into the same variable, updating its value.

3. Final Answer:

   Adds the right value to the left variable and stores the result in the variable -> Option B

4. Quick Check:

   += means add and assign [OK]

Hint: Think: add then store back in the same variable [OK]

Common Mistakes:

• Confusing += with subtraction or multiplication
• Forgetting that the variable must be mutable
• Thinking it creates a new variable instead of updating

2. Which of the following is the correct way to use the *= operator in Rust?

easy

A. let x = 5; x *= 2;

B. let mut x = 5; *= x 2;

C. let x = 5; mut x *= 2;

D. let mut x = 5; x *= 2;

Solution

1. Step 1: Check variable mutability

   In Rust, to change a variable's value, it must be declared with mut. So let mut x = 5; is correct.

2. Step 2: Correct operator usage

   The operator *= is used as x *= 2; to multiply and assign.

3. Final Answer:

   let mut x = 5; x *= 2; -> Option D

4. Quick Check:

   Mutable variable + operator syntax = correct [OK]

Hint: Declare variable mutable before using assignment operators [OK]

Common Mistakes:

• Not declaring variable as mutable
• Placing mut in wrong position
• Incorrect operator syntax like *= x 2

3. What is the output of the following Rust code?

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

medium

A. 30

B. 25

C. 20

D. 15

Solution

1. Step 1: Perform a += 5;

   Starting with a = 10, adding 5 gives a = 15.

2. Step 2: Perform a *= 2;

   Multiplying 15 by 2 updates a = 30.

3. Final Answer:

   30 -> Option A

4. Quick Check:

   10 + 5 = 15; 15 * 2 = 30 [OK]

Hint: Calculate step-by-step updates to variable [OK]

Common Mistakes:

• Adding and multiplying in wrong order
• Forgetting variable is mutable
• Misreading operator effects

4. Identify the error in this Rust code snippet:

let x = 8;
x -= 3;
println!("{}", x);

medium

A. The println! macro is used incorrectly

B. The operator -= is not valid in Rust

C. Variable x is not mutable but is modified

D. Missing semicolon after println!

Solution

1. Step 1: Check variable mutability

   The variable x is declared without mut, so it is immutable.

2. Step 2: Check assignment operator usage

   The operator -= tries to subtract and assign a new value to x, which is not allowed for immutable variables.

3. Final Answer:

   Variable x is not mutable but is modified -> Option C

4. Quick Check:

   Immutable variable cannot be changed [OK]

Hint: Mutable variables needed for assignment operators [OK]

Common Mistakes:

• Assuming all variables are mutable by default
• Thinking operator syntax is wrong
• Ignoring Rust's immutability rules

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

let mut result = 50;
let change = 7;
result %= change;
result += 3;
result *= 4;

hard

A. 16

B. 12

C. 28

D. 14

Solution

1. Step 1: Calculate result %= change;

   50 % 7 equals 1 because 7 * 7 = 49 and remainder is 1, so result = 1.

2. Step 2: Calculate result += 3;

   Add 3 to 1 gives result = 4.

3. Step 3: Calculate result *= 4;

   Multiply 4 by 4 gives result = 16.

4. Final Answer:

   16 -> Option A

5. Quick Check:

   50 % 7 = 1; 1 + 3 = 4; 4 * 4 = 16 [OK]

Hint: Apply each operator stepwise carefully [OK]

Common Mistakes:

• Miscomputing modulus operation
• Skipping steps or doing operations out of order
• Choosing wrong multiplication result