Operator precedence in Rust - Mini Project: Build & Apply

Operator precedence

📖 Scenario: Imagine you are calculating the total cost of items in a shopping cart. You want to make sure the calculations happen in the right order so you get the correct total.

🎯 Goal: Build a Rust program that calculates the total cost using operator precedence rules correctly.

📋 What You'll Learn

Create variables with exact values

Use operator precedence in an expression

Print the final calculated total

💡 Why This Matters

🌍 Real World

Calculating prices in shopping carts or invoices requires correct operator precedence to avoid mistakes.

💼 Career

Understanding operator precedence is essential for writing correct calculations in software development, especially in finance and e-commerce.

1

Create item prices

Create three variables called apple_price, banana_price, and orange_price with values 2, 3, and 4 respectively.

Rust

let apple_price = 0;
let banana_price = 0;
let orange_price = 0;
// Your code here

Hint

Use let to create variables and assign the exact numbers.

2

Create quantity variables

Create three variables called apple_qty, banana_qty, and orange_qty with values 3, 2, and 5 respectively.

Rust

let apple_price = 2;
let banana_price = 3;
let orange_price = 4;
let apple_qty = 0;
let banana_qty = 0;
let orange_qty = 0;
// Your code here

Hint

Use let to create quantity variables with the exact numbers.

3

Calculate total cost with operator precedence

Create a variable called total_cost that calculates the total price by multiplying each price by its quantity and adding all results together. Use the expression: apple_price * apple_qty + banana_price * banana_qty + orange_price * orange_qty.

Rust

let apple_price = 2;
let banana_price = 3;
let orange_price = 4;
let apple_qty = 3;
let banana_qty = 2;
let orange_qty = 5;
let total_cost = 0; // Your code here

Hint

Remember multiplication happens before addition without extra parentheses.

4

Print the total cost

Print the value of total_cost using println! with the exact text: Total cost: {total_cost} where {total_cost} is replaced by the variable value.

Rust

let apple_price = 2;
let banana_price = 3;
let orange_price = 4;
let apple_qty = 3;
let banana_qty = 2;
let orange_qty = 5;
let total_cost = apple_price * apple_qty + banana_price * banana_qty + orange_price * orange_qty;
// Your code here

Hint

Use println!("Total cost: {}", total_cost); to print the result.

Practice

1. In Rust, which operator has the highest precedence among these: +, *, -, /?

easy

A. Multiplication (*) and division (/) have higher precedence than addition (+) and subtraction (-).

B. Addition (+) and subtraction (-) have higher precedence than multiplication (*) and division (/).

C. All operators have the same precedence and are evaluated left to right.

D. Subtraction (-) has the highest precedence among all.

Solution

1. Step 1: Recall Rust operator precedence rules

   Multiplication (*) and division (/) operators have higher precedence than addition (+) and subtraction (-).

2. Step 2: Compare given operators

   Among the given operators, * and / are evaluated before + and - in expressions.

3. Final Answer:

   Multiplication (*) and division (/) have higher precedence than addition (+) and subtraction (-). -> Option A

4. Quick Check:

   *, / > +, - [OK]

Hint: Remember: multiply/divide before add/subtract [OK]

Common Mistakes:

• Thinking addition has higher precedence than multiplication
• Assuming all operators have equal precedence
• Confusing subtraction precedence with multiplication

2. Which of the following Rust expressions is correctly written to add 2 and 3, then multiply the result by 4?

easy

A. 2 + 3 * 4

B. (2 + 3) * 4

C. 2 + (3 * 4)

D. 2 * 3 + 4

Solution

1. Step 1: Understand operator precedence and parentheses

   Multiplication (*) has higher precedence than addition (+), so without parentheses, multiplication happens first.

2. Step 2: Check each expression

   (2 + 3) * 4 uses parentheses to add 2 and 3 first, then multiplies by 4, which matches the requirement.

3. Final Answer:

   (2 + 3) * 4 -> Option B

4. Quick Check:

   Parentheses force addition first [OK]

Hint: Use parentheses to change default order [OK]

Common Mistakes:

• Ignoring parentheses and relying on default precedence
• Misplacing parentheses causing wrong order
• Assuming left-to-right always applies

3. What is the output of this Rust code?

let result = 10 - 4 / 2 + 3 * 2;
println!("{}", result);

medium

A. 13

B. 10

C. 11

D. 14

Solution

1. Step 1: Apply operator precedence to expression

   Division and multiplication happen before addition and subtraction:
   4 / 2 = 2
   3 * 2 = 6

2. Step 2: Calculate left to right with results

   Expression becomes: 10 - 2 + 6
   10 - 2 = 8
   8 + 6 = 14

3. Final Answer:

   14 -> Option D

4. Quick Check:

   */ before +- gives 14 [OK]

Hint: Calculate * and / first, then + and - left to right [OK]

Common Mistakes:

• Doing operations strictly left to right ignoring precedence
• Adding before multiplying
• Miscomputing division or multiplication

4. Identify the error in this Rust expression:

let value = 5 + 3 * / 2;

medium

A. No error, expression is valid

B. Logical error: multiplication should be addition

C. Syntax error due to misplaced operator '/'

D. Runtime error due to division by zero

Solution

1. Step 1: Analyze the expression syntax

   The expression has * / operators together without a number between them, which is invalid syntax in Rust.

2. Step 2: Confirm error type

   This causes a syntax error because Rust expects an operand between operators.

3. Final Answer:

   Syntax error due to misplaced operator '/' -> Option C

4. Quick Check:

   Operators must have operands between them [OK]

Hint: Check for missing operands between operators [OK]

Common Mistakes:

• Assuming expression is valid without checking syntax
• Confusing syntax error with runtime error
• Ignoring operator placement rules

5. Given the Rust expression:

let x = 2 + 3 * 4 - 6 / 3 + (8 - 5) * 2;

What is the correct value of x after evaluation?

hard

A. 18

B. 20

C. 19

D. 21

Solution

1. Step 1: Evaluate parentheses and high precedence operators

   Calculate inside parentheses:
   (8 - 5) = 3
   Then multiplication and division:
   3 * 4 = 12
   6 / 3 = 2
   3 * 2 = 6 (from parentheses result * 2)

2. Step 2: Substitute and calculate addition and subtraction left to right

   Expression becomes: 2 + 12 - 2 + 6
   2 + 12 = 14
   14 - 2 = 12
   12 + 6 = 18

3. Final Answer:

   18 -> Option A

4. Quick Check:

   Parentheses and */ first, then +- left to right [OK]

Hint: Calculate parentheses first, then * and /, then + and - left to right [OK]

Common Mistakes:

• Ignoring parentheses and calculating in wrong order
• Doing addition before multiplication
• Mixing up left-to-right evaluation for + and -