Operator precedence in C++ - 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: You will write a small C++ program that uses arithmetic operators with different precedence. You will see how the order of operations affects the final result.

📋 What You'll Learn

Create variables with exact values

Use a configuration variable for a discount rate

Write an expression that uses multiple operators to calculate the final price

Print the final calculated price

💡 Why This Matters

🌍 Real World

Calculating prices with discounts is common in shopping apps and billing systems.

💼 Career

Understanding operator precedence helps avoid bugs in financial calculations and software development.

1

Create initial variables

Create three variables: price1 with value 50, price2 with value 30, and price3 with value 20.

C++

// Your code here

Hint

Use int type and assign the exact values to each variable.

2

Add a discount rate variable

Add a variable called discount of type double and set it to 0.1 (which means 10% discount).

C++

int price1 = 50;
int price2 = 30;
int price3 = 20;
// Your code here

Hint

Use double type for decimal values.

3

Calculate total price with discount

Create a variable totalPrice of type double and set it to the expression (price1 + price2 + price3) * (1 - discount). This calculates the sum of prices and applies the discount.

C++

int price1 = 50;
int price2 = 30;
int price3 = 20;
double discount = 0.1;
// Your code here

Hint

Use parentheses to ensure addition happens before multiplication.

4

Print the final total price

Write a std::cout statement to print the text Total price: followed by the value of totalPrice.

C++

int price1 = 50;
int price2 = 30;
int price3 = 20;
double discount = 0.1;
double totalPrice = (price1 + price2 + price3) * (1 - discount);
// Your code here

Hint

Use std::cout with << to print text and variables.

Practice

1. Which operator has the highest precedence in the expression 3 + 4 * 5?

easy

A. Addition (+)

B. Subtraction (-)

C. Multiplication (*)

D. Division (/)

Solution

1. Step 1: Identify operators in the expression

   The expression has addition (+) and multiplication (*).

2. Step 2: Recall operator precedence rules

   Multiplication (*) has higher precedence than addition (+), so it is evaluated first.

3. Final Answer:

   Multiplication (*) -> Option C

4. Quick Check:

   Multiplication before addition = Multiplication (*) [OK]

Hint: Multiplication and division come before addition and subtraction [OK]

Common Mistakes:

• Thinking addition runs before multiplication
• Ignoring operator precedence rules
• Assuming left-to-right always applies

2. Which of the following expressions is syntactically correct in C++?

easy

A. int x = 5 + (3 * 2);

B. int x = 5 + 3 *;

C. int x = 5 + *3;

D. int x = 5 + 3 * 2);

Solution

1. Step 1: Check each expression for syntax errors

   int x = 5 + *3; has an invalid use of * before 3. int x = 5 + 3 *; ends with * which is incomplete. int x = 5 + 3 * 2); has an unmatched closing parenthesis.

2. Step 2: Confirm correct syntax

   int x = 5 + (3 * 2); uses parentheses correctly and has valid operators and operands.

3. Final Answer:

   int x = 5 + (3 * 2); -> Option A

4. Quick Check:

   Valid syntax with parentheses and operators = int x = 5 + (3 * 2); [OK]

Hint: Check for balanced parentheses and proper operator placement [OK]

Common Mistakes:

• Misplacing operators without operands
• Unbalanced parentheses
• Using operators incorrectly

3. What is the output of the following code?

int a = 2, b = 3, c = 4;
int result = a + b * c / a - b;
std::cout << result;

medium

A. 5

B. 2

C. 4

D. 3

Solution

1. Step 1: Evaluate multiplication and division first

   b * c = 3 * 4 = 12; then 12 / a = 12 / 2 = 6.

2. Step 2: Evaluate addition and subtraction left to right

   a + 6 = 2 + 6 = 8; then 8 - b = 8 - 3 = 5.

3. Final Answer:

   5 -> Option A

4. Quick Check:

   Multiplication/division before addition/subtraction = 5 [OK]

Hint: Do * and / before + and -, left to right [OK]

Common Mistakes:

• Adding before multiplying
• Ignoring division precedence
• Calculating subtraction before addition

4. Find the error in the following code snippet:

int x = 10;
int y = 5;
int z = x + y * / 2;
std::cout << z;

medium

A. Division by zero error

B. Syntax error due to misplaced operator

C. Logical error in calculation

D. No error, code runs fine

Solution

1. Step 1: Analyze the expression for syntax correctness

   The expression x + y * / 2 has two operators (*) and (/) together without an operand, which is invalid syntax.

2. Step 2: Confirm the error type

   This causes a syntax error because the compiler cannot parse the expression correctly.

3. Final Answer:

   Syntax error due to misplaced operator -> Option B

4. Quick Check:

   Invalid operator sequence causes syntax error [OK]

Hint: Check operator placement for missing operands [OK]

Common Mistakes:

• Assuming code runs despite syntax errors
• Confusing syntax error with runtime error
• Ignoring operator adjacency rules

5. Consider the expression int result = 10 - 3 * 2 + 4 / 2;. How can you change it using parentheses to make addition happen before multiplication and division?

hard

A. int result = 10 - ((3 * 2) + 4) / 2;

B. int result = ((10 - 3) * 2 + 4) / 2;

C. int result = 10 - ((3 * 2) + (4 / 2));

D. int result = 10 - (3 * (2 + 4)) / 2;

Solution

1. Step 1: Understand original precedence

   Original expression evaluates multiplication and division before addition and subtraction.

2. Step 2: Use parentheses to force addition before multiplication/division

   int result = 10 - (3 * (2 + 4)) / 2; groups (2 + 4) first, so addition happens before multiplication and division.

3. Final Answer:

   int result = 10 - (3 * (2 + 4)) / 2; -> Option D

4. Quick Check:

   Parentheses change order to add before multiply/divide [OK]

Hint: Use parentheses to group additions before other ops [OK]

Common Mistakes:

• Misplacing parentheses changing meaning
• Not grouping addition properly
• Assuming left-to-right overrides precedence