Assignment operators in C++ - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the assignment operator (=) in C++?

The assignment operator (=) is used to copy the value from the right side to the variable on the left side.

beginner

Explain the combined assignment operator += with an example.

The operator += adds the right side value to the left side variable and assigns the result back to the left variable.
Example: int x = 5; x += 3; // x becomes 8

beginner

List three compound assignment operators in C++ besides +=.

Three compound assignment operators are:
- -= (subtract and assign)
- *= (multiply and assign)
- /= (divide and assign)

intermediate

What happens if you write a = b = c; in C++?

This is called chained assignment. It assigns the value of c to b, then assigns b's value to a. Both a and b end up with the value of c.

intermediate

Why should you be careful when using the assignment operator inside conditions?

Using assignment (=) inside conditions can cause bugs because it assigns a value instead of comparing. For comparison, use ==. For example, if (x = 5) assigns 5 to x and always evaluates true, which is usually unintended.

What does the operator *= do in C++?

A Divides the left variable by the right and assigns the result

B Subtracts the right value from the left variable

C Adds the right value to the left variable

D Multiplies the left variable by the right and assigns the result

The operator *= multiplies the left variable by the right value and stores the result back in the left variable.

What is the result of this code?
int a = 2; a += 3;

A a is 5

B a is 6

C a is 3

D a is 2

The operator += adds 3 to a's current value 2, so a becomes 5.

Which operator is used to assign a value to a variable?

A =

B +=

C ==

D !=

The single equals sign (=) is the assignment operator in C++.

What does this code do?
int x = 1, y = 2; x = y = 5;

A Only x becomes 5

B x and y both become 5

C Only y becomes 5

D x becomes 2, y becomes 5

The assignment is chained: y is assigned 5, then x is assigned y's value, so both become 5.

Why is this condition problematic?
if (a = 10)

A It compares a to 10

B It checks if a is not equal to 10

C It assigns 10 to a and always evaluates true

D It causes a syntax error

Using = assigns 10 to a, so the condition is always true, which is usually a mistake.

Explain how compound assignment operators work and give two examples.

Think about how they combine an operation and assignment in one step.

Describe what happens in chained assignments and why it might be useful.

Consider how values flow from right to left.

Practice

1. Which of the following assignment operators adds the right value to the left variable and stores the result back in the left variable?

easy

A. +=

B. =

C. -=

D. *=

Solution

1. Step 1: Understand the meaning of +=

   The operator += adds the right side value to the left side variable and updates the variable with the new value.

2. Step 2: Compare with other operators

   = assigns a value, -= subtracts, and *= multiplies. Only += adds and assigns.

3. Final Answer:

   += -> Option A

4. Quick Check:

   Add and assign = += [OK]

Hint: Remember: += means add then assign [OK]

Common Mistakes:

• Confusing = with +=
• Thinking -= adds values
• Mixing *= with addition

2. Which of the following is the correct syntax to multiply variable x by 5 and store the result back in x?

easy

A. x =* 5;

B. x *= 5;

C. x = 5*;

D. x = 5 *;

Solution

1. Step 1: Identify the correct assignment operator syntax

   The operator to multiply and assign is *=, so x *= 5; is correct.

2. Step 2: Check other options for syntax errors

   x =* 5; and x = 5*; are invalid syntax. x = 5 *; is incomplete.

3. Final Answer:

   x *= 5; -> Option B

4. Quick Check:

   Multiply and assign = *= [OK]

Hint: Use operator before = for combined assignment [OK]

Common Mistakes:

• Placing * after = instead of before
• Leaving incomplete expressions
• Confusing with simple assignment =

3. What is the output of the following code?

int a = 10;
a += 5;
a -= 3;
std::cout << a;

medium

A. 12

B. 15

C. 8

D. 10

Solution

1. Step 1: Calculate a += 5;

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

2. Step 2: Calculate a -= 3;

   Subtracting 3 from 15 gives a = 12.

3. Final Answer:

   12 -> Option A

4. Quick Check:

   10 + 5 - 3 = 12 [OK]

Hint: Apply each assignment step by step [OK]

Common Mistakes:

• Adding and subtracting in wrong order
• Confusing += and -= effects
• Ignoring the updated value after first operation

4. Identify the error in the following code snippet:

int x = 7;
x =+ 3;
std::cout << x;

medium

A. Syntax error due to incorrect operator

B. x will be 10

C. x will be 3

D. No error, output is 10

Solution

1. Step 1: Analyze x =+ 3;

   This is not the addition assignment +=. Instead, =+ means assign positive 3 to x.

2. Step 2: Determine the value of x after assignment

   x becomes 3, not 10, because =+ is just assignment with unary plus.

3. Final Answer:

   3 -> Option C

4. Quick Check:

   =+ is assignment, not addition [OK]

Hint: Watch for =+ vs += difference [OK]

Common Mistakes:

• Confusing =+ with += operator
• Expecting addition instead of assignment
• Assuming output is sum without checking syntax

5. Given int n = 20;, which code snippet correctly divides n by 4 and then subtracts 2 from the result, updating n accordingly?

hard

A. n = n - 2 / 4;

B. n -= 2; n /= 4;

C. n = (n - 2) / 4;

D. n /= 4; n -= 2;

Solution

1. Step 1: Understand the required operations order

   First divide n by 4, then subtract 2 from the result, updating n each time.

2. Step 2: Check each option

   n /= 4; n -= 2; does n /= 4; then n -= 2;, matching the requirement. n -= 2; n /= 4; reverses order. n = (n - 2) / 4; subtracts first due to parentheses (18 / 4 = 4). n = n - 2 / 4; subtracts incorrectly.

3. Step 3: Confirm correct update

   n /= 4; n -= 2; updates n stepwise as requested.

4. Final Answer:

   n /= 4; n -= 2; -> Option D

5. Quick Check:

   Divide then subtract = n /= 4; n -= 2; [OK]

Hint: Follow operation order exactly [OK]

Common Mistakes:

• Reversing operation order
• Misunderstanding combined expressions
• Incorrect operator precedence assumptions