Increment and decrement operators in C++ - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the increment operator (++) do in C++?

The increment operator (++) increases the value of a variable by 1.

intermediate

Explain the difference between prefix (++x) and postfix (x++) increment operators.

Prefix (++x) increases the value first, then uses it. Postfix (x++) uses the current value first, then increases it.

beginner

What does the decrement operator (--) do in C++?

The decrement operator (--) decreases the value of a variable by 1.

intermediate

How does the postfix decrement operator (x--) behave in an expression?

It uses the current value of x in the expression, then decreases x by 1 after.

advanced

Can increment and decrement operators be used with non-integer types like floats or pointers?

Yes. Increment and decrement operators can be used with floats (increase/decrease by 1.0) and pointers (move to next/previous memory address).

What is the value of x after this code? int x = 5; int y = ++x;

A x = 5, y = 6

B x = 6, y = 6

C x = 6, y = 5

D x = 5, y = 5

Prefix ++x increases x first to 6, then assigns y = 6.

What is the value of y after this code? int x = 5; int y = x++;

A y = 6

B y = 4

C y = 7

D y = 5

Postfix x++ uses x's current value (5) for y, then increments x.

Which operator decreases a variable's value by 1?

A --

B ++

C +=

D -=

The decrement operator (--) decreases the value by 1.

If int x = 10; what is the value of x after x--?

A 9

B 10

C 11

D Undefined

x-- decreases x by 1, so x becomes 9.

Can you use ++ operator on a pointer variable?

A No, ++ only works on integers.

B Yes, but it resets the pointer to null.

C Yes, it moves the pointer to the next memory location.

D No, pointers cannot be changed.

Incrementing a pointer moves it to the next element in memory.

Describe how prefix and postfix increment operators differ in behavior.

Think about when the variable changes relative to its use.

Explain how decrement operators work and give an example with both prefix and postfix forms.

Compare with increment operators but in reverse.

Practice

1. What does the ++ operator do in C++?

easy

A. Multiplies the variable by 2

B. Subtracts 1 from the variable

C. Adds 1 to the variable

D. Divides the variable by 2

Solution

1. Step 1: Understand the increment operator

   The ++ operator increases the value of a variable by 1.

2. Step 2: Compare with other options

   Other options describe different operations like subtraction or multiplication, which are not what ++ does.

3. Final Answer:

   Adds 1 to the variable -> Option C

4. Quick Check:

   Increment (++) means add 1 [OK]

Hint: Remember ++ means add one, -- means subtract one [OK]

Common Mistakes:

• Confusing ++ with --
• Thinking ++ multiplies or divides
• Mixing prefix and postfix effects

2. Which of the following is the correct syntax for prefix decrement of a variable x by 1 using the decrement operator?

easy

A. --x;

B. x--;

C. x-1;

D. x--1;

Solution

1. Step 1: Identify prefix decrement syntax

   The prefix decrement operator is --x;, which decrements the value first.

2. Step 2: Check other options

   x--; is postfix. x-1; subtracts but does not modify x. x--1; is invalid.

3. Final Answer:

   --x; -> Option A

4. Quick Check:

   Prefix decrement is -- before variable [OK]

Hint: Prefix decrement: -- before variable [OK]

Common Mistakes:

• Writing x--1 which is invalid
• Using x-1; which does not change x
• Confusing decrement with subtraction

3. What is the output of the following code?

int x = 5;
int y = x++ + ++x;
std::cout << y;

medium

A. 11

B. 12

C. 13

D. 14

Solution

1. Step 1: Evaluate x++ and ++x values

   Initially, x = 5. x++ uses 5 then increments to 6. Then ++x increments 6 to 7 and uses 7.

2. Step 2: Calculate the sum

   Sum is 5 (from x++) + 7 (from ++x) = 12.

3. Step 3: Confirm final value of y

   So, y = 12.

4. Final Answer:

   12 -> Option B

5. Quick Check:

   Postfix uses old value, prefix increments first [OK]

Hint: Postfix uses old value; prefix increments before use [OK]

Common Mistakes:

• Adding incremented values twice
• Mixing prefix and postfix effects
• Ignoring order of operations

4. Find the error in the following code snippet:

int a = 3;
int b = ++a + a--;
std::cout << b;

medium

A. Undefined behavior due to modifying variable twice without sequence point

B. No error; output is 8

C. Syntax error due to invalid operator usage

D. Runtime error due to decrement on uninitialized variable

Solution

1. Step 1: Analyze variable modifications

   The variable a is modified twice in the same expression: once by ++a and once by a--.

2. Step 2: Understand sequence point rules

   In C++, modifying a variable more than once between sequence points causes undefined behavior.

3. Final Answer:

   Undefined behavior due to modifying variable twice without sequence point -> Option A

4. Quick Check:

   Multiple ++/-- on same var in one expression causes undefined behavior [OK]

Hint: Avoid multiple ++/-- on same variable in one expression [OK]

Common Mistakes:

• Assuming output is always predictable
• Ignoring undefined behavior rules
• Thinking syntax error occurs here

5. Given the code below, what will be the final value of sum?

int sum = 0;
int i = 0;
while (i++ < 5) {
  sum += i;
}

hard

A. 10

B. 20

C. 25

D. 15

Solution

1. Step 1: Trace the loop iterations and values of i

   Initially, i = 0. Condition i++ < 5 uses i then increments. Iterations:
   1) i=0 (check 0<5 true), then i=1, sum+=1
   2) i=1 (check 1<5 true), then i=2, sum+=2
   3) i=2 (check 2<5 true), then i=3, sum+=3
   4) i=3 (check 3<5 true), then i=4, sum+=4
   5) i=4 (check 4<5 true), then i=5, sum+=5
   6) i=5 (check 5<5 false), loop ends

2. Step 2: Calculate sum

   Sum = 1+2+3+4+5 = 15

3. Final Answer:

   15 -> Option D

4. Quick Check:

   Postfix increments after comparison; sum 1 to 5 [OK]

Hint: Postfix increments after check; sum counts 1 to 5 [OK]

Common Mistakes:

• Assuming i increments before comparison
• Adding wrong range of numbers
• Confusing prefix and postfix in loop condition