Increment and decrement operators in C++ - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of mixed pre and post increment

What is the output of this C++ code snippet?

C++

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

A 13

B 11

C 10

D 12

💡 Hint

Remember that post-increment returns the value before incrementing, pre-increment increments first.

x++ returns 5 then x becomes 6; ++x increments x to 7 then returns 7; sum is 5 + 7 = 12.

❓ Predict Output

intermediate

Effect of multiple increments in one expression

What is the output of this C++ code?

C++

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

A 14

B 15

C 12

D 13

💡 Hint

Evaluate increments from left to right, updating 'a' after each operation.

++a increments a to 4 and returns 4; a++ returns 4 then increments a to 5; ++a increments a to 6 and returns 6; sum = 4 + 4 + 6 = 14.

❓ Predict Output

advanced

Output with decrement operators in complex expression

What is the output of this C++ code?

C++

int i = 4;
int j = i-- - --i + i--;
std::cout << j << std::endl;

A 4

B 3

C 2

D 5

💡 Hint

Remember post-decrement returns value before decrement, pre-decrement decrements first.

i-- returns 4 then i=3; --i decrements i to 2 returns 2; i-- returns 2 then i=1; sum = 4 - 2 + 2 = 4.

❓ Predict Output

advanced

Value of variable after increments and decrements

What is the value of 'x' after running this code?

C++

int x = 10;
x = ++x + x++ - --x + x--;
std::cout << x << std::endl;

A 20

B 22

C 19

D 21

💡 Hint

Track the value of x carefully after each increment or decrement.

Stepwise: ++x increments x to 11 returns 11; x++ returns 11 then x=12; --x decrements x to 11 returns 11; x-- returns 11 then x=10; sum=11+11-11+11=22; x assigned 22.

❓ Predict Output

expert

Output of complex expression with increments and decrements

What is the output of this C++ code snippet?

C++

int n = 5;
int result = n++ + ++n - --n + n-- + n;
std::cout << result << std::endl;

A 27

B 28

C 17

D 30

💡 Hint

Evaluate each increment/decrement carefully and update 'n' after each operation.

n++ returns 5 (n=6); ++n increments n to 7 returns 7; --n decrements n to 6 returns 6; n-- returns 6 (n=5); n=5; sum=5+7-6+6+5=17.

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