Increment and decrement operators in C++ - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to increment the variable count by 1.

C++

int count = 5;
count[1];

Drag options to blanks, or click blank then click option'

A++

B--

C+=

D-

Attempts:

3 left

💡 Hint

Common Mistakes

Using the decrement operator -- instead of increment.

Using += without specifying the amount.

The increment operator ++ increases the value of count by 1.

2fill in blank

medium

Complete the code to decrement the variable value by 1.

C++

int value = 10;
value[1];

Drag options to blanks, or click blank then click option'

A--

B++

C+=

D-

Attempts:

3 left

💡 Hint

Common Mistakes

Using the increment operator ++ instead of decrement.

Using -= without specifying the amount.

The decrement operator -- decreases the value of value by 1.

3fill in blank

hard

Fix the error in the code to correctly increment num.

C++

int num = 7;
num = num[1];

Drag options to blanks, or click blank then click option'

A++

B- 1

C--

D+ 1

Attempts:

3 left

💡 Hint

Common Mistakes

Using ++ after assignment operator.

Using decrement operators instead of increment.

To increment num by 1 using assignment, you add 1 with + 1.

4fill in blank

hard

Fill both blanks to create a map of numbers to their squares using increment in the loop.

C++

std::map<int, int> squares;
for (int i = 1; i [1] 5; i[2]) {
    squares[i] = i * i;
}

Drag options to blanks, or click blank then click option'

A<=

B<

C++

D--

Attempts:

3 left

💡 Hint

Common Mistakes

Using i <= 5 causes the loop to run one extra time.

Using decrement operator -- in the loop.

The loop runs while i is less than 5, and i++ increments i by 1 each time.

5fill in blank

hard

Fill all three blanks to create a map of uppercase letters to their ASCII codes, incrementing the letter.

C++

std::map<char, int> ascii_map;
for (char ch = 'A'; ch [1] 'Z'; ch[2]) {
    ascii_map[[3]] = static_cast<int>(ch);
}

Drag options to blanks, or click blank then click option'

A<=

B++

Cch

D--

Attempts:

3 left

💡 Hint

Common Mistakes

Using < code < 'Z' > causes 'Z' to be excluded.

Using decrement operator -- instead of increment.

Using wrong variable as map key.

The loop runs while ch is less than or equal to 'Z', increments ch with ++, and uses ch as the key in the map.

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