Increment and decrement operators - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of mixed increment operators

What is the output of the following C code?

C

int main() {
    int a = 5;
    int b = a++ + ++a;
    printf("%d\n", b);
    return 0;
}

A 10

B 11

C 12

D 13

💡 Hint

Remember that a++ uses the value before increment, ++a increments before use.

Initially, a=5. a++ returns 5 then a becomes 6. ++a increments a to 7 then returns 7. So b = 5 + 7 = 12.

❓ Predict Output

intermediate

Value of variable after increments

What is the value of variable x after executing this code?

C

int main() {
    int x = 3;
    x = ++x + x++ + ++x;
    printf("%d\n", x);
    return 0;
}

A 14

B 13

C 12

D 15

💡 Hint

Evaluate each increment carefully and track x's value step by step.

Step 1: ++x increments x to 4 and returns 4. Step 2: x++ returns 4 then increments x to 5. Step 3: ++x increments x to 6 and returns 6. Sum: 4 + 4 + 6 = 14, but x is assigned this sum, so x=14. However, the expression uses x on left and right, so the final x is 14. But the question asks for x after assignment, which is 14.

🔧 Debug

advanced

Identify the error in increment usage

Which option will cause a compilation error in C?

C

int main() {
    int a = 5;
    int b = /* expression */;
    printf("%d\n", b);
    return 0;
}

A b = ++a + a++;

B b = a+++++a;

C b = a++ + ++a;

D b = a++ + a++ + ++a;

💡 Hint

Look carefully at the operator sequences and how C parses them.

Option B has 'a+++++a' which is invalid because C cannot parse '+++++' as valid operators. It causes a syntax error. Other options are valid expressions with increments.

❓ Predict Output

advanced

Output of complex increment and decrement

What is the output of this C program?

C

int main() {
    int i = 2;
    int j = i++ + --i + i-- + ++i;
    printf("%d\n", j);
    return 0;
}

A 8

B 9

C 10

D 11

💡 Hint

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

Start i=2 1) i++ returns 2, i=3 2) --i decrements i to 2, returns 2 3) i-- returns 2, i=1 4) ++i increments i to 2, returns 2 Sum: 2 + 2 + 2 + 2 = 8 So output is 8, option A. Correct answer is C.

❓ Predict Output

expert

Final value after chained increments and decrements

What is the value of variable x after this code runs?

C

int main() {
    int x = 1;
    x = x++ + ++x + --x + x--;
    printf("%d\n", x);
    return 0;
}

A 9

B 7

C 6

D 8

💡 Hint

Evaluate each increment/decrement carefully and update x after each step.

Start x=1 1) x++ returns 1, x=2 2) ++x increments x to 3, returns 3 3) --x decrements x to 2, returns 2 4) x-- returns 2, x=1 Sum: 1 + 3 + 2 + 2 = 8 But x is assigned this sum, so x=8 Option D is 8, so correct answer is C.

Practice

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

easy

A. Adds 1 to the variable

B. Subtracts 1 from the variable

C. Multiplies the variable by 2

D. Divides the variable by 2

Solution

1. Step 1: Understand the meaning of ++

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

2. Step 2: Compare with other options

   Subtracting, multiplying, or dividing are not what ++ does.

3. Final Answer:

   Adds 1 to the variable -> Option A

4. Quick Check:

   ++ means add 1 [OK]

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

Common Mistakes:

• Confusing ++ with --
• Thinking ++ multiplies
• Mixing increment with assignment

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

easy

A. x--;

B. x-+;

C. x+-;

D. --x;

Solution

1. Step 1: Identify prefix decrement syntax

   The prefix decrement operator is --x;, which decrements x by 1.

2. Step 2: Check options carefully

   A is postfix x--;, B x-+; and C x+-; invalid syntax, D is correct prefix.

3. Final Answer:

   --x; -> Option D

4. Quick Check:

   Prefix decrement --x; is correct syntax [OK]

Hint: Look for correct -- syntax before or after variable [OK]

Common Mistakes:

• Using invalid operators like -+ or +-
• Confusing prefix and postfix syntax
• Missing semicolon

3. What is the output of this C code?

int x = 5;
printf("%d", x++);
printf(" %d", x);

medium

A. 5 5

B. 5 6

C. 6 6

D. 6 5

Solution

1. Step 1: Understand postfix increment x++

   Postfix means use the current value first, then add 1. So printf("%d", x++) prints 5, then x becomes 6.

2. Step 2: Print updated value of x

   The second printf prints the new value of x, which is 6.

3. Final Answer:

   5 6 -> Option B

4. Quick Check:

   Postfix prints old value, then increments [OK]

Hint: Postfix ++ prints old value, prefix ++ prints new value [OK]

Common Mistakes:

• Confusing postfix with prefix
• Expecting increment before print
• Ignoring space in output

4. Find the error in this code snippet:

int a = 3;
int b = ++a++;
printf("%d", b);

medium

A. Syntax error due to invalid use of increment operators

B. b will be 4

C. b will be 5

D. No error, prints 3

Solution

1. Step 1: Analyze the expression ++a++

   This expression tries to apply prefix and postfix increment operators together on the same variable, which is invalid syntax in C.

2. Step 2: Understand compiler rules

   C does not allow chaining increments like this; it causes a syntax error.

3. Final Answer:

   Syntax error due to invalid use of increment operators -> Option A

4. Quick Check:

   Invalid chaining of ++ operators causes syntax error [OK]

Hint: Don't chain prefix and postfix ++ on same variable [OK]

Common Mistakes:

• Thinking it increments twice
• Ignoring syntax error
• Assuming valid output

5. Consider this code:

int arr[3] = {1, 2, 3};
int i = 0;
int sum = 0;
while (i < 3) {
    sum += arr[i++];
}
printf("%d", sum);

What is the output and why?

hard

A. 6 because i increments before accessing each element

B. 5 because i increments before accessing each element

C. 6 because i increments after accessing each element

D. 5 because i increments after accessing each element

Solution

1. Step 1: Understand postfix increment in arr[i++]

   The value of i is used first to access the array element, then i is incremented.

2. Step 2: Calculate sum step-by-step

   Elements accessed: arr[0]=1, arr[1]=2, arr[2]=3; sum = 1+2+3 = 6.

3. Final Answer:

   6 because i increments after accessing each element -> Option C

4. Quick Check:

   Postfix increments after use, sum is 6 [OK]

Hint: Postfix increments after use, so sum all elements correctly [OK]

Common Mistakes:

• Confusing prefix and postfix increments
• Calculating wrong sum
• Assuming i increments before access