Logical operators - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of combined logical AND and OR

What is the output of this C code snippet?

C

#include <stdio.h>

int main() {
    int a = 5, b = 0, c = 3;
    int result = a && b || c;
    printf("%d", result);
    return 0;
}

A 0

B 1

C 3

D 5

💡 Hint

Remember that in C, logical AND (&&) and OR (||) return 1 or 0, not the original values.

The expression a && b evaluates to 0 because b is 0 (false). Then 0 || c evaluates to 1 because c is non-zero (true). So the final result is 1.

❓ Predict Output

intermediate

Logical NOT operator effect

What will this C program print?

C

#include <stdio.h>

int main() {
    int x = 0;
    printf("%d", !x);
    return 0;
}

A 1

B 0

C -1

D Garbage value

💡 Hint

Logical NOT (!) returns 1 if the operand is zero, else 0.

Since x is 0 (false), !x is 1 (true). So the program prints 1.

❓ Predict Output

advanced

Short-circuit evaluation in logical AND

What is the output of this C code?

C

#include <stdio.h>

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

A 1 5

B 0 6

C 0 5

D 1 6

💡 Hint

Logical AND stops evaluating if the first operand is false.

Since a is 0 (false), ++b is not evaluated due to short-circuit. So result is 0 and b remains 5.

❓ Predict Output

advanced

Short-circuit evaluation in logical OR

What will this program print?

C

#include <stdio.h>

int main() {
    int a = 1, b = 5;
    int result = a || (++b);
    printf("%d %d", result, b);
    return 0;
}

A 1 5

B 0 5

C 1 6

D 0 6

💡 Hint

Logical OR stops evaluating if the first operand is true.

Since a is 1 (true), ++b is not evaluated due to short-circuit. So result is 1 and b remains 5.

🧠 Conceptual

expert

Understanding logical operator precedence

Given the expression in C: int x = 0 || 1 && 0 || 1; What is the value of x after this line executes?

A 0

B 2

C Compiler error

D 1

💡 Hint

Remember that && has higher precedence than || in C.

The expression is evaluated as 0 || (1 && 0) || 1 = 0 || 0 || 1 = 1.

Practice

1. Which logical operator in C means AND and returns true only if both conditions are true?

easy

A. !

B. ||

C. &&

D. &

Solution

1. Step 1: Understand logical AND operator

   The logical AND operator (&&) returns true only if both conditions it connects are true.

2. Step 2: Compare with other operators

   The || operator means OR, ! means NOT, and & is bitwise AND, not logical AND.

3. Final Answer:

   && -> Option C

4. Quick Check:

   AND = && [OK]

Hint: AND operator is double ampersand && [OK]

Common Mistakes:

• Confusing & (bitwise) with && (logical)
• Mixing OR and AND operators
• Using ! for AND

2. Which of the following is the correct syntax to negate a condition in C?

easy

A. condition !

B. !condition

C. not condition

D. condition != true

Solution

1. Step 1: Identify negation syntax in C

   The NOT operator in C is a single exclamation mark placed before the condition: !condition.

2. Step 2: Check other options

   condition ! places ! after condition which is invalid, C uses 'not' which is not C syntax, D is a comparison, not negation.

3. Final Answer:

   !condition -> Option B

4. Quick Check:

   NOT = ! before condition [OK]

Hint: Negate condition with ! before it [OK]

Common Mistakes:

• Putting ! after condition
• Using 'not' keyword
• Confusing != with negation

3. What is the output of this C code?

int a = 5, b = 10;
if (a > 3 && b < 15) {
    printf("True\n");
} else {
    printf("False\n");
}

medium

A. No output

B. False

C. Compilation error

D. True

Solution

1. Step 1: Evaluate first condition (a > 3)

   a is 5, which is greater than 3, so this condition is true.

2. Step 2: Evaluate second condition (b < 15)

   b is 10, which is less than 15, so this condition is true.

3. Step 3: Apply logical AND (&&)

   Both conditions are true, so the combined condition is true, entering the if block.

4. Final Answer:

   True -> Option D

5. Quick Check:

   True && True = True [OK]

Hint: Both conditions true means && condition is true [OK]

Common Mistakes:

• Assuming OR instead of AND
• Misreading variable values
• Ignoring logical operator precedence

4. Find the error in this C code snippet:

int x = 0;
if (x = 5 || x == 0) {
    printf("Yes\n");
}

medium

A. Using assignment '=' instead of comparison '==' in if condition

B. Missing semicolon after printf statement

C. Incorrect use of logical AND instead of OR

D. Variable x is not declared

Solution

1. Step 1: Check the if condition syntax

   The condition uses 'x = 5 || x == 0'. The '=' is assignment, not comparison.

2. Step 2: Understand effect of assignment in condition

   Assignment returns the assigned value (5), which is true, so condition always true, likely a bug.

3. Final Answer:

   Using assignment '=' instead of comparison '==' in if condition -> Option A

4. Quick Check:

   Use '==' for comparison, not '=' [OK]

Hint: Use '==' to compare, '=' assigns value [OK]

Common Mistakes:

• Confusing '=' and '=='
• Ignoring logical operator precedence
• Assuming code won't compile

5. Given the code below, what will be printed?

int a = 0, b = 1, c = 2;
if (!(a || b) && c > 1) {
    printf("Inside if\n");
} else {
    printf("Inside else\n");
}

hard

A. Inside else

B. Inside if

C. Compilation error

D. No output

Solution

1. Step 1: Evaluate inner OR condition (a || b)

   a is 0 (false), b is 1 (true), so a || b is true.

2. Step 2: Apply NOT operator !(a || b)

   NOT true is false, so this part is false.

3. Step 3: Evaluate c > 1

   c is 2, which is greater than 1, so true.

4. Step 4: Combine with AND: false && true

   False AND anything is false, so condition is false, else block runs.

5. Final Answer:

   Inside else -> Option A

6. Quick Check:

   !(true) && true = false [OK]

Hint: NOT true is false; AND with false is false [OK]

Common Mistakes:

• Ignoring NOT operator effect
• Misreading OR condition
• Assuming if block runs when condition false