10 C Logical Interview Questions and Answers

C is a foundational programming language that has significantly influenced many modern languages. Known for its efficiency and control over system resources, C is widely used in system programming, embedded systems, and performance-critical applications. Its syntax and structure have laid the groundwork for many other languages, making it an essential skill for developers aiming to understand the core principles of programming.

This article offers a curated selection of C logical interview questions designed to test and enhance your problem-solving abilities. By working through these questions, you will gain a deeper understanding of logical constructs in C, preparing you to tackle complex challenges in technical interviews with confidence.

C Logical Interview Questions and Answers

1. Explain the difference between the logical AND (&&) and logical OR (||) operators.

The logical AND (&&) and logical OR (||) operators evaluate boolean expressions in C.

• The logical AND (&&) operator returns true only if both operands are true.
• The logical OR (||) operator returns true if at least one operand is true.

Example:

#include <stdio.h>

int main() {
    int a = 5;
    int b = 10;
    int c = 0;

    if (a > 0 && b > 0) {
        printf("Both a and b are positive.\n");
    }

    if (a > 0 || c > 0) {
        printf("At least one of a or c is positive.\n");
    }

    return 0;
}

In this example:

• The first if statement prints the message because both a and b are greater than 0.
• The second if statement prints the message because a is greater than 0.

2. Write a conditional statement that checks if a number is both positive and even.

To check if a number is both positive and even, use a conditional statement:

#include <stdio.h>

int main() {
    int number = 4; // Example number

    if (number > 0 && number % 2 == 0) {
        printf("The number is both positive and even.\n");
    } else {
        printf("The number is not both positive and even.\n");
    }

    return 0;
}

3. Write an expression that checks if a number is either negative or greater than 100, but not equal to 50.

To check if a number is either negative or greater than 100, but not equal to 50:

int number = /* some value */;
if ((number < 0 || number > 100) && number != 50) {
    // The number is either negative or greater than 100, but not equal to 50
}

In this expression:

• number < 0 checks if the number is negative.
• number > 100 checks if the number is greater than 100.
• number != 50 ensures the number is not 50.

4. Write a loop that continues until a variable is both non-zero and less than 10.

To write a loop that continues until a variable is both non-zero and less than 10:

#include <stdio.h>

int main() {
    int var = 0; // Initialize the variable

    while (var == 0 || var >= 10) {
        // Perform some operations
        printf("Enter a number: ");
        scanf("%d", &var);
    }

    printf("Loop ended. Variable is now %d\n", var);
    return 0;
}

5. Write a function that returns true if both input parameters are non-zero.

To write a function that returns true if both input parameters are non-zero:

#include <stdbool.h>

bool areBothNonZero(int a, int b) {
    return (a != 0 && b != 0);
}

6. Explain De Morgan’s laws and provide an example of how they can be used to simplify a logical expression.

De Morgan’s laws transform logical expressions into equivalent forms, simplifying them. For example:

if (!(A && B)) {
    // Do something
}

Can be simplified to:

if (!A || !B) {
    // Do something
}

Similarly:

if (!(A || B)) {
    // Do something
}

Can be simplified to:

if (!A && !B) {
    // Do something
}

7. Optimize the following code snippet: if ((a > 0 && b < 0) || (c == 0 && d != 0)) { /* do something */ }

To optimize the given code snippet:

if ((a > 0 && b < 0) || (c == 0 && d != 0)) {
    /* do something */
}

Reorder conditions for efficiency if one is more likely true:

if ((c == 0 && d != 0) || (a > 0 && b < 0)) {
    /* do something */
}

This reordering can improve performance if the first condition is more frequently true.

8. Explain the XOR logical operation and provide an example of its use.

The XOR operation returns true if the inputs are different. Example:

#include <stdio.h>

int main() {
    int a = 5; // 0101 in binary
    int b = 3; // 0011 in binary

    int result = a ^ b; // XOR operation

    printf("Result of %d XOR %d is %d\n", a, b, result); // Output: 6 (0110 in binary)

    return 0;
}

In this example, XOR is used to compare and manipulate binary values.

9. Write a complex conditional statement that combines at least three different logical conditions.

#include <stdio.h>

int main() {
    int a = 5;
    int b = 10;
    int c = 15;

    if ((a < b && b < c) || (a == 5 && c != 20) || !(b == 10)) {
        printf("The complex condition is true.\n");
    } else {
        printf("The complex condition is false.\n");
    }

    return 0;
}

10. Describe a real-world scenario where logical operators are used and explain how you would implement it.

Logical operators are used in scenarios where multiple conditions need evaluation. For example, in an access control system, access might be granted if a user has a valid ID and the current time is within allowed hours.

Example:

#include <stdio.h>
#include <stdbool.h>

bool hasValidID = true;
bool isWithinAccessHours = false;

int main() {
    if (hasValidID && isWithinAccessHours) {
        printf("Access Granted\n");
    } else {
        printf("Access Denied\n");
    }
    return 0;
}

In this example, the program checks if the user has a valid ID and if the current time is within allowed hours. If both conditions are true, access is granted.