Top C Interview Questions(2024)

Question: What is the difference between struct and union in C?

Answer:

In C, both struct and union are used to group different data types together under a single name, but they differ significantly in terms of memory allocation and usage. Here are the key differences between struct and union:

1. Memory Allocation:

• struct:

  • In a struct, all the members are allocated their own memory, and each member can hold a value simultaneously. The total size of the structure is the sum of the sizes of its members, and possibly some padding to align the data types according to memory alignment rules.
  • Each member has its own memory location, so they can store different values at the same time.

  Example:

  struct Example {
      int i;      // 4 bytes
      char c;     // 1 byte
      float f;    // 4 bytes
  };

  The total memory required for struct Example would be at least 4 + 1 + 4 = 9 bytes, but due to padding (for alignment), it might be larger (e.g., 12 bytes on some systems).

• union:

  • In a union, all members share the same memory location. Only one member can hold a value at any given time. The memory size of the union is the size of its largest member. The total size of the union is the size of its largest member (not the sum of the sizes of all members).
  • Since all members share the same memory, a union can only store one of its members’ values at a time. When you assign a value to one member, the value of the other members will be overwritten.

  Example:

  union Example {
      int i;      // 4 bytes
      char c;     // 1 byte
      float f;    // 4 bytes
  };

  The total memory required for union Example would be the size of the largest member, which is 4 bytes (since int and float typically take 4 bytes). Thus, it will take 4 bytes of memory regardless of the number of members.

2. Usage:

• struct:

  • A struct is used when you need to store different data types and want all of them to hold values simultaneously. It is ideal when you want to represent an entity or object that has multiple attributes that should all be stored together (e.g., a Person with name, age, and salary).

  Example:

  struct Person {
      char name[50];
      int age;
      float salary;
  };

• union:

  • A union is used when you need to store different types of data, but only one of them needs to be stored at a time. It is ideal when you want to save memory and are only interested in storing one of the possible values (e.g., storing an integer, float, or character in a variable, but not more than one at a time).

  Example:

  union Data {
      int i;
      float f;
      char c;
  };

3. Access to Members:

• struct:

  • In a struct, all members can be accessed independently. You can modify and use the values of all members at the same time.

  Example:

  struct Person p1;
  p1.age = 25;
  p1.salary = 50000.50;
  strcpy(p1.name, "John Doe");

• union:

  • In a union, you can only access the most recently assigned member, and all other members will contain garbage or the last assigned value. This is because they all share the same memory location.

  Example:

  union Data data;
  data.i = 10;     // Assigning value to int
  printf("%d\n", data.i);  // Prints 10
  
  data.f = 3.14;   // Assigning value to float (overwrites int)
  printf("%f\n", data.f);  // Prints 3.14
  
  printf("%d\n", data.i);  // Prints garbage or the last assigned value

4. Memory Efficiency:

• struct:

  • struct consumes more memory because each member has its own separate memory allocation.
  • If you have a large number of members, the memory consumption can add up quickly.

• union:

  • union is more memory efficient because it only needs enough memory for the largest member. However, you can only use one member at a time.

5. Size of Structure vs Union:

• struct: The size of a struct is the sum of the sizes of its individual members, plus any padding added by the compiler for alignment.
• union: The size of a union is the size of its largest member.

6. Example of struct and union:

#include <stdio.h>

struct ExampleStruct {
    int i;      // 4 bytes
    char c;     // 1 byte
    float f;    // 4 bytes
};

union ExampleUnion {
    int i;      // 4 bytes
    char c;     // 1 byte
    float f;    // 4 bytes
};

int main() {
    struct ExampleStruct structVar;
    union ExampleUnion unionVar;

    // Assign values to struct members
    structVar.i = 10;
    structVar.c = 'A';
    structVar.f = 3.14;

    printf("Struct Example:\n");
    printf("i: %d, c: %c, f: %.2f\n", structVar.i, structVar.c, structVar.f);
    printf("Size of struct: %lu bytes\n\n", sizeof(struct ExampleStruct));

    // Assign values to union members
    unionVar.i = 10;
    printf("Union Example (after assigning to i):\n");
    printf("i: %d\n", unionVar.i);

    unionVar.c = 'A';
    printf("Union Example (after assigning to c):\n");
    printf("c: %c\n", unionVar.c);

    printf("Size of union: %lu bytes\n", sizeof(union ExampleUnion));

    return 0;
}

Output:

Struct Example:
i: 10, c: A, f: 3.14
Size of struct: 12 bytes

Union Example (after assigning to i):
i: 10
Union Example (after assigning to c):
c: A
Size of union: 4 bytes

7. Key Differences Summary:

Feature	struct	union
Memory Allocation	Each member gets its own memory	All members share the same memory
Size	Size = sum of sizes of all members + padding	Size = size of largest member
Access	All members can be accessed at once	Only the most recently assigned member can be accessed
Use Case	Storing different data that needs to be accessed simultaneously	Storing different types of data, but only one at a time
Memory Efficiency	Less memory-efficient	More memory-efficient

Conclusion:

• Use struct when you need to store multiple types of data and access all the members simultaneously. Structures are ideal for representing complex objects or records where each field needs to hold a separate value.

• Use union when you need to store different types of data in the same memory space, but only one value at a time. Unions save memory but limit the ability to store multiple values at once.

Question: What are the different types of loops in C?

Answer:

In C, loops are used to execute a block of code repeatedly based on certain conditions. There are three main types of loops:

1. for Loop:

The for loop is used when you know in advance how many times you want to execute a statement or a block of statements.

Syntax:

for (initialization; condition; increment/decrement) {
    // Code to be executed
}

• Initialization: Executed once at the beginning. It typically defines and initializes the loop counter variable.
• Condition: Checked before each iteration. If the condition evaluates to true, the loop continues. If it’s false, the loop stops.
• Increment/Decrement: Executed after each iteration. It typically updates the loop counter.

Example:

#include <stdio.h>

int main() {
    // Print numbers from 1 to 5 using a for loop
    for (int i = 1; i <= 5; i++) {
        printf("%d\n", i);
    }
    return 0;
}

Output:

1
2
3
4
5

2. while Loop:

The while loop is used when you want to repeat a block of code an unknown number of times, but you have a condition that must be met for the loop to run. The condition is evaluated before each iteration.

Syntax:

while (condition) {
    // Code to be executed
}

• Condition: Checked before each iteration. The loop continues to execute as long as the condition evaluates to true.

Example:

#include <stdio.h>

int main() {
    int i = 1;
    // Print numbers from 1 to 5 using a while loop
    while (i <= 5) {
        printf("%d\n", i);
        i++;  // Increment i
    }
    return 0;
}

Output:

1
2
3
4
5

3. do-while Loop:

The do-while loop is similar to the while loop, but the condition is evaluated after the loop’s body has executed. This ensures that the code inside the loop is executed at least once, even if the condition is false initially.

Syntax:

do {
    // Code to be executed
} while (condition);

• Condition: Checked after each iteration. The loop will always execute the code block at least once.

Example:

#include <stdio.h>

int main() {
    int i = 1;
    // Print numbers from 1 to 5 using a do-while loop
    do {
        printf("%d\n", i);
        i++;  // Increment i
    } while (i <= 5);
    return 0;
}

Output:

1
2
3
4
5

Comparison of Loops:

Feature	for loop	while loop	do-while loop
Condition Check	Before each iteration	Before each iteration	After each iteration
Execution	Executes a fixed number of times	Executes as long as the condition is true	Executes at least once, even if the condition is false
Usage	When the number of iterations is known	When the number of iterations is not known	When you need to execute the loop body at least once
Syntax	for (initialization; condition; increment/decrement)	while (condition)	do { } while (condition);

4. Nested Loops:

In addition to these basic loops, you can use loops inside other loops, which are called nested loops. They are often used when working with multi-dimensional arrays or solving complex problems like matrix manipulations.

Example:

#include <stdio.h>

int main() {
    // Print a 3x3 matrix using nested for loops
    for (int i = 1; i <= 3; i++) {
        for (int j = 1; j <= 3; j++) {
            printf("%d ", i * j);
        }
        printf("\n");
    }
    return 0;
}

Output:

1 2 3 
2 4 6 
3 6 9 

5. break and continue Statements:

• break: Used to exit a loop prematurely, regardless of the loop’s condition.

  Example:

  for (int i = 1; i <= 10; i++) {
      if (i == 5) {
          break;  // Exit the loop when i equals 5
      }
      printf("%d ", i);
  }
  // Output: 1 2 3 4

• continue: Used to skip the current iteration of a loop and move to the next iteration.

  Example:

  for (int i = 1; i <= 5; i++) {
      if (i == 3) {
          continue;  // Skip the iteration when i equals 3
      }
      printf("%d ", i);
  }
  // Output: 1 2 4 5

Conclusion:

• The for loop is useful when you know the number of iterations in advance.
• The while loop is best when the number of iterations is unknown, and the loop needs to continue as long as a condition is true.
• The do-while loop ensures that the loop body is executed at least once, making it suitable for scenarios where you need the loop to run regardless of the condition.

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