C Data type - void

Linguistic meaning of void is nothing. Size of void data type is meaningless question.

What will be output of following c code?
#include<stdio.h>
int main(){
    int size;
    size=sizeof(void);
    printf("%d",size);
    return 0;
}

Output: Compilation error

If we will try to find the size of void data type complier will show an error message “not type allowed”. We cannot use any storage class modifier with void data type so void data type doesn’t reserve any memory space. Hence we cannot declare any variable as of void type i.e.
void num;   //compilation error

Use of void data type

1. To declare generic pointer
2. As a function return type
3. As a function parameter.    

Write a C program to check prime number or not without using recursion

#include<stdio.h>

int isPrime(int);

int main(){

    int num,prime;

    printf("Enter a positive number: ");
    scanf("%d",&num);

    prime = isPrime(num);

   if(prime==1)
        printf("%d is a prime number",num);
   else
      printf("%d is not a prime number",num);

   return 0;
}

int isPrime(int num){

    int i=2;

    while(i<=num/2){
         if(num%i==0)
             return 0;
         else
             i++;
    }

    return 1;
}

Prime number program in c using recursion

#include<stdio.h>

int isPrime(int);

int main(){

    int num,prime;

    printf("Enter a positive number: ");
    scanf("%d",&num);

    prime = isPrime(num);

   if(prime==1)
        printf("%d is a prime number",num);
   else
      printf("%d is not a prime number",num);

   return 0;
}

int isPrime(int num){

    static int i=2;

    if(i<=num/2){
         if(num%i==0)
             return 0;
         else{
             i++;
             isPrime(num);
         }
    }

    return 1;
}

C interview questions and answers

1. Write a c program without using any semicolon which output will : Hello word.


Solution: 1
void main(){
    if(printf("Hello world")){
    }
}
Solution: 2
void main(){
    while(!printf("Hello world")){
    }
}
Solution: 3
void main(){
    switch(printf("Hello world")){
    }
}

2. Write a C program to Swap two variables without using third variable.

int main()
{
    int a=5,b=10;
//process one
    a=b+a;
    b=a-b;
    a=a-b;
    printf("a= %d  b=  %d",a,b);

//process two
    a=5;
    b=10;
    a=a+b-(b=a);
    printf("\na= %d  b=  %d",a,b);
//process three
    a=5;
    b=10;
    a=a^b;
    b=a^b;
    a=b^a;
    printf("\na= %d  b=  %d",a,b);
  
//process four
    a=5;
    b=10;
    a=b-~a-1;
    b=a+~b+1;
    a=a+~b+1;
    printf("\na= %d  b=  %d",a,b);
  
//process five
    a=5,
    b=10;
    a=b+a,b=a-b,a=a-b;
    printf("\na= %d  b=  %d",a,b);
    getch();

}

3. What is dangling pointer in c?

If any pointer is pointing the memory address of any variable but after some variable has deleted from that memory location while pointer is still pointing such memory location. Such pointer is known as dangling pointer and this problem is known as dangling pointer problem.

Initially:

Later: 

4. What is wild pointer in c ?

A pointer in c which has not been initialized is known as wild pointer.

Example:

(q)What will be output of following c program?

void main(){
int *ptr;
printf("%u\n",ptr);
printf("%d",*ptr);

}

Output: Any address
Garbage value

Here ptr is wild pointer because it has not been initialized.
There is difference between the NULL pointer and wild pointer. Null pointer points the base address of segmentwhile wild pointer doesn’t point any specific memory location.

5. What is the meaning of prototype of a function ?

Prototype of a function
Answer: Declaration of function is known as prototype of a function. Prototype of a function means
(1) What is return type of function?
(2) What parameters are we passing?
(3) For example prototype of printf function is:
int printf(const char *, …);
I.e. its return type is int data type, its first parameter constant character pointer and second parameter is ellipsis i.e. variable number of arguments.


6. What are merits and demerits of array in c?

(a) We can easily access each element of array.
(b) Not necessity to declare two many variables.
(c) Array elements are stored in continuous memory location.

Demerit:
(a) Wastage of memory space. We cannot change size of array at the run time.
(b) It can store only similar type of data.