some tricky c interview questions to practice

some tricky c interview questions to practice. Learn c and c++ programming interview skill. predicting output of sample program is way of refreshing your basic concepts.

1.what could be output of below c code?

#include <stdio.h>

int main()
{
   printf ("%x" ,-1<<4);
   return 0;
}

output:
fffffff0

Technically left-shifting a negative integer invokes Undefined Behaviour. That means -1<<4 is UB
C99 [6.5.7/4] says

The result of E1 << E2 is E1 left-shifted E2 bit positions; vacated bits 
are filled with zeros. If E1 has an unsigned type, the value of the result 
is E1× 2E2, reduced modulo one more than the maximum value representable in
 the result type. If E1 has a signed type and nonnegative value, and E1× 2E2
 is representable in the result type, then that is the resulting value; 
otherwise, the behavior is un-defined.

2. write c code for the below problem.

Given a number n, give me the numbers (among 3..5 and an even number of numbers) whose adding would return the original number.
The resulting numbers should be as balanced as possible, meaning that instead of returning 3 and 5, for instance, return 4 and 4. Ex:

7 = 3 + 4
16 = 4 + 4 + 4 + 4 rather than 3 + 5 + 4 + 4
24 = 12 + 12 or 6 + 6 + 6 + 6

3. will the below code compile??

#include <stdio.h>
#include <time.h>

int f()
{
   return 2;
}

int main()
{
    static int i = f();
    printf("%d\n", i);
}

ans: NO, 
test_1.c:11:5: error: initializer element is not constant
     static int i = f();
     ^
For all static variables (and global variables), the compiler must ensure 
that they are initialized before main() is called. Even static variables 
that aren't explicitly set to a value by the programmer, must be initialized
 to zero or NULL

4. What’s the output of the following program?

#include <stdio.h>

void Unknown(int *p, int num);
void HardToFollow(int *p, int q, int *num);

void Unknown(int *p, int num) {
  int *q;

  q = &num;
  *p = *q + 2;
  num = 7;
}


void HardToFollow(int *p, int q, int *num) {
  *p = q + *num;
  *num = q;
  num = p;
  p = &q;
  Unknown(num, *p);
}

int main() 
{
  int *q;
  int trouble[3];

  trouble[0] = 1;
  q = &trouble[1];
  *q = 2;
  trouble[2] = 3;

  HardToFollow(q, trouble[0], &trouble[2]);
  Unknown(&trouble[0], *q);

  printf("%d %d %d \n", *q,*trouble,*(trouble+2));
  return 0;
}

5. Write a c program to find size of structure without using sizeof operator?

#include <stdio.h>
 
struct  foo {
    float y;
    char z;
};
 
int main()
{
    struct foo p;
    struct foo *q = &p;
    int size = (char*)(q+1)  - (char*)q;
    printf("%d\n",size);
    return 0;
}

6. How can you find if a variable is allocated in stack or heap?

7. What could be the output of below program?

#include<stdio.h>

int main()
{
  printf("%lu\n",sizeof(2.4));
  return 0;
}

8. Add two numbers without using + or – operator in c.

#include<stdio.h>

int add(int, int);
int main() {
	int a,b,sum;
	printf("enter two numbers:");
	scanf("%d %d",&a,&b);
	sum=add(a,b);
	printf("%d\n",sum);
    return 0;
}

int add(int a, int b) {
	int sum, carry;
	if (b == 0)
		return a;
	else
		sum = a ^ b; // add without carrying
	carry = (a & b) << 1; // carry, but don’t add
	return add(sum, carry); // recursion
}

Ref:

https://www.quora.com/How-do-I-add-two-numbers-without-using-any-operator-C-C++

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