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Sample Technical Questions

C Sample Question

141. main()
     {
     		char *p="GOOD";
     	char a[ ]="GOOD";
     printf("\n sizeof(p) = %d, sizeof(*p) = %d, strlen(p) = %d", sizeof(p), sizeof(*p), strlen(p));
     	printf("\n sizeof(a) = %d, strlen(a) = %d", sizeof(a), strlen(a));
     }

     Answer:

     sizeof(p) = 2, sizeof(*p) = 1, strlen(p) = 4
     sizeof(a) = 5, strlen(a) = 4

     Explanation:

     sizeof(p) => sizeof(char*) => 2
     sizeof(*p) => sizeof(char) => 1
     Similarly,
     sizeof(a) => size of the character array => 5
     When sizeof operator is applied to an array it returns the sizeof the array and it is not the same as the sizeof the pointer variable. Here the sizeof(a) where a is the character array and the size of the array is 5 because the space necessary for the terminating NULL character should also be taken into account.

142. #define DIM( array, type) sizeof(array)/sizeof(type)
     main()
     {
     int arr[10];
     printf("The dimension of the array is %d", DIM(arr, int));    
     }

     Answer:

     10

     Explanation:

     The size of integer array of 10 elements is 10 * sizeof(int). The macro expands to sizeof(arr)/sizeof(int) => 10 * sizeof(int) / sizeof(int) => 10.

143. int DIM(int array[]) 
     {
     return sizeof(array)/sizeof(int );
     }
     main()
     {
     int arr[10];
     printf("The dimension of the array is %d", DIM(arr));    
     }

     Answer:

     1

     Explanation:

     Arrays cannot be passed to functions as arguments and only the pointers can be passed. So the argument is equivalent to int * array (this is one of the very few places where [] and * usage are equivalent). The return statement becomes, sizeof(int *)/ sizeof(int) that happens to be equal in this case.

144. main()
     {
     	static int a[3][3]={1,2,3,4,5,6,7,8,9};
     	int i,j;
     	static *p[]={a,a+1,a+2};
     		for(i=0;i<3;i++)
     	{
     			for(j=0;j<3;j++)
     			printf("%d\t%d\t%d\t%d\n",*(*(p+i)+j),
     			*(*(j+p)+i),*(*(i+p)+j),*(*(p+j)+i));
     		}
     }

     Answer:

     			1       1       1       1
     			2       4       2       4
     			3       7       3       7
     			4       2       4       2
     			5       5       5       5
     			6       8       6       8
     			7       3       7       3
     			8       6       8       6
     			9       9       9       9

     Explanation:

     *(*(p+i)+j) is equivalent to p[i][j].

145. main()
     {
     		void swap();
     	int x=10,y=8;     
     		swap(&x,&y);
     	printf("x=%d y=%d",x,y);
     }
     void swap(int *a, int *b)
     {
        *a ^= *b,  *b ^= *a, *a ^= *b; 
     }

     Answer:

     x=10 y=8

     Explanation:

     Using ^ like this is a way to swap two variables without using a temporary variable and that too in a single statement.
     Inside main(), void swap(); means that swap is a function that may take any number of arguments (not no arguments) and returns nothing. So this doesn't issue a compiler error by the call swap(&x,&y); that has two arguments.
     This convention is historically due to pre-ANSI style (referred to as Kernighan and Ritchie style) style of function declaration. In that style, the swap function will be defined as follows, void swap()

     int *a, int *b
     {
        *a ^= *b,  *b ^= *a, *a ^= *b; 
     }

     where the arguments follow the (). So naturally the declaration for swap will look like, void swap() which means the swap can take any number of arguments.

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