Computer Knowledge — MCQ Practice Questions & Answers

Question 1

What will be the output of the following C code? int main() { int x = 5; int *p = &x; int **q = &p; printf("%d", **q + 1); return 0; }

Accepted Answer

Correct answer: A. 6 — The correct answer is 6 because **q is a pointer to pointer that points to p, which points to x. When we dereference **q, we get the value of x (which is 5), and then adding 1 gives us 6, which is printed. The key concept here is understanding pointer dereferencing: a single asterisk dereferences one level, so double asterisks dereference two levels, bringing us from q to p to the actual value stored in x. Any other answer would be incorrect because it would either misunderstand the double dereferencing operation or incorrectly interpret what value is being accessed—for instance, if someone only dereferenced once or forgot to add 1, they would get a different result.

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B. 5

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C. Compilation error

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D. Garbage value

Question 2

What is the output of the following code snippet? int main() { char arr[] = "GATE"; char *p = arr; printf("%d
", sizeof(arr)); printf("%d", sizeof(p)); return 0; }

Accepted Answer

Correct answer: C. 5 and 8 (on 64-bit system) — The output is 5 and 8 because sizeof(arr) returns 5 bytes (4 characters plus 1 null terminator in the string "GATE"), while sizeof(p) returns 8 bytes on a 64-bit system since p is a pointer variable and pointers occupy 8 bytes in 64-bit architecture. The key concept is that sizeof() behaves differently for arrays versus pointers: when applied to an array, it returns the total memory allocated for the entire array, but when applied to a pointer, it only returns the size of the pointer itself, not what it points to. Other options would be incorrect because they either miscalculate the array size by forgetting the null terminator, use incorrect pointer sizes (like 4 bytes for 32-bit systems), or confuse the size of the array with the size of the pointer variable.

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A. 4 and 4

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B. 5 and 4

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D. 4 and 8 (on 64-bit system)

Question 3

Which of the following operations has an average time complexity of O(1) in a Hash Table with good hash function and low collision rate?

Accepted Answer

Correct answer: C. Both insertion and search — Hash Tables achieve O(1) average-case time complexity for both insertion and search operations when using a well-designed hash function that minimizes collisions and maintains a good load factor. Deletion also operates in O(1) average time. However, in worst-case scenarios with poor hash functions or high collision rates, these operations can degrade to O(n). The key to performance is maintaining low collision rates through techniques like chaining or open addressing.

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A. Insertion only

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B. Search only

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D. Deletion only

Question 4

Which data structure is most suitable for implementing a priority queue where elements with higher priority need to be dequeued first?

Accepted Answer

Correct answer: C. Min-Heap or Max-Heap — A Heap (either Min-Heap or Max-Heap) is the optimal data structure for implementing priority queues with O(log n) insertion and deletion time complexity. In a Max-Heap, the element with the highest priority is always at the root, enabling efficient extraction of the maximum element. Arrays and Linked Lists would require O(n) time for priority-based operations, while Graphs are unsuitable for this purpose. Heaps are fundamental in algorithms like Dijkstra's and Prim's for finding shortest paths.

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A. Array

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B. Linked List

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D. Graph

Question 5

What is the output of the following C code? int x = 5; int y = ++x + x++; printf("%d", y);

Accepted Answer

Correct answer: D. Undefined behavior — This code exhibits undefined behavior because x is modified twice between sequence points without an intervening sequence point. The result depends on the compiler's implementation.

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A. 11

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B. 12

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C. 13

Question 6

What is the difference between scanf() and gets() functions?

Accepted Answer

Correct answer: A. scanf() can read formatted input while gets() reads only strings — scanf() reads formatted input based on format specifiers and stops at whitespace. gets() reads a string until a newline is encountered. Note: gets() is deprecated due to buffer overflow vulnerabilities.

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B. gets() is safer than scanf()

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C. There is no difference

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D. scanf() stops at whitespace while gets() doesn't

Question 7

What is the correct way to allocate memory for a single integer using malloc()?

Accepted Answer

Correct answer: D. Both A and B — Both A and B are correct. Option A uses explicit type casting which is optional in C (not in C++). Option B avoids casting. Using sizeof(int) is preferred over hardcoding 4, as int size may vary across systems.

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A. int *ptr = (int *)malloc(sizeof(int));

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B. int *ptr = malloc(sizeof(int));

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C. int *ptr = (int *)malloc(4);

Question 8

What is the output of the following C code? int x = 5; int *ptr = &x; printf("%d %d", *ptr, x);

Accepted Answer

Correct answer: A. 5 5 — *ptr dereferences the pointer to access the value at the address it points to, which is x = 5. So both *ptr and x print 5.

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B. Address Address

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C. Garbage value 5

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D. Compilation error

Computer Knowledge - MCQ Practice Questions