C Programming — Dynamic Memory Questions & Answers

Q.1 Medium Dynamic Memory

What is the correct way to store variable-length strings dynamically?

A char str[MAX]; gets(str);

B char *str = (char*)malloc(length+1); fgets(str, length+1, stdin);

C char *str; scanf("%s", str);

D char str[] = malloc(100);

Correct Answer: B. char *str = (char*)malloc(length+1); fgets(str, length+1, stdin);

EXPLANATION

Proper way requires dynamic allocation with malloc, adding 1 for null terminator, and using fgets for safe input.

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Q.2 Medium Dynamic Memory

In competitive programming for graph problems, if adjacency list uses dynamic arrays, what is the time complexity of adding an edge?

A O(1) amortized

B O(n)

C O(log n)

D O(n²)

Correct Answer: A. O(1) amortized

EXPLANATION

Using dynamic arrays (vectors in C++) with doubling strategy provides O(1) amortized time for appending edges.

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Q.3 Medium Dynamic Memory

Which scenario is most likely to cause a segmentation fault in dynamic memory?

A Calling malloc() multiple times

B Accessing memory beyond allocated size

C Freeing NULL pointer

D Calling realloc() on allocated memory

Correct Answer: B. Accessing memory beyond allocated size

EXPLANATION

Buffer overflow by accessing beyond allocated boundaries causes segmentation fault as it accesses invalid memory pages.

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Q.4 Medium Dynamic Memory

For implementing a dynamic circular buffer of size n, how many pointers are minimally required?

A 1 pointer

B 2 pointers

C 3 pointers

D 4 pointers

Correct Answer: B. 2 pointers

EXPLANATION

A circular buffer requires head and tail pointers minimum to track current positions in the circular structure.

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Q.5 Medium Dynamic Memory

What happens when realloc() is called on a pointer allocated by malloc() with a larger size?

A Old memory is freed and new memory is allocated

B Returns NULL always

C Reallocates in same location or new location, preserving old data

D Causes undefined behavior

Correct Answer: C. Reallocates in same location or new location, preserving old data

EXPLANATION

realloc() attempts to expand memory in place if possible; if not, allocates new block, copies old data, and frees old block.

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Q.6 Medium Dynamic Memory

Which of the following correctly deallocates a 2D dynamically allocated array created as int **arr?

A free(arr);

B for(int i=0;i

C delete arr;

D free(**arr);

Correct Answer: B. for(int i=0;i

EXPLANATION

A 2D array created with double pointers requires freeing each row first, then the pointer array itself to avoid memory leaks.

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Q.7 Medium Dynamic Memory

What happens when you try to free a pointer that was not allocated by malloc/calloc/realloc?

A Undefined behavior - likely crash or memory corruption

B The program ignores the free() call

C It frees some random memory safely

D Compilation error occurs

Correct Answer: A. Undefined behavior - likely crash or memory corruption

EXPLANATION

Calling free() on a pointer that wasn't returned by malloc/calloc/realloc causes undefined behavior, potentially corrupting the heap and crashing the program.

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Q.8 Medium Dynamic Memory

Which function should be used to allocate memory for an array of 50 structures of size 'sizeof(struct Node)' and initialize to zero?

A calloc(50, sizeof(struct Node))

B malloc(50 * sizeof(struct Node))

C realloc(NULL, 50)

D alloc(50, sizeof(struct Node))

Correct Answer: A. calloc(50, sizeof(struct Node))

EXPLANATION

calloc(50, sizeof(struct Node)) allocates memory for 50 elements of struct Node size and initializes all bytes to zero, which is ideal when you need initialized memory.

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Q.9 Medium Dynamic Memory

What is the advantage of dynamic allocation over static allocation in competitive programming?

A Flexibility to use memory based on input size, avoiding stack limits

B Faster execution speed

C No need to include headers

D Automatic memory cleanup

Correct Answer: A. Flexibility to use memory based on input size, avoiding stack limits

EXPLANATION

Dynamic allocation allows using memory proportional to actual input size, preventing TLE from stack overflow with large static arrays and enabling solutions for variable-sized problems.

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Q.10 Medium Dynamic Memory

In graph algorithms with dynamic adjacency lists, what's the memory requirement for a sparse graph with V vertices and E edges?

A O(V + E)

B O(V²)

C O(E²)

D O(V * E)

Correct Answer: A. O(V + E)

EXPLANATION

Using dynamic adjacency lists, you allocate V list pointers and E edge nodes total, giving O(V + E) space complexity, which is optimal for sparse graphs.

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