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C Programming with DSA Interview Questions in Ahmedabad

36 practice questions with answers for C Programming with DSA interviews in Ahmedabad.

C Programming with DSA jobs in Ahmedabad

Ahmedabad and GIFT City have a growing market for IT services, fintech and product start-ups. Companies here value hands-on skills and the ability to work across tools, especially in smaller teams.

Below are 36 questions that match the mix of service, product and domain employers hiring c developers in Ahmedabad.

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01

Use qsort with a function pointer to sort an array of integers in descending order.

Pointers and Memory ManagementMedium
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Write a comparator that returns negative, zero or positive. int cmpDesc(const void *a, const void *b) { int x = *(const int *)a; int y = *(const int *)b; return (y > x) - (y < x); } int arr[] = {4, 1, 9, 3}; qsort(arr, 4, sizeof arr[0], cmpDesc); Returning y - x can overflow, so the comparison form is safer.
02

A doubly linked list delete function leaves the list broken when deleting the head. What is missing?

Linked ListsMedium
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When deleting the head there is no previous node, so the head pointer itself must move to head->next, and the new head's prev must be set to NULL. Similarly, deleting the tail needs the new tail's next set to NULL. The fix is to check node->prev and node->next for NULL and update head or tail pointers accordingly, or use a sentinel node to avoid these cases.
03

Write BFS on a graph stored as an adjacency matrix.

Trees and GraphsMedium
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Use a queue and a visited array. void bfs(int n, int adj[n][n], int start) { int visited[n], queue[n], front = 0, rear = 0; for (int i = 0; i < n; i++) visited[i] = 0; visited[start] = 1; queue[rear++] = start; while (front < rear) { int u = queue[front++]; printf("%d ", u); for (int v = 0; v < n; v++) { if (adj[u][v] && !visited[v]) { visited[v] = 1; queue[rear++] = v; } } } }
04

How would you find the kth smallest element in an unsorted array efficiently?

Sorting, Searching and Dynamic ProgrammingHard
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Sorting takes O(n log n). A max heap of size k gives O(n log k), good when k is small or data streams in. Quickselect partitions like quicksort but recurses into one side only, giving O(n) average and O(n squared) worst case, which random pivots make unlikely. Median of medians guarantees O(n) worst case but is slower in practice.
05

What are bit fields in C?

Structures, Unions and File HandlingMedium
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Bit fields let struct members use a specified number of bits, such as unsigned int ready : 1; unsigned int mode : 3;, which saves memory for flags. They are common in embedded code and protocol headers. However, bit ordering, padding and whether plain int bit fields are signed are implementation defined, so for portable hardware register or wire format access, explicit masks and shifts are safer.
06

Count the number of islands in a grid of land and water cells. What approach do you take?

Trees and GraphsMedium
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Treat each land cell as a graph node connected to its four neighbours. Scan the grid, and when you find unvisited land, increment the count and flood fill with DFS or BFS to mark the whole island visited. Time and space are O(rows times cols). For very large grids, use iterative BFS to avoid recursion depth issues, or union-find.
07

Write Kadane's algorithm in C.

Arrays and StringsMedium
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Track the best sum ending here and the overall best. long maxSubarray(const int *a, int n) { long cur = a[0], best = a[0]; for (int i = 1; i < n; i++) { cur = (cur + a[i] > a[i]) ? cur + a[i] : a[i]; if (cur > best) best = cur; } return best; } Input {-2, 1, -3, 4, -1, 2, 1, -5, 4} returns 6 for subarray 4, -1, 2, 1.
08

Rotate an array to the right by k positions in O(n) time and O(1) space.

Arrays and StringsMedium
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Use three reversals. void rev(int *a, int l, int r) { while (l < r) { int t = a[l]; a[l++] = a[r]; a[r--] = t; } } void rotate(int *a, int n, int k) { k %= n; if (k == 0) return; rev(a, 0, n - 1); rev(a, 0, k - 1); rev(a, k, n - 1); } For 1 2 3 4 5 with k = 2 the result is 4 5 1 2 3.
09

What is structure padding and how can you reduce it?

Structures, Unions and File HandlingMedium
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The compiler inserts padding bytes between members so each one is aligned, which can make a struct larger than the sum of its members. struct { char a; int b; char c; } is usually 12 bytes instead of 6. Reduce padding by ordering members from largest to smallest alignment. Packing with __attribute__((packed)) removes padding but can slow access or fault on some CPUs.
10

What is the difference between an array and a pointer?

Pointers and Memory ManagementMedium
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An array is a block of contiguous elements whose size is fixed and known to the compiler; sizeof gives the full array size. A pointer is a variable holding an address; sizeof gives the pointer size. An array name decays to a pointer to its first element in most expressions, but it cannot be reassigned. When passed to a function, an array always becomes a pointer, losing its length.
11

What is the two-pointer technique and when is it used?

Arrays and StringsEasy
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Two pointers keep two indexes that move through an array according to a rule, often from both ends of a sorted array or at different speeds. It solves pair sum on sorted arrays, removing duplicates in place, reversing, merging sorted arrays and partitioning. It usually gives O(n) time and O(1) extra space instead of nested loops.
12

Which gcc flags would you use to build and debug a C program safely?

C Basics, Data Types and OperatorsMedium
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A good development build is: gcc -std=c17 -Wall -Wextra -Wpedantic -g -O1 \ -fsanitize=address,undefined \ -o app main.c utils.c -Wall and -Wextra turn on useful warnings, -g adds debug symbols for gdb, and sanitizers catch memory errors and undefined behaviour at runtime. For release, drop sanitizers and use -O2.
13

What is the difference between char s[] = "hello" and char *s = "hello"?

Arrays and StringsEasy
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char s[] = "hello" creates a modifiable array of 6 bytes, usually on the stack, initialised with a copy of the literal. char *s = "hello" makes a pointer to a string literal stored in read-only memory; modifying it is undefined behaviour and often crashes. Declare such pointers as const char * so the compiler catches accidental writes.
14

What is a deque and how can it be implemented in C?

Stacks, Queues and HashingEasy
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A deque, or double ended queue, supports insertion and removal at both the front and the rear in O(1). In C it is commonly implemented as a circular array with front and rear indexes that wrap using modulo, or as a doubly linked list. It is the core of the sliding window maximum algorithm and can act as both a stack and a queue.
15

Explain Kadane's algorithm.

Arrays and StringsMedium
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Kadane's algorithm finds the maximum sum contiguous subarray in O(n). It keeps the best sum ending at the current index: cur = max(a[i], cur + a[i]), and updates the global best with best = max(best, cur). If the running sum becomes worse than starting fresh, it starts a new subarray. Initialise with the first element to handle arrays where all numbers are negative.
16

What is a monotonic stack?

Stacks, Queues and HashingMedium
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A monotonic stack keeps elements in increasing or decreasing order by popping elements that break the order before pushing a new one. Each element is pushed and popped at most once, so problems run in O(n). It solves next greater element, daily temperatures, stock span and largest rectangle in a histogram, which would otherwise need nested loops.
17

Implement union-find with path compression and union by rank.

Trees and GraphsHard
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Two small functions cover most uses. int parent[MAXN], rnk[MAXN]; void makeSet(int n) { for (int i = 0; i < n; i++) { parent[i] = i; rnk[i] = 0; } } int find(int x) { if (parent[x] != x) parent[x] = find(parent[x]); return parent[x]; } int unite(int a, int b) { a = find(a); b = find(b); if (a == b) return 0; if (rnk[a] < rnk[b]) { int t = a; a = b; b = t; } parent[b] = a; if (rnk[a] == rnk[b]) rnk[a]++; return 1; }
18

Freeing a linked list causes a crash. What is the common mistake?

Linked ListsMedium
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A common bug is freeing the current node and then reading current->next, which is a use-after-free. Save the next pointer first, then free the node, then move on. Also make sure no node is freed twice and that the caller's head pointer is set to NULL afterwards so it is not used again. Valgrind or AddressSanitizer points to the exact line.
19

What is dynamic programming?

Sorting, Searching and Dynamic ProgrammingMedium
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Dynamic programming solves problems that have overlapping subproblems and optimal substructure by storing results of subproblems instead of recomputing them. Top-down memoisation caches recursive calls, while bottom-up tabulation fills a table iteratively. Classic examples are Fibonacci, 0/1 knapsack, longest common subsequence, coin change and edit distance. Defining the state and transition clearly is the hardest part.
20

Explain the memory layout of a C program.

C Basics, Data Types and OperatorsMedium
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From low to high address, a typical process has the text segment holding code, the initialised data segment for globals and statics with values, BSS for uninitialised or zero globals, the heap growing upward for malloc memory, and the stack growing downward for function frames and local variables. Knowing this helps explain stack overflow, memory leaks and where variables live.
21

Your stack implementation crashes with deep recursion converted to an explicit stack. What would you check?

Stacks, Queues and HashingMedium
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Check for stack overflow of your own array: push must verify top < capacity and either fail or grow the array with realloc. Check pop on an empty stack returns an error instead of reading index -1. If using realloc, make sure no pointers into the old buffer are kept. Add asserts in debug builds and test with the largest expected input.
22

A program crashes with a segmentation fault. How do you find the cause?

Pointers and Memory ManagementMedium
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Compile with -g and run under gdb; when it crashes, use bt to see the call stack and print variables in the failing frame. Common causes are dereferencing NULL, using freed memory, out-of-bounds array access and stack overflow from deep recursion. Building with -fsanitize=address gives a precise report of the bad access and where the memory was allocated or freed.
23

Tell me about a time you explained a complex data structure to your team.

Linked ListsMedium
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Situation: our team needed an LRU cache in a C service and several members had not built one. Task: align everyone before coding. Action: I drew the hash table and doubly linked list on a whiteboard, walked through get and put with a small example and shared a short test suite covering edge cases. Result: the team implemented it in two days with no major review issues.
24

What are the main stages of compiling a C program?

C Basics, Data Types and OperatorsEasy
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Preprocessing expands #include files and macros and removes comments. Compilation turns the preprocessed code into assembly. Assembly converts it into an object file of machine code. Linking combines object files with libraries such as libc to produce the executable. With gcc you can stop at each stage using -E, -S and -c, which helps when debugging macro or linker errors.
25

Write a function that swaps two integers using pointers.

Pointers and Memory ManagementEasy
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Pass addresses so the function can modify the caller's variables. void swap(int *a, int *b) { int temp = *a; *a = *b; *b = temp; } int main(void) { int x = 3, y = 7; swap(&x, &y); printf("%d %d\n", x, y); // 7 3 return 0; } C passes everything by value, so pointers are needed to change caller data.
26

Why is gets() dangerous and what should you use instead?

Arrays and StringsEasy
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gets() reads a line into a buffer without knowing its size, so long input overflows the buffer and can be used for code execution attacks. It was removed in C11. Use fgets(buf, sizeof buf, stdin), which stops at the buffer size and keeps the newline, then strip the newline if needed. For formatted input, give scanf field widths like %19s.
27

What is the difference between strcpy, strncpy and snprintf for copying strings?

Arrays and StringsMedium
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strcpy copies until the null terminator with no bounds check, so it can overflow. strncpy limits the count but does not add a terminator if the source is too long and pads with zeros otherwise, which surprises many developers. snprintf(dst, size, "%s", src) always terminates and returns the length it wanted, so you can detect truncation. Prefer snprintf or a checked helper.
28

What is topological sorting and where is it used?

Trees and GraphsHard
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A topological sort orders the vertices of a directed acyclic graph so every edge goes from earlier to later. Kahn's algorithm repeatedly removes vertices with zero in-degree using a queue; DFS adds vertices to the order after finishing their descendants. If not all vertices are processed, the graph has a cycle. It is used for build systems, course prerequisites and task scheduling.
29

Write a C function to check whether a number is an Armstrong number.

C Basics, Data Types and OperatorsEasy
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Sum of digits raised to the number of digits equals the number. int isArmstrong(int n) { int digits = 0, t = n, sum = 0; while (t) { digits++; t /= 10; } t = n; while (t) { int d = t % 10, p = 1; for (int i = 0; i < digits; i++) p *= d; sum += p; t /= 10; } return sum == n; } For example 153 = 1 + 125 + 27.
30

How do you prioritise between writing fast code and readable code?

Linked ListsMedium
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Situation: a teammate replaced a clear linked list loop with a complex XOR trick to save memory. Task: decide what to keep. Action: I measured memory and speed, found the saving was under one percent and proposed keeping the readable version with a comment, while optimising a real hotspot the profiler showed. Result: we kept clear code and got a measurable 20 percent speedup elsewhere.
31

You need to implement a queue using two stacks. How does it work and what is its cost?

Stacks, Queues and HashingMedium
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Use an in-stack for enqueue and an out-stack for dequeue. Enqueue always pushes to the in-stack. Dequeue pops from the out-stack, and only when it is empty moves all elements from the in-stack to it, which reverses their order so the oldest comes out first. Each element moves at most twice, so operations are O(1) amortised even though a single dequeue can be O(n).
32

What is a circular linked list and where is it used?

Linked ListsEasy
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In a circular linked list the last node points back to the first, so there is no NULL end. You can start at any node and visit all nodes, and keeping only a tail pointer gives O(1) access to both ends. It is used in round-robin CPU scheduling, multiplayer turn order and circular buffers. Traversal loops must stop when they return to the starting node.
33

What is a hash table and how are collisions handled?

Stacks, Queues and HashingMedium
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A hash table maps keys to array slots using a hash function, giving O(1) average insert, search and delete. Collisions happen when two keys map to the same slot. Separate chaining stores a linked list per slot. Open addressing probes for another free slot using linear probing, quadratic probing or double hashing. Keep the load factor low and resize when it grows to maintain speed.
34

Tell me about a time you optimised an algorithm in a real project.

Sorting, Searching and Dynamic ProgrammingMedium
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Situation: a C tool comparing two inventory files of 5 lakh rows took 25 minutes. Task: make it run in a nightly window of 2 minutes. Action: I found nested loops doing O(n squared) matching, sorted both files with qsort and merged them with two pointers, which is O(n log n). Result: runtime fell to 40 seconds, and the approach became our standard for file reconciliation jobs.
35

You need to find the shortest route between two cities on a map with distances. Which algorithm do you use and why?

Trees and GraphsMedium
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Road distances are non-negative, so Dijkstra's algorithm with a min heap is the right choice, giving O((V + E) log V). If you only need one target, stop when it is popped from the heap. For very large maps, A star with a straight-line distance heuristic explores fewer nodes. If some edges had negative costs, Bellman-Ford would be needed.
36

Explain storage classes in C.

C Basics, Data Types and OperatorsMedium
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auto is the default for local variables, stored on the stack with block lifetime. register hints the variable should live in a CPU register, and its address cannot be taken. static gives a local variable lifetime for the whole program, or limits a global to its file. extern declares a variable defined in another file. Storage class decides scope, lifetime and linkage.

Frequently asked questions

Which companies hire C Programming with DSA professionals in Ahmedabad?

Companies with large offices in Ahmedabad include many IT services and product firms. Use their company pages below to practise employer-specific sets.

Are C Programming with DSA interviews in Ahmedabad different from other cities?

The questions depend more on the company than the city. Ahmedabad's mix of employers decides the style: Companies here value hands-on skills and the ability to work across tools, especially in smaller teams

How do I find C Programming with DSA jobs in Ahmedabad?

Shortlist companies with Ahmedabad offices, keep your CV focused on C Programming with DSA projects, apply on company career pages and job portals with the Ahmedabad filter, and practise the questions on this page before each interview.

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