Infosys C Programming with DSA Interview Questions & Answers
Curated interview practice questions with answers. These are practice scenarios, not a verified list of past interview questions.
About Infosys hiring for C Programming with DSA roles
Infosys is an Indian multinational IT services and consulting company offering digital transformation, cloud, application management, testing and data services to global clients. It is known for its Mysuru training campus where new hires go through foundation programmes. In India it recruits for roles such as systems engineer, test engineer, data analyst and data engineer, often through campus and off-campus drives.
For C Programming with DSA roles, this Infosys practice set focuses most on C Basics, Data Types and Operators (7), Pointers and Memory Management (6), Structures, Unions and File Handling (4), Linked Lists (3). It has 14 easy, 15 medium, 1 hard questions, so you can start easy and work up to the harder ones.
Typical pattern based on public candidate experiences. Rounds vary by role, team and year.
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01
Freeing a linked list causes a crash. What is the common mistake?
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.
02
What is a deque and how can it be implemented in C?
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.
03
A search on a sorted array of 10 lakh elements is done with a linear loop. How would you justify changing it?
Linear search takes up to 10 lakh comparisons, while binary search needs about 20, because log base 2 of 10 lakh is roughly 20. If searches are frequent, this is a huge saving at zero memory cost. If data changes often, keeping it sorted costs inserts, so consider a balanced tree or hash table. Measure with realistic workloads before and after.
04
What is the difference between a complete, full and perfect binary tree?
A full binary tree has every node with zero or two children. A complete binary tree has all levels filled except possibly the last, which is filled from left to right; heaps use this shape so they fit an array. A perfect binary tree has all internal nodes with two children and all leaves at the same level, so it has 2 to the power h+1 minus 1 nodes.
05
Write a C function to check whether a number is an Armstrong number.
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.
06
You need to find whether any two numbers in an unsorted array sum to a target. Compare approaches.
Brute force checks all pairs in O(n squared). Sorting first with qsort and then using two pointers takes O(n log n) time and O(1) extra space but loses original indexes. A hash set gives O(n) time with O(n) memory; in C you would implement a simple open addressing table. Choose based on size, memory limits and whether indexes must be returned.
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.
08
What is a circular linked list and where is it used?
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.
09
Find the lowest common ancestor of two nodes in a binary search tree.
Use the BST order to walk down from the root.
TNode *lcaBST(TNode *root, int a, int b) {
while (root) {
if (a < root->key && b < root->key) root = root->left;
else if (a > root->key && b > root->key) root = root->right;
else return root;
}
return NULL;
}
The first node where a and b split is the LCA. Time O(h) and space O(1).
10
What is the difference between const int *p, int *const p and const int *const p?
const int *p is a pointer to a constant int: you cannot change the value through p, but p can point elsewhere. int *const p is a constant pointer: p always points to the same address, but the value can change. const int *const p fixes both. Read declarations from right to left to understand them. const pointers to data are common in function parameters.
11
What is the difference between break and continue in C?
break immediately exits the nearest enclosing loop or switch statement, and execution continues after it. continue skips the rest of the current loop iteration and moves to the next one; in a for loop the update expression still runs. In nested loops both affect only the innermost loop, so a flag or goto is needed to exit outer loops early.
12
Find the next greater element for every array element using a monotonic stack.
Keep indexes of elements still waiting for a greater value.
void nextGreater(const int *a, int n, int *res) {
int st[n], top = -1;
for (int i = 0; i < n; i++) {
res[i] = -1;
while (top >= 0 && a[st[top]] < a[i]) {
res[st[top--]] = a[i];
}
st[++top] = i;
}
}
For 4 5 2 25 the result is 5 25 25 -1, in O(n) time.
13
A program reads a file with while (!feof(fp)) and processes the last line twice. Why?
feof() becomes true only after a read has already failed at end of file. The loop checks feof before reading, so the final failed read leaves the old buffer content, which is processed again. Loop on the read function's return value instead, such as while (fgets(line, sizeof line, fp) != NULL), and check ferror after the loop.
14
How do you convert an infix expression to postfix?
Use the shunting yard algorithm with an operator stack. Output operands directly. For an operator, pop operators of higher precedence, or equal precedence if left associative, to the output, then push it. Push '(' and on ')' pop until the matching '('. At the end, pop all remaining operators. For example a + b * c becomes a b c * +. Postfix is easy to evaluate with a stack.
Alignment means placing data at addresses that are multiples of its size or required boundary, such as 4 for int. CPUs read aligned data faster, and some architectures fault on misaligned access. Compilers add padding in structs to align members. You can check alignment with _Alignof and request it with _Alignas or aligned_alloc in C11.
Adding an integer to a pointer moves it by that many elements, not bytes, so p + 1 on an int pointer advances by sizeof(int). Subtracting two pointers into the same array gives the number of elements between them as ptrdiff_t. Arithmetic is valid only within an array or one past its end; going further is undefined behaviour. Void pointer arithmetic is not allowed in standard C.
A null pointer holds NULL and points to nothing; dereferencing it crashes. A dangling pointer points to memory that has been freed or a local that went out of scope. A wild pointer is uninitialised and holds a random address. A void pointer is a generic pointer that can hold any object address but must be cast before dereferencing, as malloc returns void *.
18
Use a tagged union to store either an int, a float or a string value.
Using arithmetic or XOR through pointers:
void swap(int *a, int *b) {
if (a == b) return;
*a = *a ^ *b;
*b = *a ^ *b;
*a = *a ^ *b;
}
The a == b check matters because XOR swapping a variable with itself sets it to zero. In real code a temporary variable is clearer and just as fast.
20
What is the difference between text and binary file modes?
In text mode, the C library may translate line endings, for example \r\n on Windows to \n, and a Ctrl+Z byte can mean end of file. Binary mode reads and writes bytes exactly as stored. Use text mode with fprintf and fgets for human readable files and binary mode with fread and fwrite for images, structs and any exact byte format. On Linux both behave the same.
21
Insert a node at the end of a singly linked list using a double pointer.
A pointer to pointer handles the empty list without special cases.
int append(Node **head, int value) {
Node *n = malloc(sizeof *n);
if (!n) return -1;
n->data = value;
n->next = NULL;
while (*head) head = &(*head)->next;
*head = n;
return 0;
}
Call it as append(&head, 5). Keep a tail pointer if you append often.
A pointer is a variable that stores the memory address of another variable. int *p = &x; makes p hold the address of x, and *p reads or writes the value at that address. Pointers allow passing large data efficiently, modifying caller variables, working with arrays and strings, building dynamic data structures like linked lists and allocating memory at runtime.
23
What does the static keyword do in different contexts?
Inside a function, a static local variable keeps its value between calls and is initialised only once. At file level, a static global variable or function has internal linkage, so it is visible only in that source file, which helps hide implementation details. Static variables are stored in the data or BSS segment, not the stack, and default to zero.
24
A recursive Fibonacci solution times out for n = 50. How do you fix it?
Naive recursion recomputes the same subproblems repeatedly, giving about O(1.6 to the n) calls. Memoise results in an array so each value is computed once, giving O(n), or iterate bottom-up keeping only the last two values for O(1) space. Use unsigned long long, since fib(50) exceeds the int range. Matrix exponentiation gives O(log n) for very large n.
25
A program crashes with a segmentation fault. How do you find the cause?
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.
C stores multidimensional arrays in row-major order: all elements of row 0, then row 1 and so on, in one contiguous block. Element a[i][j] of an array with C columns is at offset i * C + j. Traversing row by row is cache friendly and much faster than column by column for large matrices. When passing a 2D array to a function, every dimension except the first must be specified.
27
Write C code to count the number of set bits in an integer.
Brian Kernighan's trick clears the lowest set bit each step.
int countBits(unsigned int n) {
int count = 0;
while (n) {
n &= n - 1;
count++;
}
return count;
}
It runs once per set bit. GCC and Clang also offer __builtin_popcount(n), which uses a CPU instruction when available.
28
Define a struct for a student and print all students with marks above 75.
Use an array of structs and the dot operator.
typedef struct {
int roll;
char name[50];
float marks;
} Student;
void printToppers(const Student *s, int n) {
for (int i = 0; i < n; i++) {
if (s[i].marks > 75.0f)
printf("%d %s %.1f\n", s[i].roll, s[i].name, s[i].marks);
}
}
Passing a const pointer avoids copying the whole array.
C has no string type. A string is a char array ending with the null character '\0', which marks its end. char s[] = "hi" creates an array of 3 bytes: 'h', 'i' and '\0'. Library functions like strlen and printf scan until the null terminator, so forgetting it, or writing past the array, causes reading of random memory and crashes.
30
What is the difference between stack and heap allocation?
Stack memory holds local variables, is allocated and freed automatically when functions enter and return, and is very fast but limited in size, often 1 to 8 MB. Heap memory is allocated with malloc and freed manually, can be large and outlive the function, but is slower and can fragment or leak. Large or variable sized data and data that must outlive a function belong on the heap.
Tips to crack the Infosys C Programming with DSA interview
Practise pseudocode output questions and puzzle-style reasoning, which commonly appear in Infosys online tests and HackWithInfy style contests.
Revise SQL joins, normalisation and one programming language well enough to write and explain simple programs on paper.
Mention willingness to learn new technologies and relocate, since Infosys emphasises its training programme and flexible project allocation.
Keep a one-minute story ready for every project on your CV: problem, your part, tools, result.
Practise the hard questions on this page aloud; Infosys interviewers usually ask follow-ups on your first answer.
Frequently asked questions
How many rounds are there in the Infosys C Programming with DSA interview?
Candidates usually go through 4 stages: Online assessment (aptitude, reasoning + pseudocode); Coding test; Technical interview; HR round. The exact rounds change with the role, team, location and your experience, so confirm the process with the recruiter.
Which topics should I prepare for the Infosys C Programming with DSA interview?
This practice set leans on C Basics, Data Types and Operators (7), Pointers and Memory Management (6), Structures, Unions and File Handling (4), Linked Lists (3). Revise those first, then practise explaining one or two of your own projects end to end, because most Infosys interviewers ask you to walk through real work.
Are these the exact questions asked at Infosys?
No. These are curated practice questions that match the skills and difficulty usually tested in it services interviews like Infosys's. They are not a leaked or verified list of past questions.
Where does Infosys hire C Programming with DSA professionals in India?
Infosys has large teams in Bengaluru, Mysuru, Pune, Hyderabad, Chennai. Openings and locations change often, so check the Infosys careers page and filter by city and experience.
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