Easy C Programming with DSA Interview Questions

Show answer

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.

What are the basic data types in C and their typical sizes?

Show answer

C has char (1 byte), short (usually 2), int (usually 4), long (4 on Windows, 8 on 64-bit Linux), long long (at least 8), float (4) and double (8). The standard only fixes minimum ranges, so sizes depend on the platform. Use sizeof to check and the fixed-width types from stdint.h such as int32_t and uint8_t when exact sizes matter.

What is the difference between a declaration and a definition in C?

Show answer

A declaration tells the compiler a name and its type, such as extern int count; or a function prototype, without allocating storage. A definition actually creates the object or provides the function body, such as int count = 0;. A variable can be declared many times but defined only once across the program, otherwise the linker reports a multiple definition error.

What does the static keyword do in different contexts?

Show answer

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.

What is the difference between ++i and i++?

Show answer

++i increments i and then yields the new value, while i++ yields the old value and then increments. As a standalone statement both have the same effect, and modern compilers generate the same code. The difference matters inside expressions: if i is 5, a = i++ sets a to 5, while a = ++i sets a to 6. Never use both on the same variable in one expression.

A program prints garbage values for a local variable. What is the likely cause?

Show answer

Local automatic variables are not initialised in C, so they hold whatever bytes were on the stack. Reading them is undefined behaviour and gives random results. Initialise every variable at declaration, such as int sum = 0;. Compiling with -Wall -Wuninitialized and running under Valgrind or with -fsanitize=memory helps detect such reads early.

Write a C program to swap two numbers without using a third variable.

Show answer

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.

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

Show answer

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.

Write a header guard and explain why it is needed.

Show answer

A header guard stops a header from being included twice in one translation unit, which would cause redefinition errors. #ifndef MATH_UTILS_H #define MATH_UTILS_H int add(int a, int b); #endif Most compilers also support #pragma once, which is shorter but not part of the C standard.

Why do you want to work in C and systems programming?

Show answer

Situation: during college I built a small shell in C for an operating systems lab. Task: make it handle pipes and background jobs. Action: I learned fork, exec and file descriptors and debugged crashes with gdb. Result: I enjoyed understanding how software talks to hardware and the OS, and I want roles in embedded, systems or performance critical software where C skills matter.

What is the difference between break and continue in C?

Show answer

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.

What is a pointer in C?

Show answer

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.

What are null, dangling, wild and void pointers?

Show answer

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 *.

What is the difference between malloc, calloc, realloc and free?

Show answer

malloc(size) allocates uninitialised memory. calloc(n, size) allocates memory for n elements and sets all bytes to zero, and it checks n times size for overflow. realloc(ptr, size) resizes a block, possibly moving it, and returns the new address. free(ptr) releases memory back to the allocator. All allocators return NULL on failure, so always check the result.

What is the difference between stack and heap allocation?

Show answer

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.

Write a function that dynamically allocates an array of n integers and fills it with squares.

Show answer

Check the allocation and let the caller free it. int *makeSquares(size_t n) { int *arr = malloc(n * sizeof *arr); if (arr == NULL) return NULL; for (size_t i = 0; i < n; i++) { arr[i] = (int)(i * i); } return arr; } // caller int *a = makeSquares(10); if (a) { /* use */ free(a); }

Write a function that swaps two integers using pointers.

Show answer

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.

What does sizeof return for a pointer and for an array?

Show answer

sizeof on a pointer returns the size of the pointer itself, typically 8 bytes on a 64-bit system, regardless of what it points to. sizeof on an array declared in the same scope returns the total bytes of all elements, so sizeof arr / sizeof arr[0] gives the element count. Inside a function receiving an array parameter, sizeof returns pointer size, so pass the length separately.

How are strings represented in C?

Show answer

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.

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

Show answer

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.

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

Show answer

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.

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

Show answer

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.

An interview asks you to find the missing number from 1 to n in an array of n-1 numbers. What approaches work?

Show answer

Sum approach: expected sum n(n+1)/2 minus the actual sum gives the missing number, using a long type to avoid overflow. XOR approach: XOR all numbers from 1 to n and all array elements; pairs cancel and the missing number remains, with no overflow risk. Both are O(n) time and O(1) space, and XOR is the safer choice for very large n.

Reverse a string in place in C.

Show answer

Swap characters from both ends moving inward. void reverse(char *s) { size_t n = strlen(s); if (n < 2) return; for (size_t i = 0, j = n - 1; i < j; i++, j--) { char t = s[i]; s[i] = s[j]; s[j] = t; } } Time O(n) and O(1) extra space. The string must be a modifiable array, not a literal.

Merge two sorted arrays into a third sorted array.

Show answer

Walk both arrays with two indexes. void merge(const int *a, int n, const int *b, int m, int *out) { int i = 0, j = 0, k = 0; while (i < n && j < m) out[k++] = (a[i] <= b[j]) ? a[i++] : b[j++]; while (i < n) out[k++] = a[i++]; while (j < m) out[k++] = b[j++]; } Time O(n + m). Using <= keeps the merge stable.

Implement your own strcmp function.

Show answer

Compare character by character until a difference or the end. int my_strcmp(const char *a, const char *b) { while (*a && *a == *b) { a++; b++; } return (unsigned char)*a - (unsigned char)*b; } Casting to unsigned char matches the standard behaviour for characters above 127. It returns zero if equal, negative if a sorts first and positive otherwise.

Write a C function to count words in a sentence.

Show answer

Count transitions from space to non-space. #include <ctype.h> int countWords(const char *s) { int count = 0, inWord = 0; for (; *s; s++) { if (isspace((unsigned char)*s)) { inWord = 0; } else if (!inWord) { inWord = 1; count++; } } return count; } This handles multiple spaces, tabs and leading or trailing spaces correctly.

How did you improve your problem solving skills?

Show answer

Situation: in my second year I could not solve even easy array problems in time. Task: become placement ready in six months. Action: I followed a topic-wise sheet, solved two problems daily in C, wrote down the pattern behind each one and revised mistakes every Sunday. Result: I solved over 300 problems, cleared three online coding tests and now explain complexity confidently.

What does it mean that arrays decay to pointers when passed to functions?

Show answer

When you pass an array to a function, C passes only a pointer to its first element, not a copy of the array. So void f(int a[]) is the same as void f(int *a), sizeof a inside gives the pointer size and changes to elements affect the caller's array. Always pass the length as a separate parameter, such as f(arr, n).

What is a structure in C and why is it used?

Show answer

A structure groups related variables of different types under one name, such as struct Student { int roll; char name[50]; float marks; };. It models real records, lets functions take or return one object instead of many parameters and is the base for data structures like linked lists and trees. Members are accessed with the dot operator or with -> through a pointer.

What is the difference between a structure and a union?

Show answer

In a structure each member has its own memory, so the size is at least the sum of members plus padding, and all members hold values at once. In a union all members share the same memory, so its size equals the largest member and only one member is meaningful at a time. Unions save memory and are used for variant types and in embedded protocol parsing.

What is typedef and how is it used with structures?

Show answer

typedef creates an alias for a type. typedef struct Node { int data; struct Node *next; } Node; lets you write Node *head instead of struct Node *head. It is also useful for function pointer types and platform specific types. Some style guides avoid hiding pointers behind typedefs because it makes ownership and const usage less clear.

What is an enum in C and what are its advantages?

Show answer

An enum defines a set of named integer constants, such as enum Color { RED, GREEN, BLUE };, where values start at 0 unless set. It makes code readable and lets compilers warn about missing cases in a switch with -Wswitch. Enum values are int sized in practice, and C23 allows specifying the underlying type. Use them for states, commands and error codes.

A file opened with fopen is NULL in production but fine on your machine. How do you debug?

Show answer

Always check the return value and print the reason with perror or strerror(errno), which shows errors like No such file or directory or Permission denied. Common causes are a relative path resolved from a different working directory, missing permissions for the service user, a full disk or too many open files. Use absolute or configured paths and close every file you open.

Define a struct for a student and print all students with marks above 75.

Show answer

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.

How do you pass a structure to a function efficiently?

Show answer

Passing a struct by value copies every member, which is costly for large structs. Pass a pointer instead, and add const when the function should only read it, such as void print(const Student *s). Access members with s->name. Small structs of a few bytes can be passed by value safely, and returning small structs by value is also fine.

What is a linked list and how is it different from an array?

Show answer

A linked list is a chain of nodes where each node holds data and a pointer to the next node, allocated separately on the heap. Unlike an array, it grows easily and inserts or deletes in O(1) once you have the position, but it has O(n) access by index, extra memory per node for pointers and poor cache locality. Arrays win for random access and iteration speed.

What are the types of linked lists?

Show answer

A singly linked list has a next pointer only and can be traversed one way. A doubly linked list adds a prev pointer, allowing backward traversal and O(1) deletion of a known node, at the cost of more memory. A circular linked list links the last node back to the first, useful for round-robin scheduling. Variants include lists with sentinel or dummy head nodes.

What is the time complexity of common linked list operations?

Show answer

Access or search by value is O(n). Insert or delete at the head is O(1). Insert at the tail is O(1) if you keep a tail pointer, otherwise O(n). Deleting a given node is O(1) in a doubly linked list but O(n) in a singly linked list, since you must find the previous node. Finding length is O(n) unless you store a count.

How do you find the middle of a linked list in one pass?

Show answer

Use slow and fast pointers starting at the head. Move slow by one node and fast by two nodes each step. When fast reaches the end or NULL, slow is at the middle. For an even number of nodes this gives the second middle; start fast at head->next to get the first middle instead. It takes O(n) time and O(1) space.

A function to insert at the head does not change the list in main. Why?

Show answer

C passes arguments by value, so the function receives a copy of the head pointer; changing that copy does not affect main. Pass a pointer to the head, as in void push(Node **head, int x), and assign *head = newNode, or return the new head and assign it in the caller with head = push(head, x). This is a very common fresher interview mistake.

Describe a time you failed in an interview or test and what you learned.

Show answer

Situation: in my first product company interview I froze on a linked list reversal. Task: improve before the next opportunity. Action: I practised pointer problems daily, always drawing boxes and arrows before coding, and did mock interviews with friends where I spoke my thinking aloud. Result: two months later I solved a harder linked list problem calmly and got the offer.

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

Show answer

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.

What is a stack and where is it used?

Show answer

A stack is a last in, first out structure with push, pop and peek operations, all O(1). It is used for function call frames, undo features, expression evaluation, balanced bracket checking, backtracking and depth first search. In C you implement it with an array and a top index, or with a linked list when the size is not known in advance.

What is a queue and what are its variants?

Show answer

A queue is first in, first out with enqueue at the rear and dequeue at the front, both O(1). Variants are the circular queue, which reuses array space; the deque, which allows insertion and removal at both ends; and the priority queue, which removes the highest priority element first. Queues are used in BFS, task scheduling, print spooling and buffering data between producers and consumers.

Implement a stack using an array in C.

Show answer

Track the top index and check bounds. #define MAX 100 typedef struct { int data[MAX]; int top; } Stack; void init(Stack *s) { s->top = -1; } int push(Stack *s, int x) { if (s->top == MAX - 1) return -1; s->data[++s->top] = x; return 0; } int pop(Stack *s, int *out) { if (s->top < 0) return -1; *out = s->data[s->top--]; return 0; } Returning an error code avoids undefined reads on empty stacks.

Check whether brackets in an expression are balanced using a stack.

Show answer

Push opening brackets and match on closing ones. int isBalanced(const char *s) { char st[1000]; int top = -1; for (; *s; s++) { char c = *s; if (c == '(' || c == '[' || c == '{') { if (top == 999) return 0; st[++top] = c; } else if (c == ')' || c == ']' || c == '}') { if (top < 0) return 0; char o = st[top--]; if ((c == ')' && o != '(') || (c == ']' && o != '[') || (c == '}' && o != '{')) return 0; } } return top == -1; }

How do you handle working with a teammate who writes buggy code?

Show answer

Situation: a teammate's stack and queue module in our project kept failing tests. Task: fix the quality issue without blaming. Action: I offered to pair on one bug, showed how drawing the array and indexes revealed the off-by-one error and suggested we both add edge case tests before pushing. Result: his code quality improved quickly and our module passed all tests at the review.

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

Show answer

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.

What is a binary search tree and what are its operation costs?

Show answer

A binary search tree keeps every node's left subtree smaller and right subtree larger than the node. Search, insert and delete follow one path from the root, so they cost O(h), where h is the height. A balanced tree has h of about log n, but inserting sorted data creates a skewed tree with h equal to n. In-order traversal of a BST gives keys in sorted order.

Explain the tree traversal orders.

Show answer

In-order visits left, node, right and gives sorted output for a BST. Pre-order visits node, left, right and is used to copy or serialise a tree. Post-order visits left, right, node and is used to delete a tree or evaluate expression trees. Level-order visits nodes level by level using a queue. The first three are depth first and use recursion or an explicit stack.

What is the difference between a complete, full and perfect binary tree?

Show answer

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.

Insert a value into a binary search tree recursively in C.

Show answer

Go left or right until an empty spot is found. typedef struct TNode { int key; struct TNode *left, *right; } TNode; TNode *insert(TNode *root, int key) { if (root == NULL) { TNode *n = malloc(sizeof *n); n->key = key; n->left = n->right = NULL; return n; } if (key < root->key) root->left = insert(root->left, key); else if (key > root->key) root->right = insert(root->right, key); return root; } Duplicates are ignored here.

Find the height of a binary tree in C.

Show answer

Height is one plus the larger subtree height. int height(const TNode *root) { if (root == NULL) return 0; int lh = height(root->left); int rh = height(root->right); return 1 + (lh > rh ? lh : rh); } This counts nodes on the longest root to leaf path; return -1 for empty trees if height is defined in edges. Time O(n).

How do you explain the time complexity of your solution to an interviewer?

Show answer

Situation: in a technical round I solved a tree problem but the interviewer questioned its efficiency. Task: justify the complexity clearly. Action: I explained that each node is visited once and does constant work, so time is O(n), and that recursion depth equals tree height, giving O(h) space, worst case O(n) for a skewed tree. Result: the interviewer agreed and moved to a harder follow-up.

What is Big O notation?

Show answer

Big O describes how an algorithm's time or memory grows as input size grows, ignoring constants and lower terms. Common classes are O(1), O(log n), O(n), O(n log n), O(n squared) and O(2 to the n). It usually describes the worst case. For example, binary search is O(log n), so a million elements need about 20 steps. Big Omega and Big Theta describe lower and tight bounds.

How does binary search work and what are common bugs?

Show answer

Binary search works on sorted data by comparing the middle element with the target and discarding half the range each step, giving O(log n). Common bugs are computing mid as (low + high) / 2, which can overflow, so use low + (high - low) / 2; wrong loop conditions causing infinite loops; and off-by-one errors when finding first or last occurrences. Test with zero, one and two elements.

What is recursion and what are its risks?

Show answer

Recursion is when a function calls itself on a smaller subproblem until reaching a base case. It expresses tree traversal, divide and conquer and backtracking naturally. Risks are stack overflow when depth is large, repeated work when subproblems overlap, like naive Fibonacci, and function call overhead. Use memoisation for overlapping subproblems and iterative versions when depth may be huge.

A recursive Fibonacci solution times out for n = 50. How do you fix it?

Show answer

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.

A search on a sorted array of 10 lakh elements is done with a linear loop. How would you justify changing it?

Show answer

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.

Write binary search in C that returns the index of the target or -1.

Show answer

Use an overflow safe midpoint. int binarySearch(const int *a, int n, int target) { int lo = 0, hi = n - 1; while (lo <= hi) { int mid = lo + (hi - lo) / 2; if (a[mid] == target) return mid; if (a[mid] < target) lo = mid + 1; else hi = mid - 1; } return -1; } Time O(log n). The array must be sorted in ascending order.

How do you keep improving your DSA skills while working full time?