C Programming with DSA Last-Minute Revision

30 quick revision notes for C Programming with DSA

1. A C program goes through preprocessing, compilation, assembly and linking before it becomes an executable.
2. Sizes of int and long depend on the platform, so use stdint.h types like int32_t when exact size matters.
3. Local variables are not initialised automatically, while global and static variables default to zero.
4. A static local variable keeps its value between calls, and a static global is visible only in its file.
5. Signed integer overflow, out-of-bounds access and use of freed memory are undefined behaviour in C.
6. Macros are text substitution with no type checks; prefer static inline functions for small helpers.
7. const int *p protects the value, int *const p fixes the pointer, and const int *const p fixes both.
8. Pointer arithmetic moves by element size, so p + 1 on an int pointer advances by sizeof(int) bytes.
9. An array name decays to a pointer when passed to a function, so always pass the length separately.
10. malloc returns uninitialised memory, calloc zeroes it, and every allocator returns NULL on failure.
11. Store realloc's result in a temporary pointer first, or the original block leaks when realloc fails.
12. Set pointers to NULL after free to avoid dangling pointers and accidental use-after-free bugs.
13. C strings end with the null character, and strlen counts characters before it, not the array size.
14. Never use gets; use fgets with the buffer size and snprintf for safe, bounded string formatting.
15. Struct padding aligns members, so order fields from largest to smallest alignment to reduce size.
16. In a union all members share the same memory, so its size equals the size of the largest member.
17. Loop on the return value of fgets or fread instead of feof to avoid processing the last line twice.
18. Use fixed-width types and explicit byte order when writing binary data shared between different machines.
19. Use a pointer to the head pointer when a linked list function must change the head of the list.
20. Floyd's slow and fast pointers detect a cycle in O(n) time and O(1) space and also find its start.
21. A circular queue reuses array slots with modulo arithmetic and tracks fullness with a count.
22. Hash tables give O(1) average lookup and must resize when the load factor grows too high.
23. A binary heap stored in an array has children of index i at positions 2i plus 1 and 2i plus 2.
24. In-order traversal of a binary search tree prints keys in sorted ascending order.
25. BFS finds shortest paths in unweighted graphs, while Dijkstra handles non-negative weighted edges.
26. Topological sort works only on directed acyclic graphs and can also be used to detect cycles.
27. Compute the binary search midpoint as low plus (high minus low) divided by two to avoid overflow.
28. Quicksort averages O(n log n) but degrades to O(n squared) with poor pivots; merge sort is stable.
29. Dynamic programming needs overlapping subproblems and optimal substructure; define state and transition first.
30. Compile with -Wall -Wextra -g and -fsanitize=address,undefined to catch memory and behaviour bugs early.

Night-before checklist

• Revise your saved questions on this page once, aloud.
• Read the quick notes and mark any you cannot explain in one line.
• Rehearse a two-minute self-introduction and two project stories.
• Keep CV, ID, laptop charger and interview link ready; sleep on time.