👶➡️👩‍💻 C Programming — 30 Days, 120 Questions (Explained Like You're 5)

Made for someone who has never written a line of code. 🧸 Every idea is explained like you're 5 — no jargon left unexplained, a picture (diagram) for each concept, a real-life analogy, a step-by-step build, a dry-run table, and common mistakes to avoid.

These are the classic placement / campus-interview questions asked by TCS, Infosys, Wipro, Amazon, Microsoft, Adobe, Accenture, Cognizant, Capgemini, Deloitte & Paytm — organized as a 30-day self-learning plan (120 questions). All code is in the C language.

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🗺️ How to use this repo (reading order)

1. 📖 Start with the Concepts Primer — what is a program, a variable, a loop, an array, a function, recursion, ASCII, a struct… Read this first if you've never coded.
2. 📅 Open a day's lesson in docs/ — each day teaches 4 questions the explain-like-I'm-5 way: analogy → concepts → plan → diagram → step-by-step build → full program → dry run → pitfalls → interview tip.
3. 💻 Run the code in src/ — from Day 15 onward, every question is also a standalone, compilable .c file you can build and run yourself.

💡 New to all this? Read 00_concepts_primer.md, then go day by day. Already know the basics? Jump straight to Day 13 (arrays) or Day 15 (where the runnable code starts).

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📚 The 30-Day Map

Lessons (markdown explainers) exist for all 30 days / 120 questions. Standalone runnable C files start at Day 15 (the “💻 Code” column).

Day	Topic you'll learn	Qs	Lesson	💻 Code
1	First loops: counting, %, / on a number	Q1–Q4	day01	—
2	Pulling a number apart digit by digit	Q5–Q8	day02	—
3	Divisibility, primes, GCD & LCM	Q9–Q12	day03	—
4	Fibonacci & Armstrong numbers	Q13–Q16	day04	—
5	Special numbers & factors	Q17–Q20	day05	—
6	Number systems & bits (binary)	Q21–Q24	day06	—
7	Recursion (a function that calls itself)	Q25–Q28	day07	—
8	Nested loops & patterns (1)	Q29–Q32	day08	—
9	Patterns (2): reverse & hollow	Q33–Q36	day09	—
10	Pyramids (centered patterns)	Q37–Q40	day10	—
11	Functions (1): your own reusable blocks	Q41–Q44	day11	—
12	Functions (2): wrapping earlier logic	Q45–Q48	day12	—
13	Arrays (1): a row of boxes	Q49–Q52	day13	—
14	Arrays (2): searching & frequencies	Q53–Q56	day14	—
15	Arrays (3): rearranging elements	Q57–Q60	day15	src/day15
16	Arrays (4): missing numbers, pairs	Q61–Q64	day16	src/day16
17	Two arrays: merge, union, intersection	Q65–Q68	day17	src/day17
18	Sorting & searching	Q69–Q72	day18	src/day18
19	Matrices (1): a grid of numbers	Q73–Q76	day19	src/day19
20	Matrices (2): multiply, symmetric, sums	Q77–Q80	day20	src/day20
21	Strings (1): text is an array of chars	Q81–Q84	day21	src/day21
22	Strings (2): palindromes, words, frequency	Q85–Q88	day22	src/day22
23	Strings (3): repeating chars & anagrams	Q89–Q92	day23	src/day23
24	Strings (4): rotation, compression, cleaning	Q93–Q96	day24	src/day24
25	Mixing arrays & strings	Q97–Q100	day25	src/day25
26	Mini apps (1): menus, conditions & loops	Q101–Q104	day26	src/day26
27	Mini apps (2): records with structs	Q105–Q108	day27	src/day27
28	Mini apps (3): bigger menu-driven systems	Q109–Q112	day28	src/day28
29	Mini apps (4): menu-driven toolkits	Q113–Q116	day29	src/day29
30	Capstone mini projects	Q117–Q120	day30	src/day30

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🧠 Every lesson follows the same kid-friendly recipe

For each question you get:

1. What is it asking? — the problem in plain words + a tiny example.
2. Real-life analogy 💡 — relate it to something you already know.
3. New ideas you'll meet here — every concept named & explained in one line.
4. The plan (in human words) — the logic before any code.
5. Diagram — a Mermaid flowchart (+ ASCII box pictures for arrays/strings/matrices).
6. Build it step by step — the program grows in small pieces, with output after each step.
7. Full program — the complete, compilable C code.
8. Dry run — a trace table following one example line by line.
9. Sample input / output — a real run.
10. Watch out! ⚠️ — common beginner mistakes.
11. Speed (complexity) — Big-O in kid words.
12. Interview tip 🎯.

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💻 How to compile & run the C programs

You need a C compiler (gcc). Then, for any file under src/:

# Compile one program
gcc -std=c11 -Wall -Wextra -o prog src/day15/q57_reverse_array.c -lm

# Run it
./prog          # on Windows:  prog.exe

Compile every program at once (Linux/macOS/Git-Bash):

bash scripts/compile_all.sh

✅ Every .c file in this repo is automatically compiled by GitHub Actions (see the badge at the top) with gcc -std=c11 -Wall -Wextra — so if the badge is green, all programs build cleanly.

Don't have a compiler installed?

• Windows: install MinGW-w64 or run scoop install gcc.
• Online (zero install): paste any program into onlinegdb.com or programiz.com.

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🗂️ Repository layout

c-programming-placement-prep/
├── README.md                 ← you are here
├── 00_concepts_primer.md     ← read this FIRST (all fundamentals)
├── docs/                     ← 30 day-by-day lessons (all 120 questions)
│   ├── day01.md ... day30.md
├── src/                      ← runnable C programs (Day 15 → Day 30)
│   ├── day15/ ... day30/
│   └── (one qNN_name.c per question)
├── scripts/compile_all.sh    ← compile every program at once
└── .github/workflows/        ← CI that compiles all C files on every push

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🏢 Which companies ask these?

Pattern of company	Typical questions here
TCS, Infosys, Wipro	numbers, loops, patterns, functions, matrices
Amazon, Deloitte, Capgemini	arrays, searching, sorting, merging
Microsoft, Adobe, Paytm	strings, characters, anagrams
TCS, Accenture, Cognizant	mini projects & menu-driven systems

(Each lesson lists the exact companies for that question.)

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🙌 Tips for absolute beginners

• Don't rush. Do 4 questions a day — that's the whole point of the 30-day plan.
• Type the code yourself (don't copy-paste). Your fingers learn too.
• If a line confuses you, open the Concepts Primer and re-read that idea.
• Run the program, then change one thing and predict what happens. That's how you really learn.

“The expert in anything was once a beginner.” You've got this. 💪

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📄 License

Released under the MIT License — free to use, learn from, and share.