Hello World Is a Lie — Build a Real Program in RUDRA (Chapter 2)

Hello world teaches you nothing. It has no state, no decisions, no loops, no bugs. It's a greeting card, not a program.

Chapter 2 is where RUDRA gets real. We build a Number Summarizer: feed it 1..n, it prints each number's even/odd status, totals them up, and classifies the sum. Small enough to fit in your head. Real enough to teach you everything.

The program

fn sum_to(n: int) -> int { let mut total = 0; for i in 1..(n + 1) { total = total + i; } return total; } fn is_even(x: int) -> bool { return x % 2 == 0; } fn main() { let n = 5; print("RUDRA number summary"); for i in 1..(n + 1) { if is_even(i) { print("{i} is even"); } else { print("{i} is odd"); } } let s = sum_to(n); print("sum: {s}"); if is_even(s) { print("the sum is even"); } else { print("the sum is odd"); } }

The ideas that do all the work

let vs let mut — variables are immutable by default. Reassign a plain let and the compiler stops you with the exact line number. Mutability is opt-in, which means every mutation in your code is a deliberate decision.
Ranges end exclusive — 1..6 means 1 through 5. This WILL bite you exactly once; after that you'll never forget. The idiom for "1 through n" is 1..(n + 1).
The accumulator — total = total + i; inside the loop. (No += in RUDRA's book — the explicit form keeps beginners honest about what's happening.)
Functions with types — fn sum_to(n: int) -> int. Parameters and return types are written out. The compiler checks every call.

The honest workflow

There's no keyboard-input API in the book at this stage — so n lives in the source. Change it and re-run:

rudrac summary.rux --run

Edit → run → read output → repeat. n = 5, 10, 20… each run teaches you something.

The 3 bugs you will definitely write

Debugging method: problem → behavior → reason → fix → retest.

Off-by-one — you write 1..n and n silently vanishes from the sum. Runs fine. Wrong answer. The exclusive end strikes.
Missing mut — you reassign a plain let. Compiler error, exact line given. Add mut, move on.
= vs == — assignment inside a condition. Compiler error; conditions need comparison.

Every one of these has bitten a professional this month. You're in good company.

Refactor like you mean it

Repeated logic becomes a function (is_even, sum_to). Names say what they mean. One idea per function. And the rule that keeps you honest: refactor only with features you've already learned — no cleverness debt.

Proof it works

n	sum	parity
1	1	odd
5	15	odd
10	55	odd
20	210	even

Real runs, real numbers.

Keep going

🎥 Start the course — Chapter 1 video: https://www.youtube.com/watch?v=-adNF7Fi0Pg

📥 Download RUDRA (free): https://github.com/rakshanex/rudra

💬 WhatsApp channel: https://whatsapp.com/channel/0029Vb8hknnGE56kO24pbq47

Next chapters build on this foundation — the course follows the official book, chapter by chapter.

Question: which of the 3 bugs has bitten YOU most recently — off-by-one, missing mut, or = vs ==? Confess in the comments.

Piyush Kumar — Founder & Director, RAKSHANEX TECHNOLOGIES. Cybersecurity specialist (pentesting, red teaming, SOC, DFIR). Builder of RUDRA, OmniCore OS, AgentTrace. Author of "The RUDRA Programming Language" and four cybersecurity books.