Coding-decoding: Reasoning Ability Notes for Haryana Forest Guard

Coding‑Decoding – Study Notes (Haryana Forest Guard – Reasoning Ability)

Coding‑Decoding questions test your ability to spot patterns in sequences of numbers, letters, words or symbols.
You are given a partial code (or decode it) and must find the missing element, the next element, or the value that corresponds to a given key.

1. Simple Explanation

Term	What it means	Example
Code	The part that is written (e.g., a sequence of numbers or letters).	12, 18, 24, …
Decode	The part that you need to find (e.g., the next number, a missing code, or the word corresponding to a code).	30
Pattern	The rule that links the code elements (addition, multiplication, alternation, etc.).	+6 each step

Goal: Identify the underlying rule and use it to answer the question.

2. Key Points (What to Look For)

Category	Typical Pattern	How to Spot It
Arithmetic	+constant, –constant, ×constant, ÷constant	Look at differences or ratios between consecutive elements.
Alternating	Two or more sequences interleaved	Separate odd & even positions and check each for a pattern.
Cyclic	Repeats after a fixed number of terms	Count how many terms before the pattern repeats.
Positional	Position in alphabet, digit sum, factorial, etc.	Compare each element with its index.
Composite	Combination of two or more operations	Break the sequence into parts and test each part.
Word/Letter	First letter of months, days, countries, etc.	Recognise common word lists.

Tip: Always try two simple patterns first: difference and ratio. If both are constant, you’re almost done.

3. Worked Example

Question

Sequence: 5, 10, 20, 40, …
What is the 6th term?

Step	Action	Result
1	Check differences: 10–5 = 5, 20–10 = 10, 40–20 = 20 → not constant.
2	Check ratios: 10/5 = 2, 20/10 = 2, 40/20 = 2 → constant ratio 2.
3	Apply rule: Multiply previous term by 2.
4	Find 6th term: 40 × 2 = 80.

Answer: 80

4. Another Example – Decoding a Word

Question

Code: 8, 3, 6, 9, 2
Decode to the word.

Step	Action	Result
1	Convert numbers to letters (A=1, B=2, …, Z=26).	8=H, 3=C, 6=F, 9=I, 2=B
2	Look for a meaningful word or pattern.	HCFIB – looks odd.
3	Check if the sequence is reversed or shifted.	Reverse → BIFCH → still odd.
4	Try shifting each letter by 2 positions back (H->F, C->A, …).	F A D G Z → FADGZ – still no.
5	Recognise that the numbers might represent positions of letters in a given word.	Suppose the word is “HARDY” → positions 8, 1, 18, 4, 25. Not match.
6	Try mapping to a common word list – e.g., months: Jan=1, Feb=2, …	8=Aug, 3=Mar, 6=Jun, 9=Sep, 2=Feb → Aug-Mar-Jun-Sep-Feb – not helpful.
7	Consider that the code is actually a simple substitution: Add 0, subtract 5, add 3, subtract 2, add 0.	Not a pattern.

Solution (common approach):
The numbers are the positions of letters in the word “HARDY” but scrambled:
H(8), A(1), R(18), D(4), Y(25) – not matching.
Since no clear pattern emerges, we might need additional context (e.g., a keyword or a table).

Bottom line: When a pattern isn’t obvious, back‑track, try alternative mappings, or refer to the key provided in the question.

5. Quick Revision Summary

Topic	Quick Notes
Identify the pattern	Start with differences, then ratios.
Separate sequences	Odd vs. even positions for alternating patterns.
Check for cycles	Repeat after 3–5 terms often.
Use positional tricks	A=1, B=2 …, Nth digit of Pi, etc.
Word codes	Convert numbers ➜ letters; check common word lists.
Work backwards	If the answer is given, reverse engineer the rule.
Practice	Solve 10–15 coding‑decoding questions daily.

Final tip: In exam conditions, time‑management is key. Spend the first 30 % of your time on the problem; if you’re stuck, move on and return if time permits.

Practice Problem (Try It)

Sequence: 3, 9, 27, 81, …
Question: What is the 7th term?
Hint: Think of multiplication by a constant.

Happy studying! 🎯