Coding‑Decoding – Study Notes (Jammu & Kashmir Police SI)
Why it matters
Coding‑decoding questions test your pattern recognition, logical deduction, and speed.
In the J&K Police SI exam, they usually appear in the Reasoning Ability section, with 10–15 questions per paper.
1. Simple Explanation
- Coding: A system that represents data (letters, numbers, words) in a different form.
- Decoding: Reversing the system to recover the original data.
- In reasoning tests, you are given a set of symbols (e.g., letters, numbers, words) and asked to find the next item in the sequence, or to match one symbol to another.
Example
A B C D E -> ?
You must identify the transformation rule that turns each symbol into the next one.
2. Key Points & Strategies
| # | Strategy | How to Use |
|---|---|---|
| 1 | Look for positional patterns | Check if the next item is a simple shift (e.g., +1, +2) or a jump (e.g., +2, +4). |
| 2 | Check for word‑based patterns | Words may be related by meaning (synonyms, antonyms), parts of speech, or word length. |
| 3 | Identify numeric mapping | Letters ↔ numbers (A=1, B=2, …) or numeric sequences (1, 4, 9, …). |
| 4 | Observe parity or alternation | Even‑odd, prime‑composite, or alternating patterns (A ↔ Z, B ↔ Y). |
| 5 | Examine letter positions in the alphabet | Rotations (ROT‑n), mirror images (A↔Z), or skipping letters. |
| 6 | Consider phonetic or morphological clues | Same starting letter, same suffix, or sound similarity. |
| 7 | Guess and test | If unsure, try a few plausible patterns and see if they fit all given pairs. |
Tips
- Always write down the given pairs first.
- Use a single‑line table to track transformations.
- If more than one pattern fits, choose the simplest (Occam’s razor).
- Keep a mental or written “rule bank” of common transformations (e.g., “add 1 to ASCII value”, “reverse alphabet”).
- Practice with mock tests; time yourself.
3. Types of Coding‑Decoding Questions
| Type | Description | Typical Example |
|---|---|---|
| Letter‑to‑Letter | Single letters mapped to other letters. | A B C → ? |
| Word‑to‑Word | Words mapped to other words (synonyms, antonyms, etc.). | SUN → MOON |
| Letter‑to‑Number | Letters represented by numbers. | A → 1 |
| Number‑to‑Letter | Numbers translated to letters. | 1 → A |
| Alphabet‑Skipping | Skipping a fixed number of letters. | A C E G … |
| Mirror/Reverse | Reverse order or mirror image. | A ↔ Z |
| Positional (Odd/Even) | Alternate patterns on odd/even positions. | A B A B … |
| Rhythm/Pattern | Repeating or cyclical patterns. | ABCDABCD … |
4. Common Patterns
| Pattern | Example | Explanation |
|---|---|---|
| +n Shift | A B C → B C D (n=+1) |
Each letter is replaced by the next one in the alphabet. |
| +n Skip | A C E G … (skip 1) |
Letters are separated by a fixed number of positions. |
| Mirror | A ↔ Z, B ↔ Y |
The alphabet is reversed. |
| Alternating | A B C D → B D F H (A→B, B→D, C→F, D→H) |
Even positions shift by +2, odd by +1. |
| Word Length | CAT → DOG (3 letters each) |
Words of equal length. |
| Synonym/Antonym | HOT → COLD |
Opposites. |
| Number → Alphabet | 1 2 3 → A B C |
A=1, B=2, etc. |
| Alphabetical Position Sum | A + B = 3 → C |
Sum of positions gives next letter. |
5. Worked Example
Question
The following coding is given:
A → 2
B → 5
C → 10
D → ?
Find the coded value for D.
Step‑by‑Step Solution
| Symbol | Code | Observation |
|---|---|---|
| A | 2 | A = 1 → 2 = 1² + 1 |
| B | 5 |