Coding‑Decoding – Study Notes
Target: Manipur Police Sub‑Inspector (SI) – Reasoning Ability
Style: Beginner‑friendly, short sentences, everyday analogies, no heavy jargon.
1. What is Coding‑Decoding?
- A type of reasoning problem.
- You encode (or code) a series of items into a new form.
- Then you are asked to decode – find the original item or the next item in the sequence.
2. Everyday Analogy
| Real‑world | Code | Decoded |
|---|---|---|
| Classroom seating | 1 → A, 2 → B, 3 → C | 2 → B |
Think of a lock that turns a number into a letter. Once you know the rule, you can read the lock or create your own.
3. Simple Explanation
- Look at the given list.
Example:A, C, E, G. - Find a pattern.
Here, each letter is the next odd letter in the alphabet. - Apply the rule to new items to decode or encode.
4. How to Approach a Question
| Step | What to do |
|---|---|
| 1. Observe | Write the items in order. |
| 2. Identify changes | Count steps in numbers, letters, or shapes. |
| 3. Test a rule | Check if all changes follow one rule. |
| 4. Confirm | Use the rule on the last item; the result should match the answer. |
| 5. Answer | Write the decoded item or the next item, whichever is asked. |
5. Common Coding‑Decoding Patterns
| Pattern | Example | How to spot it |
|---|---|---|
| Alphabet shift | A, C, E, G → +2 each step |
Count letter positions. |
| Number‑letter mapping | 1 → A, 2 → C, 3 → E |
Translate numbers to letters. |
| Reverse sequence | Z, Y, X → backward |
Notice decreasing order. |
| Skip pattern | A, D, G, J → +3 each step |
Add the same number each time. |
| Alternate pattern | 1, 3, 1, 3 |
Two different rules used alternately. |
| Shape or size change | ◻︎, ◯, ◻︎, ◯ |
Look at shapes or sizes, not letters. |
6. Quick Tips
- Write it down – a quick sketch helps see the pattern.
- Use the alphabet as a number line – A=1, B=2, … Z=26.
- Check the extremes – first and last items often reveal the rule.
- Work backward – if decoding, reverse the rule.
- Beware of trick questions – sometimes the pattern changes after a few items.
7. Tiny Example
Question:
Decode the series: B, D, F, H.
What comes next?
Solution
- Observe: B → D → F → H
- Pattern: Each letter moves 2 steps forward in the alphabet.
- Apply: H + 2 = J
- Answer: J
8. Practice Questions
| # | Series | What is the next item? | Hint |
|---|---|---|---|
| 1 | C, F, I, L |
+3 each step (letters). | |
| 2 | 2, 5, 8, 11 |
+3 each step (numbers). | |
| 3 | A, C, F, J |
Not a simple shift – look for a second pattern. | |
| 4 | Z, X, V, T |
Decreasing by 2 letters. | |
| 5 | 1, 4, 9, 16 |
Squares of 1, 2, 3, 4. | |
| 6 | ◻︎, ◯, ◻︎, ◯ |
Alternate shapes. | |
| 7 | 3, 3, 5, 5, 7, 7 |
Numbers repeat twice. | |
| 8 | A, C, F, H, K |
Find the rule. |
Answer Key (for your check)
- O (each letter +3)
- 14 (+3)
- O (skip +3 then +4)
- R (decrease by 2)
- 25 (5²)
- ◻︎ (alternate)
- 9 (next odd number repeating)
- L (pattern: +2, +3, +2, +3)
9. Recap
- Coding‑decoding is turning items into another form and back.
- Find the rule by looking at how each item changes.
- Apply the rule to decode or predict the next item.
- Use simple tools: alphabet as numbers, counting steps, and sketching.
Keep practicing, and you’ll spot patterns faster and answer correctly in the exam!