Coding‑Decoding – Study Notes
(State Agriculture Officer: Reasoning Ability – Chapter 13)
Coding‑decoding is a type of reasoning problem where a set of symbols (letters, numbers, shapes, words, etc.) is transformed into another set by a hidden rule or pattern.
The goal is to discover the rule and use it to decode (or encode) any given item.
1. Simple Explanation
| Step | What to Do | Why it matters |
|---|---|---|
| 1️⃣ Identify the unit of coding | Are we dealing with letters, numbers, words, or shapes? | The unit determines the range of possibilities. |
| 2️⃣ Look for obvious patterns | Direct mapping (A→1, B→2…), shift, reverse, skip, etc. | Most coding problems use a simple rule. |
| 3️⃣ Test the pattern on multiple items | Pick 3–4 items, apply the rule, check consistency. | Guarantees that the rule isn’t accidental. |
| 4️⃣ Encode or decode the target item | Apply the rule to find the answer. | Final answer. |
Tip: When stuck, try reverse decoding (take the code and look for a simple transformation back to the original).
2. Key Points
| Concept | What it Means | Example |
|---|---|---|
| Direct mapping | One symbol → one symbol (often position‑based). | A=1, B=2, C=3 |
| Shift (Caesar) cipher | Each symbol is shifted by a fixed number. | R → U (shift +3) |
| Reversal | Order of symbols is reversed. | ABCD → DCBA |
| Skipping | Every nth symbol is taken or omitted. | A B C D E F → B D F (skip 1) |
| Alternating patterns | Two or more rules applied in succession. | 1st letter → +2, 2nd letter → –1, 3rd letter → +3, etc. |
| Position‑dependent | The rule depends on the symbol’s position in the word. | 1st → +1, 2nd → –2, 3rd → +3 |
| Number‑letter interchange | Numbers represent letters or vice versa. | 12 → L (12th letter) |
| Word‑based encoding | Whole words are encoded by a rule (e.g., first letter, last letter). | RAIN → R+I+N = 18+9+14 = 41 |
| Shape/colour coding | Non‑alphabetical symbols (e.g., shapes, colours). | ● → 1, ○ → 2 |
| Multiple‑step coding | Two or more transformations in sequence. | A → 1 → 10 (A=1, add 9) |
3. Worked Example
Problem
ABCD → 2413
EFGH → 5678
IJKL → ?
Step‑by‑Step Solution
| Item | Decoding Rule | Explanation |
|---|---|---|
ABCD → 2413 |
Swap the 1st with 3rd, 2nd with 4th | 1st (A) ↔ 3rd (C) → 2,4; 2nd (B) ↔ 4th (D) → 1,3 |
EFGH → 5678 |
Same as above | 1st (E)=5 ↔ 3rd (G)=7 → 5,7; 2nd (F)=6 ↔ 4th (H)=8 → 6,8 |
IJKL → ? |
Apply the same rule | 1st (I)=9 ↔ 3rd (K)=11 → 9,11; 2nd (J)=10 ↔ 4th (L)=12 → 10,12 |
Answer: IJKL → 9 11 10 12 → 911012
Verification
ABCD: 1↔3 gives positions 2,4; 2↔4 gives 1,3 →2413.EFGH: 1↔3 → 5,7; 2↔4 → 6,8 →5678.
Thus the rule is consistent.
4. Quick Revision Summary
- Identify the coding unit: letter, number, word, shape, etc.
- Search for basic transformations: shift, reverse, skip, position‑dependent.
- Test the rule on multiple items to confirm consistency.
- Check for multi‑step rules: sometimes a symbol undergoes two or more operations.
- Decode/encode the target using the validated rule.
- Double‑check: ensure the answer matches all patterns seen.
5. Quick Revision Checklist
| ✔️ | Item |
|---|---|
| 1 | Recognize direct letter‑number mappings (A=1, B=2, …). |
| 2 | Spot shift patterns (+1, –2, +3, etc.). |
| 3 | Identify reversal or mirroring. |
| 4 | Notice skipping or selecting specific positions. |
| 5 | Look for alternating or |