AFCAT Reasoning Ability – Coding‑Decoding
Key Idea
Coding‑decoding questions ask you to find the rule that transforms one set of items into another. Once you identify the pattern, you can decode the answer for any given set.
1. Simple Explanation
| Term | What it means | Example |
|---|---|---|
| Coding | A set of items that are encoded using a hidden rule. | A B C |
| Decoding | The corresponding set after applying the rule. | 2 4 6 |
| Rule | The hidden pattern that links coding to decoding. | “Multiply the alphabet position by 2.” |
Goal:
Given a coding set and parts of its decoding, determine the missing element in the decoding.
2. Key Points & Strategies
| Strategy | How to Use It | When it Helps |
|---|---|---|
| Look for numeric patterns | Convert letters to numbers (A=1, B=2…) or numbers to letters. |
Most standard coding‑decoding problems. |
| Check for alternating operations | Apply different operations to odd and even positions. | Example: odd positions +1, even positions ×2. |
| Identify shifts | Add/subtract a constant or rotate alphabet. | Example: A → C (shift +2). |
| Consider positional values | Use position in the set, like nth prime, nth square, etc. |
Example: 1st → 1, 2nd → 4, 3rd → 9. |
| Look for word relationships | Count letters, find anagrams, or use synonyms. | Example: “SUN” → “SUNSET” (add 3 letters). |
| Use elimination | If the answer choices are given, rule out impossible options quickly. | When time‑constrained. |
| Cross‑check | Verify that the rule works for all given pairs before finalising. | Avoids mistakes from a coincidental match. |
Helpful Reminders
| Tip | Why it matters |
|---|---|
| Always check the first pair | The first pair usually provides the simplest clue to the rule. |
| Watch for “reverse” coding | Sometimes decoding is the reverse of coding (e.g., 1 2 3 → 3 2 1). |
| Mind the “carry” in arithmetic | When adding digits, carry‑over can change the pattern. |
| Convert back to alphabet only at the end | Avoid confusion by keeping numbers during the pattern search. |
3. Worked Example
Question
Coding:
A C E G
Decoding:2 6 10 ?
Find the missing number.
Step‑by‑Step
| Step | What we do | Result |
|---|---|---|
| 1. Convert letters to numbers | A=1, C=3, E=5, G=7 |
1 3 5 7 |
| 2. Look for a pattern | Check arithmetic differences: 3-1=2, 5-3=2, 7-5=2 → constant +2. |
|
| 3. Apply same operation to decoding | 2 → ? needs to follow the same +4 pattern? But decoding numbers: 2,6,10 → differences 4 and 4. |
|
| 4. Infer rule | Coding numbers → Decoding numbers: multiply by 2 and add 0? 1×2=2, 3×2=6, 5×2=10. |
|
| 5. Compute missing decoding | 7 × 2 = 14 |
|
| 6. Check consistency | All pairs satisfy “multiply by 2.” ✔️ |
Answer: 14
4. Quick Revision Summary
- Identify the coding set first; convert letters to numbers if necessary.
- Search for simple arithmetic or positional rules (addition, multiplication, square, prime, etc.).
- Check consistency across all provided pairs before concluding.
- Remember common tricks: alternating operations, letter shifts, word‑length changes.
- Use elimination if answer options are given.
- Always double‑check the final answer against the rule.
One‑Minute Self‑Test
| Coding | Decoding |
|---|---|
B D F |
4 8 12 |
H J L |
? |
Answer:
20(Rule: multiply letter position by 2).
Try a few more on your own to cement these strategies!