Overview
Coding-Decoding is a staple topic in the **SSC CGL** General Intelligence & Reasoning section. In every SSC CGL Tier 1 exam, you can expect **2-3 questions** from this topic. The concept involves converting information into a coded form using a specific rule or pattern, and then decoding it back.
In the SSC CGL exam, Coding-Decoding questions test your ability to detect patterns in letter shifts, number assignments, or symbol replacements. With 25 questions in the Reasoning section for 50 marks and a **negative marking of 0.50 marks**, mastering coding-decoding can secure you easy marks with minimal risk.
Key Concepts
### Types of Coding-Decoding
1. **Letter Shifting Codes**: Each letter is replaced by another letter based on a fixed shift pattern.
2. **Number/Position Codes**: Letters are replaced by their position numbers or modified position numbers.
3. **Reverse/Mirror Codes**: Words are coded using reverse alphabet positions.
4. **Condition-Based Codes**: Coding rules change based on conditions (vowel/consonant, position).
5. **Analogy-Based Codes**: A coding pattern is given for one word; apply the same to another.
6. **Symbol/Mixed Codes**: Letters replaced by symbols or a mix of numbers and symbols.
Detailed Explanation
### Type 1: Letter Shifting Codes
**Rule**: Each letter is shifted by a fixed number of positions forward or backward.
**Example**: If COME is coded as DPNF, find the code for GONE.
- C+1=D, O+1=P, M+1=N, E+1=F → Each letter shifted +1
- G+1=H, O+1=P, N+1=O, E+1=F → **HPOF**
**Variable Shift Example**: If ROAD is coded as URDG, find the code for SWAN.
- R+3=U, O+3=R, A+3=D, D+3=G → Each letter shifted +3
- S+3=V, W+3=Z, A+3=D, N+3=Q → **VZDQ**
### Type 2: Reverse Alphabet Coding
The **reverse alphabet** maps: A↔Z, B↔Y, C↔X, D↔W, E↔V... (each pair sums to 27).
**Formula**: Coded letter position = 27 - Original letter position
**Example**: If WORD is coded as DLIW:
- W(23)→D(4), O(15)→L(12), R(18)→I(9), D(4)→W(23)
- Check: 23+4=27, 15+12=27, 18+9=27, 4+23=27 ✓ (Reverse alphabet)
### Type 3: Number-Based Coding
**Example**: If CAT = 24, DOG = ?
- C(3) + A(1) + T(20) = 24 ✓
- D(4) + O(15) + G(7) = **26**
**Squared positions**: If BE = 29, CD = ?
- B²+E² = 4+25 = 29 ✓
- C²+D² = 9+16 = **25**
### Type 4: Condition-Based Coding (New Pattern in SSC CGL)
Recent SSC CGL exams have introduced condition-based coding:
**Example**: In a coding system:
- If the first letter is a vowel, reverse the word then code
- If the first letter is a consonant, shift each letter by +2
APPLE → First letter A (vowel) → Reverse = ELPPA → then apply rule
BRING → First letter B (consonant) → DTKPI (+2 shift)
### Type 5: Analogy-Based Coding
**Example**: If FRIEND is coded as HUMGPF, how is CANDLE coded?
- F(6)→H(8): +2, R(18)→U(21): +3, I(9)→M(13): +4, E(5)→G(7): +2, N(14)→P(16): +2, D(4)→F(6): +2
- Pattern: +2, +3, +4, +2, +3, +4 (repeating)
- C+2=E, A+3=D, N+4=R, D+2=F, L+3=O, E+4=I → **EDRFOI**