Introduction
Coding and Decoding is a key reasoning topic in SSC CGL Tier-I. It tests your ability to identify patterns in letter, number, or symbol arrangements and apply them to decode or encode given words. Questions are usually based on letter shifting, positional values, or symbol substitution.
Letter Shifting and Positional Patterns
In these questions, letters of a word are shifted forward or backward by a fixed number of positions in the English alphabet. Sometimes the shift alternates or follows a sequence. The alphabet is cyclic: after Z comes A, and before A comes Z. Positional values (A=1, B=2, ..., Z=26) are often used to verify patterns.
Key Points
- Always write the alphabet with positions (A=1 to Z=26) for quick reference.
- Check if the shift is constant, increasing/decreasing, or alternating between letters.
- For reverse shifts, subtract the shift value; if it goes below 1, add 26.
Solved Example 1
Question: If 'CAT' is coded as 'FDW', how is 'DOG' coded?
Solution: GRJ
Explanation: C (+3) = F, A (+3) = D, T (+3) = W. So each letter is shifted +3. For DOG: D+3=G, O+3=R, G+3=J. Hence GRJ.
Symbol and Number Coding
Here, letters are replaced by symbols, numbers, or a mix based on a given rule. The rule may be direct substitution (e.g., A=1, B=2) or based on conditions like vowels/consonants, letter positions, or word length. Sometimes a word is coded by assigning numbers to each letter and then applying an operation.
Key Points
- Identify if the code uses positional values directly or with operations (e.g., +1, -1, sum of positions).
- Check for patterns like reversing the word, interchanging letters, or using opposite letters (A↔Z, B↔Y).
- For mixed codes, break the code into parts and match with the original word.
Solved Example 1
Question: If 'PEN' is coded as '35', how is 'INK' coded? (Use A=1, B=2, ... Z=26)
Solution: 34
Explanation: P=16, E=5, N=14; sum = 16+5+14 = 35. For INK: I=9, N=14, K=11; sum = 9+14+11 = 34.
Practice Questions
Q1. If 'TIGER' is coded as 'ULJGT', what is the code for 'LION'?
Answer: GRJ
Explanation: Each letter is shifted +3: C→F, A→D, T→W. So D→G, O→R, G→J.
Q2. If 'PEN' is coded as '35', how is 'INK' coded? (Use A=1, B=2, ... Z=26)
Answer: 34
Explanation: P=16, E=5, N=14; sum=35. I=9, N=14, K=11; sum=34.
Q3. If 'MOTHER' is coded as 'NPUIFS', how is 'FATHER' coded?
Answer: GBUIFS
Explanation: Each letter is shifted +1: M→N, O→P, T→U, H→I, E→F, R→S. So F→G, A→B, T→U, H→I, E→F, R→S.
Common Mistakes
- Assuming a constant shift without checking all letters; patterns may alternate or change.
- Forgetting that the alphabet is cyclic (Z+1=A, A-1=Z).
- Mixing up positional values (e.g., A=1 vs A=0) or miscomputing sums.
Quick Revision
- Write A=1 to Z=26 for quick reference.
- Check for constant, increasing, or alternating shifts.
- For number coding, try sum, product, or difference of positional values.