SOF NSO · Bite-size lessons

Coding-Decoding

About 7 min · Logical Reasoning · English

Time worked out from the lesson's 466 words at 150 words a minute, plus a minute for each question.

Coding-Decoding is a core pattern recognition topic in NSO's Logical Reasoning section. The essence: a word or set of letters/numbers is converted using a hidden rule, and you must crack that rule to encode new words or decode given codes.

Key ideas

• Letter-Shift Coding: Each letter moves a fixed number of steps forward or backward in the alphabet. Example: A → D (shift +3), B → E (shift +3). If BOOK codes to ERRN, every letter shifted +3.
• Reverse Alphabet Coding: Replace each letter with its counterpart from the opposite end. A ↔ Z, B ↔ Y, C ↔ X, and so on. Position formula: (26 – position + 1). If CAT codes to XZG, reverse coding is at work.
• Letter-to-Number Coding: Assign each letter its alphabet position (A=1, B=2, ... Z=26) and apply arithmetic. Example: If BAD codes to 214, then B=2, A=1, D=4 — direct positional coding.
• Mixed Operations: Combine shift + reverse, or alternate rules for odd/even positions. Example: First letter +2, second letter –1, third letter +2, repeat. These require careful pattern isolation for each position.
• Word-to-Number Total Coding: Sum the alphabet positions of all letters. Example: CAT = 3+1+20 = 24. Used less often.

Watch out for

• Miscounting Alphabet Positions: Students often confuse D as 3rd or 5th letter. Fix: Write A=1, B=2, … up to the required letter whenever unsure. Don't rely on memory under exam stress.
• Ignoring Direction (Forward vs Backward Alphabet): Seeing A→Z, a student might think "shift +25" instead of recognizing reverse coding. Fix: Always check if the sum of original position + coded position = 27. If yes, it's reverse alphabet, not a shift.
• Applying Pattern Only to First Letter: In mixed coding, the rule for the first letter may differ from the rest. Assuming a uniform shift leads to wrong codes for later letters. Fix: Check the transformation of every letter individually before concluding the pattern.

Quick reference

• Letter shift ±n: Add/subtract n to each letter's position; wrap around after 26.
• Reverse alphabet: Position + opposite position = 27. E ↔ V, J ↔ Q, etc.
• Letter-to-number: Direct position (A=1…Z=26) or sum all positions.
• Mixed patterns: Check odd vs even positions, or vowel vs consonant separately.
• Always verify pattern on all letters before answering.

Check yourself: 3 questions

This lesson is cut from the topic's note, not rewritten. Fact-checked on 21 Oct 2026: an AI-assisted check of this note against cited sources; the errors it found were corrected that day. Some errors may remain. The questions are from Shishya's validated question bank.

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