RRB NTPC · Bite-size lessons

Coding and Decoding

About 7 min · General Intelligence and Reasoning · English

Time worked out from the lesson's 505 words at 150 words a minute, plus a minute for each question.

Coding and Decoding is a reasoning topic in RRB NTPC, testing your ability to identify patterns and apply logical rules consistently. In these questions, a word, number, or phrase is transformed according to a hidden rule, and you must either decode a given code or encode a new word using the same rule. This topic directly evaluates pattern recognition, attention to detail, and logical consistency — skills critical for administrative and technical roles in Indian Railways.

Key ideas

• Letter Position Values: Every letter has a position in the English alphabet: A=1, B=2, C=3...Z=26. Many coding schemes rely on these position values, either directly or after arithmetic operations.
• Forward and Reverse Alphabets: In reverse alphabet coding, A↔Z, B↔Y, C↔X and so on. Position of a letter + position of its reverse counterpart always equals 27. This is a common transformation rule.
• Fixed Shift (Caesar Cipher): Each letter is replaced by a letter a fixed number of positions ahead or behind in the alphabet. For example, shift +3 means A→D, B→E, C→F.
• Conditional Rules: The coding rule changes based on a condition — e.g., "if the word starts with a vowel, reverse the entire code; if consonant, shift each letter by +2." Identifying the condition is the key challenge.
• Number-Letter Correspondence: Words may be coded as sequences of numbers corresponding to letter positions, or through arithmetic (sum, product, difference of positions).

Watch out for

• Ignoring Opposite-Letter Symmetry: Students often try +1 or -1 shifts when the actual rule is opposite letters. Always check if position_old + position_new = 27 for any pair. If yes, opposite-letter coding is in play.
• Inconsistent Rule Application: Applying a rule to only part of the word or changing the rule mid-word. The same transformation must apply to every letter (or follow a consistent positional/conditional pattern). Verify your rule with all given examples before finalizing.
• Wraparound Errors in Shifts: Forgetting that after Z comes A (forward) or before A comes Z (backward). For example, Y + 3 = B (not 28). Use modular arithmetic: (25 + 3 - 1) mod 26 + 1 = 2 = B.

Quick reference

• Alphabet positions: A=1, B=2...Z=26; opposite of n is 27-n.
• Common patterns: fixed shift (+n or -n), opposite letters, position sums, conditional rules.
• Always verify your identified rule against all given examples before applying to the question.
• Watch for wraparounds when shifting near A or Z.
• In conditional coding, distinguish vowels (A,E,I,O,U) from consonants carefully.

Check yourself: 3 questions

This lesson is cut from the topic's note, not rewritten. Fact-checked on 18 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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