Coding-Decoding — RRB NTPC Reasoning
This topic carries roughly 11% of General Intelligence & Reasoning's 30 questions. Every coding-decoding question gives you a worked example to reverse-engineer the rule from — the skill is testing candidate rules systematically against that example, not guessing.
1. What RRB NTPC actually asks
Expect letter-shift coding (each letter moves forward or backward by a fixed amount), reverse-alphabet coding (A pairs with Z, B pairs with Y, etc.), word-reversal coding, and number-substitution coding (each letter maps to a number based on its alphabet position).
2. The four common coding types
Letter-shift coding: every letter in the word shifts by the same fixed number of positions (e.g., +1: A→B, C→D).
Reverse-alphabet coding: each letter is replaced by its mirror position from the other end of the alphabet (A pairs with Z, B pairs with Y — the pair always sums to 27 in position numbers).
Word-reversal coding: the coded word is simply the original word's letters in reverse order.
Number-substitution coding: each letter is replaced by its alphabet position number (A=1, B=2, ..., Z=26), sometimes with an additional shift applied.
3. The systematic approach
- Compare the given word and its code letter by letter (or check if it's simply reversed).
- Identify the fixed rule: same shift for the whole word, or a mirror/reverse pattern.
- Verify the rule against every letter in the given example — a rule that only explains some letters isn't the actual rule.
- Apply the verified rule to the new word asked in the question.
Worked examples
Q1. If CAT is coded as DBU, how is DOG coded in the same language?
Pick an option to check your answer.
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Solution. Compare CAT to DBU letter by letter: C→D (+1), A→B (+1), T→U (+1). Every letter shifts forward by exactly 1 position.
Applying the same +1 shift to DOG: D→E, O→P, G→H, giving EPH. Answer: (a).
Q2. In a certain code, GARDEN is written as ICTFGP. How is FLOWER coded in the same language?
Pick an option to check your answer.
Show explanation
Solution. Compare GARDEN to ICTFGP: G→I (+2), A→C (+2), R→T (+2), D→F (+2), E→G (+2), N→P (+2). Every letter shifts forward by exactly 2 positions.
Applying +2 to FLOWER: F→H, L→N, O→Q, W→Y, E→G, R→T, giving HNQYGT. (a) uses a +1 shift instead, and (c) uses a +3 shift — both fail to match the verified +2 rule from the given example. Answer: (b).
5. Common traps
- Assuming the rule from only the first letter or two, without verifying it holds for the entire given word. A rule must explain every letter consistently.
- Confusing a forward shift with a backward shift. Always check the direction explicitly: does the code letter come AFTER or BEFORE the original letter in the alphabet?
- Missing a reversal combined with a shift. Some questions combine word-reversal with a letter shift — check both possibilities if a simple shift doesn't fully explain the given example.
- Losing count near the ends of the alphabet (wrapping from Z back to A, or vice versa) — write out positions explicitly rather than counting mentally.
6. Guessing strategy
Since the coding rule is always derivable from the given example, there's rarely a good reason to guess blindly here — invest the 15-20 seconds needed to verify the rule against every letter of the example, then apply it mechanically; this yields a certain answer rather than a probabilistic one.
Summary
- Coding-Decoding is roughly 11% of General Intelligence & Reasoning's 30 CBT 1 questions.
- Four common types: letter-shift, reverse-alphabet (mirror), word-reversal, and number-substitution coding.
- Always verify a candidate rule against every letter in the given example before applying it to the new word.
- Check the shift direction explicitly — forward and backward shifts are easy to confuse.
- Watch for combined rules (reversal + shift) when a simple shift doesn't fully explain the example.
- The coding rule is always fully derivable — invest the time to verify it rather than guessing.
