Coding and Decoding: Concepts, Formulas, and Clear Examples

Coding–decoding questions hide a fixed rule that maps letters or numbers to symbols (or to other letters). The rule may be positional (+1, −2), mirror (A↔Z), digit-sum, keyboard adjacency, or conditional on vowels/consonants.

Treat each stem as a small cipher: write plain alphabet line, coded line, and mark offsets or patterns on 2–3 examples before decoding the target word. Elimination beats guessing when options share prefixes.

Letter-shift ciphers

Forward shift by k

$$\text{code}(L) = \text{letter at position } (\text{pos}(L) + k) \mod 26$$

• Compare first letters of word and code across examples to find constant k.
• Wrap Z→A when k is positive.

Example — Constant +2

If CAT → ECV, each letter +2: C→E, A→C, T→V. Then DOG → FQI.

Reverse alphabet and mirror

• A=1…Z=26 mirror: code(L) at position $27 - \text{pos}(L)$.
• Sometimes only consonants shift; vowels unchanged.

Number and symbol codes

• Digits may encode position sums: A=1, B=2 → AB = 1+2 = 3 in some schemes.
• Symbols @,#,$ may mean +1, +2, +3 on preceding letter—read the legend in the question.

Conditional and mixed rules

Rule changes at space or at vowel: e.g. first letter +1, second −1 alternating. Split the word into pairs and test on two words before applying.

Word substitution codes

• If ‘sky is blue’ → ‘ptr ho cvsf’, map word-by-word when order preserved.
• Common words (is, the) repeat—use them as anchors.

Verification

• Re-encode your answer with the rule; must match pattern.
• Check k is constant across all letters unless rule says otherwise.

Step-by-step method for exam questions

1. Copy the given word→code pairs in aligned columns.
2. Compute letter differences or position deltas for each index.
3. Identify constant shift, mirror, or alternating pattern.
4. Apply to the asked word letter-by-letter.
5. For number codes, translate letters to positions first.
6. Match option; verify on a second example if provided.

Formula cheat sheet

Frequently asked questions

What if deltas differ per letter?

Look for alternating k, vowel/consonant split, or reverse-order coding.

Case sensitivity?

Aptitude papers almost always use uppercase; treat case as irrelevant unless shown.

Time saver?

Decode only first two letters of options against your rule to eliminate three choices.

Practice on this platform

Use 4 practice sets (10 MCQs, detailed solution after each answer) and 2 timed mocks (20 minutes) on this topic to convert formulas into speed and accuracy.