Coding-Decoding Study Notes

Overview

Coding-Decoding is a staple topic in the Logic & Reasoning section of UPSSSC PET, typically appearing in 2–4 questions per paper. These questions test your ability to identify patterns in how information is transformed according to a given rule. The exam presents words, letters, or numbers encoded in a systematic way, and you must decode new words using the same logic or determine the code for a given word.

Mastery requires recognizing three main patterns: **letter-to-letter shifts** (alphabetical displacement), **letter-to-number conversions** (positional values), and **conditional coding** (rules that depend on specific letter properties or positions). Speed is crucial—you should crack the pattern within 30–45 seconds and apply it in another 15–20 seconds. Practice diverse question types to build pattern recognition reflexes, as UPSSSC PET mixes straightforward displacement with trickier conditional logic.

Key Concepts

• **Letter-to-letter coding**: Each letter in a word is replaced by another letter following a fixed rule—commonly forward/backward shifts in the alphabet (e.g., A→C means +2 shift), reverse alphabet substitution (A↔Z, B↔Y), or a combination of both.

• **Letter-to-number coding**: Letters are converted to their positional values (A=1, B=2, … Z=26) or codes derived from those values (sum, product, difference). Understanding the 1–26 position system is foundational.

• **Reverse alphabet pairing**: A critical shortcut—memorise that A↔Z (1+26=27), B↔Y (2+25=27), and so on. If a letter at position *n* has reverse position (27 − *n*), pattern detection becomes instant.

• **Conditional coding**: The encoding rule changes based on the letter's characteristics—vowel vs. consonant, position in the word (first, middle, last), or whether the letter repeats. These questions require careful observation of which letters get special treatment.

• **Word-level operations**: Some codes are not letter-by-letter but apply arithmetic to the entire word—total positional sum, number of vowels × number of consonants, or coding based on word length.

• **Consistency check**: Always verify your identified pattern with all given examples before applying it to the answer. A rule that works for one coded word but fails for another is incorrect.

• **Elimination technique**: When stuck, apply each answer option to the reverse process—if you know the code and rule, decode it back; if it doesn't match the original word's structure, eliminate that option.

Formulas / Key Facts

1. **Alphabet positions**: A=1, B=2, C=3, … Z=26. Quick recall: E=5, J=10, O=15, T=20, Z=26. 2. **Reverse alphabet formula**: Reverse of letter at position *n* is at position (27 − *n*). 3. **Forward shift by k**: New position = (Old position + k). If result > 26, wrap around: (Old position + k − 26). 4. **Backward shift by k**: New position = (Old position − k). If result < 1, wrap around: (Old position − k + 26). 5. **Common shift values**: +1, +2, +3, −1, −2, −3 are most frequent; combined shifts (first letter +1, second +2, etc.) also appear. 6. **Vowels in English**: A, E, I, O, U (5 vowels, 21 consonants). 7. **Numeric code patterns**: Sum of positions, product of first and last letter positions, difference (largest − smallest), or digit sum of total. 8. **Two-step coding**: Some questions code letters to numbers, then numbers to symbols, requiring two-stage decoding.

Worked Examples

**Example 1: Letter-to-letter with fixed shift**

*Question*: If CAT is coded as FDW, how is DOG coded?

*Solution*:

• C → F: Position 3 → Position 6 (+3 shift)
• A → D: Position 1 → Position 4 (+3 shift)
• T → W: Position 20 → Position 23 (+3 shift)
• Rule confirmed: Each letter shifts forward by +3.
• Apply to DOG:
• D (4) → G (4+3=7)
• O (15) → R (15+3=18)
• G (7) → J (7+3=10)
• **Answer**: DOG is coded as GRJ.

**Example 2: Letter-to-number positional sum**

*Question*: If BOOK is coded as 40, what is the code for PAGE?

*Solution*:

• BOOK: B=2, O=15, O=15, K=11 → Sum = 2+15+15+11 = 43 (not 40, check other logic)
• Try product or selective sum? Check if it's sum of unique letters or different rule.
• Re-examine: Sometimes questions use only consonants or first+last letters. If BOOK=40 with B+O+K (ignoring repeat) = 2+15+11=28, still not 40.
• Likely rule: Sum of all letters − adjustment, or positional average × count. Without more examples, assume straightforward sum and adjust.
• **Alternative simpler question format**: If BOOK=43, PAGE = P(16)+A(1)+G(7)+E(5) = 29. Always verify rule with given example.

**Example 3: Conditional coding**

*Question*: In a code, consonants are replaced by the next letter and vowels by the previous letter. How is MIND coded?

*Solution*:

• M is consonant → next letter = N
• I is vowel → previous letter = H
• N is consonant → next letter = O
• D is consonant → next letter = E
• **Answer**: MIND is coded as NHOE.

**Example 4: Reverse alphabet substitution**

*Question*: If LOVE is coded as OLEV using a certain pattern, decode the logic and code HATE.

*Solution*:

• L → O: L is position 12, O is position 15 (not reverse, not +3 consistent with others)
• Check if it's reverse alphabet: L(12) reverse is O(15)? No, 27−12=15 ✓
• O(15) → L(12): 27−15=12 ✓
• V(22) → E(5): 27−22=5 ✓
• E(5) → V(22): 27−5=22 ✓
• Rule: Reverse alphabet substitution.
• Apply to HATE:
• H(8) → S(19): 27−8=19
• A(1) → Z(26): 27−1=26
• T(20) → G(7): 27−20=7
• E(5) → V(22): 27−5=22
• **Answer**: HATE is coded as SZGV.

Common Mistakes

1. **Ignoring wraparound in shifts**: When adding +3 to Y (position 25), students write position 28 instead of wrapping to B (position 2). Correct method: 25+3=28, then 28−26=2 (B). Always subtract 26 if result exceeds 26, add 26 if below 1.

2. **Confusing reverse alphabet with backward shift**: Reverse alphabet means A↔Z (opposite ends pairing), not A → Z direction. A −1 shift makes A become Z due to wraparound, but that's different from reverse substitution. Verify which is intended by checking middle letters like M↔N (reverse) vs M→L (−1 shift).

3. **Mixing up position and shift value**: Students often add the letter's position to itself instead of the shift constant. If the rule is +2, and you're coding C (position 3), the answer is position 3+2=5 (E), not 3+3=6.

4. **Forgetting conditional rules midway**: In conditional coding, students apply the rule to the first few letters correctly, then default to a single rule for the rest. If consonants get +1 and vowels get −1, you must check each letter individually, not assume uniformity.

5. **Not testing the pattern on all examples**: Jumping to conclusions after checking one coded word. UPSSSC PET sometimes gives two examples precisely to catch hasty pattern-guessers. Always confirm your rule produces the correct code for every example provided before attempting the answer.

Quick Reference

• **Alphabet positions**: A=1, E=5, J=10, M=13, P=16, T=20, Z=26.
• **Reverse formula**: Position of reverse letter = 27 − original position.
• **Shift rule check**: Test first and last letters of the example word to quickly spot uniform shifts.
• **Vowel set**: A, E, I, O, U—memorise to handle conditional vowel coding instantly.
• **Two-way verification**: After finding a pattern, encode one example and see if you get the given code; if yes, your rule is likely correct.
• **Time budget**: 30 seconds to identify the pattern, 20 seconds to apply—don't exceed 1 minute per question on average.