Coding–Decoding — Advanced Exam Problems

🎯 Learning Objective

Crack sentence codes, alternating position shifts and conditional codes — the three toughest coding formats in recent CBT papers.

💡 Concept

• Sentence coding: each word has a fixed code — compare sentence PAIRS, and the common word must carry the common code
• Alternating position codes: odd-position letters follow one shift, even-position letters another — test this the moment one fixed shift fails
• Conditional coding: a letter–digit table plus override conditions (vowel at the start/end etc.) — check the conditions BEFORE touching the table
• The wrap survives every rule: A − 1 = Z and Z + 1 = A, even inside mixed patterns
• Order of attack: single shift → alternating shifts → reverse/opposite → conditions and word-property rules

🧮 Key Formulas

Common word ↔ common code (pair the sentences)

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Alternating code: odd positions ± j, even positions ± k

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Conditions first, table later

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Wrap: A − 1 = Z, Z + 1 = A

✏️ Easy Example

Q. In a code language, 'mango is sweet' is written as 'ta la po' and 'sweet water here' is written as 'po ni ra'. How is 'sweet' written in that language?

1. Compare the two sentences — the only word appearing in both is 'sweet'
2. Compare the two code groups — the only code appearing in both is 'po'
3. A repeated word must carry the repeated code, so sweet = po

Answer: po

🇮🇳 Real-Life Example

Railway control-room messages compress whole phrases into fixed codes, and the same code always means the same word — exactly how sentence coding locks its word–code pairs.

📝 Exam-Level Example

Q. In a certain language, 'train seat window' is written as 'pa ni ta', 'window ticket line' is written as 'ta ru mo' and 'seat line coach' is written as 'ni mo ki'. What is the code for 'ticket'?

1. Pair sentences 1 and 2: the only common word is 'window' and the only common code is 'ta' — so window = ta, because a word keeps the same code everywhere
2. Pair sentences 1 and 3: 'seat' is the common word and 'ni' the only shared code → seat = ni
3. Pair sentences 2 and 3: 'line' is common and 'mo' is shared → line = mo
4. Return to sentence 2 — window (ta) and line (mo) are now known, so the leftover code 'ru' must belong to the leftover word 'ticket'

Answer: ru

📝 Exam-Level Example

Q. In a certain code, MOHAN is written as ONJZP. How is KAVITA written in that code?

1. Test a single shift first: M→O is +2 but O→N is −1 — one fixed shift fails on the second pair, so switch to position-wise rules
2. Split by position: the 1st, 3rd, 5th letters (M, H, N) all move +2, and the 2nd, 4th letters (O, A) move −1 — the rule verifies on every pair, and A − 1 wraps back to Z
3. Apply +2 to KAVITA's odd positions: K→M, V→X, T→V
4. Apply −1 to the even positions: A→Z (wrap), I→H, A→Z (wrap again)
5. Merge the letters back in original order: M Z X H V Z

Answer: MZXHVZ

📝 Exam-Level Example

Q. Letters A, M, P, R, T, U, D, K are coded as 4, 6, 1, 8, 2, 5, 7, 0 respectively. Condition (i): if the first letter is a vowel and the last a consonant, both are coded as the vowel's code. Condition (ii): if the first letter is a consonant and the last a vowel, their codes are swapped. What is the code for RAMPTU?

1. Scan the two ends before coding anything: first letter R is a consonant and last letter U is a vowel — condition (ii) fires, because conditions always override the plain table
2. Swap the end codes: position 1 takes U's code 5, and position 6 takes R's code 8
3. The middle letters follow the table normally: A = 4, M = 6, P = 1, T = 2
4. Assemble in order: 5 4 6 1 2 8 — condition (i) stays unused because its trigger (vowel first) never happened

Answer: 546128

🪄 Memory Trick

Fail fast in a fixed order: single shift, then alternating shifts, then reverse/opposite, then conditions. Each test takes ten seconds — the rule that survives every letter pair is your code.

⚠️ Common Mistakes

• ❌ Verifying the shift on only the first letter pair — alternating codes pass the first pair and fail the second
• ❌ Coding the letters before reading the override conditions
• ❌ In sentence coding, assuming the first code belongs to the first word — codes are usually jumbled within the sentence

🏆 Exam Tips

• ✅ In sentence coding always work with sentence PAIRS — three sentences give three pairs and three free mappings
• ✅ Write letter positions under the word before testing shifts; a +2/−1 zig-zag becomes visible instantly

📌 Summary

• Sentence codes: common word ↔ common code, pair by pair
• Alternating codes: split by odd/even positions when one shift fails
• Conditional codes: conditions first, table second
• Wrap at both ends: A − 1 = Z, Z + 1 = A