Coding-Decoding: Notes और MCQ Practice

आसान भाषा में:

Coding‑Decoding में एक pattern (rule) के अनुसार अक्षर/संख्या को बदलते हैं, जैसे shift, reverse, add/subtract, multiplication आदि. परीक्षा में अक्सर “+2”, “reverse”, “mirror”, “multiply by 2” जैसे simple operations आते हैं.

मुख्य नियम / Formulas:

• Shift Rule: New = Old + k (k = constant shift)
• Reverse Rule: Write the sequence backwards.
• Mirror Rule: A ↔ Z, B ↔ Y (i.e., 1 ↔ 26, 2 ↔ 25).
• Multiplication/Division Rule: New = Old × k या New = Old ÷ k.
• Percentage Change: % change = (New − Old) ÷ Old × 100

Quick Tricks:

1. पहले दो‑तीन pairs देखें, फिर difference या ratio निकालें – अक्सर वही rule पूरे series में चलता है.
2. “Z‑A” या “9‑0” mirror में, alphabet/number को 27‑position या 10‑position से घटाकर तुरंत original मिल जाता है.

Common Mistakes:

• Rule को केवल पहले pair पर लागू करके बाकी में भी वही मान लेना, जबकि कभी‑कभी alternating pattern होता है.
• Reverse या Mirror को लागू करने के बाद भी shift जोड़ना, जिससे answer गलत हो जाता है.

Concept

Coding‑Decoding questions test your ability to identify patterns and apply a consistent rule to transform a given set of symbols, numbers or letters into another set. The rule (the code) is applied uniformly to each element of the input to obtain the output.

Important formulas / rules / facts

• Identify the type of element: digit, alphabet (uppercase/lowercase), symbol.
• Common operations:
• Arithmetic shift: add/subtract a fixed number (e.g., 5 → 8 means +3).
• Multiplicative shift: multiply/divide by a constant.
• Reverse order or mirror the sequence.
• Position‑based change: change depends on the position (1st, 2nd, …).
• Alternate operations: odd‑position items follow one rule, even‑position items another.
• Use of ASCII values: change may be described as “move 2 steps forward in the alphabet” (A→C, B→D, …).
• When letters are involved, remember the circular nature (Z + 1 = A).
• For digits, after 9 the series may wrap to 0.

Solved Examples

Example 1 (Simple arithmetic shift)

```

Input : 4 9 2 7

Output: 7 12 5 10

```

Solution: Each number is increased by 3. 4+3=7, 9+3=12, etc.

Example 2 (Alternate operations)

```

Input : A B C D E

Output: C B E D G

```

Solution: Odd positions (1,3,5…) are moved two steps forward in the alphabet: A→C, C→E, E→G. Even positions remain unchanged.

Example 3 (Reverse and add)

```

Input : 1 3 5 7

Output: 8 6 4 2

```

Solution: 1) Reverse the order → 7 5 3 1. 2) Subtract each from 9 → 9‑7=2, 9‑5=4, 9‑3=6, 9‑1=8. Finally write in original left‑to‑right position → 8 6 4 2.

Example 4 (Position‑based multiplication)

```

Input : 2 4 6 8

Output: 2 8 18 32

```

Solution: Multiply the nth term by n. 1st term: 2×1=2, 2nd: 4×2=8, 3rd: 6×3=18, 4th: 8×4=32.

Example 5 (Alphabet to number and back)

```

Input : M O N K

Output: 13 15 14 11 → 26 30 28 22 → Z D B V

```

Step‑by‑step:

1. Convert letters to their positions (A=1): M=13, O=15, N=14, K=11.
2. Multiply each by 2: 13×2=26, 15×2=30, 14×2=28, 11×2=22.
3. Convert back to letters using modulo 26 (26→Z, 30→D, 28→B, 22→V).

Shortcuts and tricks

• Look for constant difference or ratio first; it solves most numeric codes.
• Check odd/even positions when a single rule does not fit all items.
• Write down ASCII/Alphabet positions to see hidden arithmetic.
• Reverse‑then‑operate is a common two‑step pattern.
• When letters wrap around, use modulo 26 arithmetic (e.g., (position+shift‑1) % 26 + 1).

Common mistakes to avoid

• Assuming the same operation for all positions without testing odd/even cases.
• Forgetting the circular nature of alphabets; e.g., Z+2 should become B, not a non‑existent character.
• Ignoring the possibility of two‑step codes (reverse + arithmetic) and stopping after the first obvious pattern.
• Mixing up the direction of shift (forward vs. backward) – always verify with at least two elements.
• Over‑complicating: many questions use the simplest rule that fits all given pairs.