Coding‑Decoding – RRB ALP Reasoning Ability
What you’ll learn
• How to spot common coding patterns
• Quick strategies to decode numbers and letters
• How to solve typical RRB‑style problems in seconds
1. Simple Explanation
Coding‑decoding is a reasoning sub‑topic where a code (usually a letter, number, or word) is assigned to something else (a number, a letter, a word, a position, etc.).
The goal is to invert the mapping: given the code, find the original value (or vice‑versa).
Typical coding rules:
| Rule | Example | How it works |
|---|---|---|
| Alphabet → Number | A=1, B=2, …, Z=26 | Add 0, 1, 2 … to get the numeric code |
| Number → Alphabet | 1=A, 2=B | Subtract 0, 1, 2 … |
| Position → Letter | 1st letter → A, 2nd → B | Use the position in the word |
| Reverse Mapping | Reverse alphabet: Z=1, Y=2 | Invert the standard A‑Z order |
| Pattern‑based | 1st letter of a word → code, 2nd letter → code | Follow the given rule |
2. Key Points to Remember
| # | Point | Why it matters |
|---|---|---|
| 1 | Always read the rule first | The rule tells you the mapping direction. |
| 2 | Check for “A=0” or “A=1” | Small shift (0 vs 1) can change the answer drastically. |
| 3 | Look for reverse alphabet | RRB often uses reverse coding (Z=1, Y=2). |
| 4 | Be careful with “position” rules | The rule might refer to the position in the word or in the alphabet. |
| 5 | Check for “modulus” or “plus/minus” | Some codes involve adding/subtracting a constant. |
| 6 | Always double‑check | A single mis‑read rule leads to wrong answer. |
| 7 | Practice pattern recognition | RRB tests your speed, so memorising frequent patterns helps. |
3. Worked Example
Question
In a certain code, “R” = 18, “A” = 1, “B” = 2, “C” = 3, …, “Z” = 26.
What is the code for the word “GHOST”?
Step‑by‑Step
-
Identify the rule
- The code is a direct mapping:
Alphabet → Position in alphabet. - (A=1, B=2, …, Z=26)
- The code is a direct mapping:
-
Break the word into letters
- G H O S T
-
Assign codes
Letter Alphabet position Code G 7 7 H 8 8 O 15 15 S 19 19 T 20 20 -
Write the final coded number
- Combine: 78151920
- (Some questions ask for space‑separated codes: 7 8 15 19 20)
Answer: 78151920 (or 7 8 15 19 20)
4. A More Challenging Example
Question
In a certain code, the 1st letter of a word is coded as “1”, the 2nd as “2”, the 3rd as “3”, …, the 26th as “26”.
If “K” is coded as 12, “T” as 19, what is the code for the word “ALPHA”?
Analysis
-
Rule: Position of letter in the alphabet → code number.
- A=1, B=2, …, Z=26.
-
Check given example
- K (11th) → 12 → means we add +1 to the alphabet position.
- T (20th) → 19 → means we subtract 1.
- So the rule is “add the position of the letter in the word to its alphabet position”?
Let's test:- For K: word position? Suppose word is “K” alone, word position = 1 → 11 + 1 = 12 ✔
- For T: word position = 1 → 20 + 1 = 21 ≠ 19.
- Wrong.
Let's try “alphabet position + (position in the word) – 1”?
- For K alone: 11 + 1 – 1 = 11, not 12.
Another possibility: “alphabet position + (letter’s own position in the alphabet) – 1”? No.
Ok, re‑interpret: maybe the rule is “code = alphabet position + (letter’s index in the word) – 1” but we need to know the word length. Let's test with a 2‑letter word: Suppose word “KT” → K(11)+1-1=11, T(20)+2-1=21 → not 19.
Let's think simpler: The rule may be “code = alphabet position + 1 if the letter is before M, else alphabet position – 1”. Too complicated.
Perhaps the rule is “code = alphabet position + (position of the letter in the alphabet) modulo 26”? No.
Let's step back. Maybe the example was mis‑typed. Let's solve the main part anyway: For “ALPHA”, we need to code each letter.
-
Assume the rule is direct mapping: A=1, L=12, P=16, H=8, A=1.
-
Answer: 1 12 16 8 1 →
1121681or1 12 16 8 1.
Note to learner: In RRB exams, the