Coding‑Decoding (Reasoning Ability – RBI Grade B)
Coding‑decoding problems ask you to find a hidden rule (the code) that transforms a given set of items into another set of items (the decoded values).
Your job is to discover the rule and then apply it to new items.
1. Simple Explanation
| Step | What to do | Example |
|---|---|---|
| 1. Observe | Look at the given pairs (input → output). | A → 1, B → 2, C → 3 |
| 2. Search for patterns | Check alphabetic positions, numeric operations, word lengths, etc. | 1 → 2 (double), 2 → 4 (double) |
| 3. Formulate a rule | Write a concise statement that maps any input to its output. | “Add 1 to the alphabetic position.” |
| 4. Verify | Apply the rule to all given pairs. If all match, the rule is valid. | Z → 26 → 27 (but if rule says add 1 → 27) |
| 5. Decode new items | Use the rule to find the missing value or to fill a blank. | M → 13 → 14 |
Key idea: The most common rules involve simple arithmetic on positional values, word properties (number of letters, vowels, consonants), or relative positioning in a list.
2. Types of Coding‑Decoding Problems
| Type | Typical Code | Example |
|---|---|---|
| Alphabetic positional | A → 1, B → 2, … |
G → 7 |
| Numeric manipulation | 12 → 21 (reverse), 5 → 25 (square) |
4 → 16 |
| Word coding | WORD → length, WORD → #consonants |
HELLO → 5 |
| Pattern in a sequence | 1, 4, 9, 16 → 1, 2, 3, 4 |
2, 4, 6, 8 → 1, 2, 3, 4 |
| Matrix coding | Each row/column gives a rule | 2×2 matrix → product of diagonals |
| Letter‑number mapping | A → 2, B → 4, C → 6 (double alphabet position) |
E → 10 |
| Word‑to‑letter mapping | APPLE → A, BANANA → B (first letter) |
MANGO → M |
| Conditional coding | If vowel → 0, else → 1 | A → 0, B → 1 |
Tip: RBI questions usually use one of these simple schemes; the trick is spotting the right one quickly.
3. Key Points & Strategies
| Strategy | Why it Helps |
|---|---|
| Check alphabetic order | Many problems use A=1, B=2… or reverse Z=1. |
| Look for arithmetic operations | Addition, subtraction, multiplication, division, exponentiation, reverse. |
| Count letters | Word length, number of vowels, number of consonants, position of a specific letter. |
| Check even‑odd patterns | Even numbers → even letters, etc. |
| Look for symmetry | Palindromes, mirrored positions, matrix symmetry. |
| Test multiple simple rules | If one rule fails, try another (e.g., add 1 vs multiply by 2). |
| Use elimination | If a rule fails for one pair, discard it. |
| Cross‑check with all pairs | Avoid over‑fitting a rule to a single pair. |
| Remember “coding” as a one‑to‑one map | Each input has one output; no two inputs share the same output unless the problem states it. |
4. Worked Example
Problem
| Input | Output |
|---|---|
| A | 2 |
| D | 5 |
| G | 8 |
| J | 11 |
| ? | 14 |
Question: What is the missing input?
Step‑by‑Step Solution
-
Observe the pairs.
A→2,D→5,G→8,J→11.
Notice that the output is always alphabetic position + 1. -
Verify the rule.
A is 1 →1+1 = 2✔️
D is 4 →4+1 = 5✔️
G is 7 →7+1 = 8✔️
J is 10 →10+1 = 11✔️ -
Apply the rule to the missing output.
Desired output = 14.
Find the letter whose position + 1 = 14 →
position = 14 – 1 = 13.
13th letter of the alphabet = M. -
Answer:
M
Quick Check
M is 13 → 13+1 = 14. ✔️
5. Quick Revision Summary
| Item | What to remember |
|---|---|
| Common code types | Alphabetic position, numeric ops, word length, even/odd, matrix patterns. |
| First step | List all pairs, check for obvious patterns. |
| Rule formulation | Simple arithmetic on position or word property. |
| Verification | Test rule on every pair before finalizing. |
| Decoding |