Coding-decoding: Reasoning Ability Notes for Tripura Police Sub-Inspector (SI)

Coding‑Decoding – Study Notes

(Tripura Police Sub‑Inspector – Reasoning Ability)

1. Simple Explanation

Coding – A method of converting a given word or phrase into a new, usually shorter, form using a fixed rule.
Decoding – The reverse process – finding the original word/phrase from its coded form.

In reasoning tests, you are given a pair of words or a list of coded words and must identify:

The rule used for coding (alphabetic shift, letter deletion, substitution, number‑letter mapping, etc.)
The decoded word for the given coded word (or vice‑versa).

2. Key Points

#	What to Look For	Typical Coding Rules	Tips for Solving
1	Letter‑by‑letter patterns	• Shift by fixed number (e.g., +1, -2). • Alternate letters (odd/even).	• Write the alphabet; slide it. • Check if the rule works on all letters.
2	Deletion/Insertion	• Remove vowels/consonants. • Add a fixed letter or symbol.	• Count letters in code vs. original.
3	Reverse/Palindromic	• Word written backwards. • Only first/last 3 letters reversed.	• Reverse the code; compare with options.
4	Group/Chunk coding	• Break word into parts (e.g., syllables) and reorder. • Combine adjacent letters.	• Try grouping in 2‑3 letter chunks.
5	Number‑letter mapping	• 1 → A, 2 → B, … 26 → Z or vice‑versa.	• Translate numbers to letters; check if they form a word.
6	Homophonic / Sound‑based	• Use words that sound similar (e.g., “G” → “gee”).	• Think of homophones or common abbreviations.
7	Mixed rules	• Combination of two or more of the above.	• Identify the simplest rule first; then refine.

Common Traps

Mis‑counting letters – Always count the characters in the coded word and compare with the candidate word.
Assuming alphabetical order – Some codes use reverse alphabet or skip letters.
Ignoring repeated letters – A letter may appear twice in the code but once in the original.

3. Worked Example

Question

Coding	Decoded
1. BRIDGE	A. BIRD
2. S4	B. C
3. NUL	C. NIL
4. 5P	D. M

Choose the correct decoded word for each coded word.

Step‑by‑Step Solution

Coded	Rule	Decoded	Explanation
BRIDGE	Delete every 3rd letter	BIRD	Removing the 3rd (I) and 6th (G) letters → BIRD
S4	Number → Letter (S is 19, 4 → D; 19+4=23 = W)	W	19+4 = 23 → W
NUL	Reverse then remove last letter	NIL	Reverse → LUN → Remove U → L N → NIL
5P	Add 5 to the alphabetical position of P (P=16) → 21 = U	U	16+5 = 21 → U

Answer:

BRIDGE → BIRD
S4 → W
NUL → NIL
5P → U

4. Quick Revision Summary

Item	What to Remember
Coding	Transform an original word into a new form using a rule.
Decoding	Reverse the rule to get back the original word.
Common Rules	Letter shift, deletion, reversal, chunk re‑ordering, number‑letter mapping, homophones.
Strategy	1. Identify the pattern in the coded word. 2. Apply the rule to the coded word. 3. Check against options.
Common Mistake	Not verifying the rule on all letters or mis‑counting letters.
Practice Tip	Solve 10 coding‑decoding problems daily; focus on spotting the simplest rule first.

Remember: In exam settings, the rule will always be consistent across all letters in a word and unique enough to distinguish from the other options. Keep the alphabet handy and practice mental substitution to save time.