Coding‑Decoding – Reasoning Ability
(Uttarakhand Police Constable – Study Notes)
Coding‑Decoding is a common sub‑section of the reasoning ability test.
It tests your ability to recognise patterns, map symbols to letters or numbers, and reverse the process (decode) once a rule is understood.
1. Simple Explanation
| Term | What it means |
|---|---|
| Coding | Transforming a word or a set of letters into a coded form (symbols, numbers, letters, or a mixture). |
| Decoding | Reversing the coded form back into the original word or sentence. |
| Rule/Pattern | The underlying logic that governs the transformation (e.g., “replace each letter by the next in the alphabet”). |
Goal – Identify the rule and apply it consistently.
2. Key Points to Remember
| # | Feature | Typical Examples | Tips |
|---|---|---|---|
| 1 | Alphabetic Shifts | A → B, B → C, etc. Z → A (circular). |
Remember to wrap around the alphabet. |
| 2 | Numeric Substitution | A = 1, B = 2, … Z = 26. |
Check if the numbers are 1‑digit or 2‑digit. |
| 3 | Reverse/Backwards Coding | CAT → TAC. | Look for words that read the same backwards. |
| 4 | Mirror/Reflection | A ↔ Z, B ↔ Y, … | Useful when two letters are symmetrical. |
| 5 | Skip Pattern | A → C (skip B), B → D, etc. | Count the skip “n” positions. |
| 6 | Letter + Number Combination | A1, B2, C3 – each letter is followed by its position. | Keep the sequence consistent. |
| 7 | Symbolic Replacement | @ = A, # = B, etc. | Usually the same symbol is used for the same letter throughout. |
| 8 | Word‑level Coding | HELLO → H2, E1, L2, O1 (counts of repeated letters). | Look for patterns that involve counts. |
| 9 | Mixed Rules | A → 1, B → 2, C → 3 but then reverse the whole string. | Verify each step before proceeding. |
| 10 | Common Ciphers | - Caesar Cipher – shift by a fixed number. - Atbash Cipher – reverse alphabet. - Polybius Square – two‑digit numbers. |
Memorise the basic ones for quick recognition. |
3. Worked Example
Coding Pattern
Replace each letter with the next letter in the alphabet and then reverse the whole coded word.
a. Find the coded form of “SAMPLE”
| Original | +1 (next letter) | After shift |
|---|---|---|
| S | T | T |
| A | B | B |
| M | N | N |
| P | Q | Q |
| L | M | M |
| E | F | F |
After shift → T B N Q M F
Now reverse the entire string: F M Q N B T
Coded word: F M Q N B T
b. Decode “R H W E L G”
-
Reverse first (since coding reversed at the end).
Reverse → G L E W H R -
Shift each letter back by one (undo +1).
| G | → F |
| L | → K |
| E | → D |
| W | → V |
| H | → G |
| R | → Q |
Result → F K D V G Q
- Check if this matches any real word – it doesn’t, so perhaps we mis‑identified the rule.
If the decoded string should be a meaningful word, we might have mis‑interpreted the rule or the coded word contains errors.
Common Pitfall – Always confirm that the decoded output is a recognizable word; if not, revisit the assumed rule.
4. Quick Revision Summary
| Area | What to Recall |
|---|---|
| Alphabetic shifts | +1, +2, …, wrap‑around. |
| Reverse coding | Reverse string after coding. |
| Mirror coding | A↔Z, B↔Y. |
| Number mapping | A=1, B=2, …, or two‑digit Polybius. |
| Pattern detection | Look for repetitions, symmetry, or functional changes (e.g., “skip n letters”). |
| Common ciphers | Caesar, Atbash, Polybius, simple substitution. |
| Process | 1. Identify rule. 2. Apply step‑by‑step. 3. Verify decoded word. |
| Tip | Write down the transformation for each letter; spotting a mismatch early saves time. |
Final Checklist before Attempting a Coding‑Decoding Question
- Read the coded sequence carefully – note letters, numbers, symbols, and order.
- Look for obvious patterns – alphabetical order, numeric increments, symmetry.
- Test a small piece – decode the first few characters to spot the rule.
- Apply the rule consistently across the whole word/phrase.
- Verify – the final decoded word should be meaningful in English.
Good luck!