HSSC CET · Bite-size lessons

Coding-Decoding

About 6 min · Reasoning Ability · English

Time worked out from the lesson's 355 words at 150 words a minute, plus a minute for each question.

Coding-Decoding is a topic in the Reasoning Ability section of HSSC CET and many other competitive exams. The questions test your ability to detect hidden patterns in how letters, numbers, or symbols are transformed according to a secret rule. Once you crack the code, applying it to decode or encode new words becomes straightforward.

Key ideas

• Coding means converting a word/number into a different form using a specific rule. Decoding means reversing the process to find the original.
• Letter-Letter Coding: Each letter is replaced by another letter based on a shifting rule (e.g., each letter moves +2 positions: A→C, B→D).
• Letter-Number Coding: Letters are converted to their positional values (A=1, B=2, ... Z=26) or vice versa, sometimes with arithmetic operations.
• Number-Number Coding: Numbers are transformed using operations like addition, subtraction, multiplication, digit reversal, or place-value swapping.
• Mixed/Symbol Coding: Words or letters are assigned arbitrary symbols (★, @, #) and you must map the pattern from given examples.

Watch out for

• Miscounting letter positions → Always double-check by counting from nearby anchors (A=1, J=10, M=13, T=20, Z=26).
• Forgetting cyclic wrap-around → When shifting beyond Z, continue from A. Example: Y+3 = B, not some invalid letter.
• Assuming uniform shift when pattern varies → Some codes use different shifts for each position (1st letter +1, 2nd letter +2, etc.). Check each letter pair individually.

Quick reference

• Memorize: A=1, J=10, M=13, S=19, Z=26 as anchor points for fast counting.
• Opposite pairs sum to 27: A+Z, B+Y, C+X ... useful for reverse alphabet coding.
• Always verify your detected rule on all given examples before applying to the unknown.
• Common shift values: +1, +2, +3, −1, −2.
• Number codes often involve: digit sum, square/cube root, reversal, or product of digits.

Check yourself: 3 questions

This lesson is cut from the topic's note, not rewritten. Fact-checked on 18 Oct 2026: an AI-assisted check of this note against cited sources; the errors it found were corrected that day. Some errors may remain. The questions are from Shishya's validated question bank.

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