Coding-decoding questions, with answers

The questions, with answers

1. 1.What are the main types of coding-decoding question?

   Six cover almost everything. Letter shifting: each letter moves a fixed number of places along the alphabet. Reversal: the word is written backwards, sometimes with a shift added. Letter-to-number: each letter becomes its alphabet position, or a function of it. Symbol substitution: letters or digits are replaced by symbols according to a table given in the question. Word substitution: whole words are replaced by code words, and you find a word's code by comparing sentences. Mixed: a shift that changes with position, or alternate letters shifted in opposite directions. The first step in every question is to compare the example word with its code letter by letter and ask which of these it is.

2. 2.How do you crack a letter-shift code quickly?

   Compare the first letter of the word with the first letter of its code and count the gap; then confirm with the second letter. If MANGO is coded as OCPIQ, M to O is +2 and A to C is +2, so every letter moves two forward: apply +2 to the target word. APPLE becomes CRRNG. Watch for the wrap at Z — Y + 2 is A — and for negative shifts, where the code letters come before the originals. Write the alphabet with positions once at the top of your rough sheet (A = 1 ... Z = 26); every other type on this page uses it too, and it is faster than counting on your fingers for each letter.

3. 3.How do reversal-based codes work, and how do you recognise them?

   The code is the word spelled backwards, possibly with a shift or a pair-swap on top. If DELHI is coded as IHLED, the letters are simply reversed, so MUMBAI is IABMUM. The tell is that the code contains exactly the same letters as the word. Variants: reverse and then shift each letter by one (DELHI becomes JIMFE); or swap letters in pairs, so DELHI becomes EDHLI with the lone last letter unchanged; or reverse only the first half. Check the letter set first — same letters means a reversal or reordering, different letters means a shift or substitution — and that one observation halves the possibilities before you do any work.

4. 4.How do letter-to-number and number-to-letter codes work?

   Each letter is replaced by its position in the alphabet, or by some function of it. If CAB is written 3-1-2, the code is the plain position, and DOG is 4-15-7. The variants are functions: position plus 1, position doubled, or the reverse position (A = 26, Z = 1), so that CAB becomes 24-26-25. Occasionally the numbers are summed into one code — CAB as 6 — in which case you sum the target word's positions. Going the other way, a code like 8-5-12-16 decodes to HELP. The alphabet-with-positions sheet is what makes these fast; also memorise the reverse positions of a few anchors (A/Z = 1/26, M/N = 13/14, E/V = 5/22) so you can decode reverse codes without recounting from Z.

5. 5.How do you solve word-substitution puzzles with several coded sentences?

   Find the code word that appears in every sentence containing a given English word, and only in those. If "mo ki la" means "roads are wide", "la pe su" means "rivers are long" and "ki fu ne" means "wide open field", then "are" appears in the first two sentences and "la" is the only code common to both, so "are" is la; "wide" is in the first and third, common code ki; and by elimination "roads" is mo. Word order in the code does not follow English order, so never match by position. When two words appear in exactly the same sentences, the question is unsolvable for those two — and the correct option is usually "cannot be determined" or a code shared by both.

6. 6.How do mixed and position-dependent codes work?

   The shift depends on where the letter sits. Common patterns: the first letter moves +1, the second +2, the third +3; or odd-position letters move forward and even-position letters move backward; or the word is split in half and each half is treated differently. If STAR is coded as TVDV, the shifts are +1, +2, +3, +4, so MOON becomes NQRR. The way in is to write the shift for every letter of the example, not just the first — a code whose first two letters both move +1 might still be position-dependent from the third letter on. If the shifts you find are not constant, look for a pattern in the shifts themselves, exactly as with number series.

7. 7.What are symbol and digit substitution codes, and how do you avoid mistakes in them?

   The question gives a table — 2 stands for @, 5 for %, 7 for $ — and asks you to encode a number or a word, or to spot the string that does not follow the table. The work is transcription, and the errors are transcription errors: reading the wrong row, or applying the table left to right when a rule says the first and last symbols are swapped. Read the whole set of rules before starting, since these questions often add a twist such as "if the first digit is odd, reverse the code". Then encode one symbol at a time with your finger on the table row. It is the least clever question on the paper and one of the most commonly dropped, entirely through haste.

8. 8.What is the fastest overall strategy for the coding-decoding section?

   Spend the first ten seconds classifying, not solving. Compare the example word and its code: same letters means reversal or reordering; letters all shifted by one amount means a simple shift; shifts that grow means position-dependent; numbers mean positions; several sentences mean word substitution. Then apply the rule to the target and verify with one letter you have not used yet. Keep the alphabet-with-positions sheet in view for every question. And read the question stem for the direction — "how is X written" wants you to encode, while "what does Y stand for" wants you to decode — because the most common wrong answer in this section is the right rule applied in the wrong direction.

How the diagnostic asks it

One question from the Aptitude bank, exactly as a sitting would show it. The bank has 4 on coding-decoding and 60 across Aptitude.