Coding & Decoding

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Coding is the highest-frequency reasoning topic in police recruitment papers — the 2022 SI paper alone carried five. Every question reduces to finding one rule and applying it. The prerequisite is knowing the alphabet positions cold, in both directions.

The Table You Must Memorise

ABCDEFGHIJKLM
12345678910111213
NOPQRSTUVWXYZ
14151617181920212223242526
The EJOTY shortcut. E = 5, J = 10, O = 15, T = 20, Y = 25. Anchor on the nearest of these and count a step or two — far faster than reciting from A.

The Four Types

1. Shift coding — every letter moves a fixed number of places.

2. Complement (opposite) coding — A↔Z, B↔Y, C↔X. The two positions always add to 27.

3. Substitution coding — whole words stand for other words; find them by intersecting the given sentences.

4. Numeric coding — a word maps to a number, usually via the sum of letter positions with some adjustment.

Worked Examples from Real Papers

TS Police Constable 2022 — Q191

In a certain code language 'DOMAIN' is written as 'ALJXFK'. How is 'RATION' coded?

Check the shift letter by letter: D→A, O→L, M→J, I→F, N→K — each is −3. For A→X the count wraps around the end of the alphabet (1 − 3 → 24).
Apply −3 to RATION: R→O, A→X, T→Q, I→F, O→L, N→K.
Answer: OXQFLK.

TS Police Constable 2022 — Q165

FLAME : UOZNV :: BRING : ?

Add the pairs: F(6) + U(21) = 27, L(12) + O(15) = 27, A(1) + Z(26) = 27, M(13) + N(14) = 27.
This is complement coding — replace each letter by its opposite.
BRING: B→Y, R→I, I→R, N→M, G→T.
Answer: YIRMT.
Spotting it fast: if a code letter is near the far end of the alphabet while the original is near the start, test the sum-to-27 rule before anything else.

TS Police SI 2022 — Q76

"Write exam well" is written as 'lm rs ek', "All is well" as 'ek jp pm', and "Exam is easy" as 'rs jp wk'. How is "All write exam well" written?

Compare the sentences pairwise and take the common word against the common code:
Sentences 1 & 2 share well, and share the code ek → well = ek.
Sentences 1 & 3 share exam, and share rs → exam = rs.
Sentences 2 & 3 share is, and share jp → is = jp.
Sentence 1 now leaves write = lm; sentence 2 leaves All = pm.
"All write exam well" = pm, lm, rs, ek — matching the option ek rs lm pm (order is not significant in these codes).

TS Police SI 2022 — Q73

The English alphabet is numbered 1 to 26 starting from the last letter to the first. Which option hides a meaningful word?

Reverse numbering means Z = 1, Y = 2 … A = 26. To decode, the letter at code k sits at normal position 27 − k.
Take 8-18-14-11-15-22 → 19, 9, 13, 16, 12, 5 → S, I, M, P, L, E.
Answer: 8-18-14-11-15-22, spelling SIMPLE.
Method: decode only the first two or three numbers of each option. A dead consonant cluster rules an option out immediately without decoding the rest.

TS Police Constable 2022 — Q123

In a certain code language GAME = 30 and RAT = 42. What is GREAT equal to?

Sum the letter positions first:
GAME = 7 + 1 + 13 + 5 = 26, but the code is 30 — a surplus of 4, the number of letters.
RAT = 18 + 1 + 20 = 39, code 42 — a surplus of 3, again the number of letters.
Rule: sum of positions + number of letters.
GREAT = 7 + 18 + 5 + 1 + 20 = 51, plus 5 letters = 56.
Method: when a plain sum does not match, always check the gap against the word length — it is by far the most common adjustment.

Substitution questions ask about the code world, not the real one. If "pencil is book", then the thing you draw with is called a book. Answer in the coded vocabulary, not the everyday one.

Also see: Series · Blood Relations · Direction Sense

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