Coding-Decoding for CSAT: Concepts, Patterns and Practice

Coding-decoding is the skill of uncovering the hidden rule that converts a word, a sentence, or a set of numbers into a code, and then applying that same rule to a new input. Every question hands you at least one worked pair, such as SYSTEM written as SYSMET, and asks you to encode or decode something fresh with the identical rule. In CSAT Paper II, the qualifying paper of the UPSC prelims that demands 33 percent to clear, coding questions are among the most scoring items: they test pure pattern recognition, need no memorised facts, and usually fall in under a minute once the method is internalised.

What a code really is

A code is a fixed, repeatable transformation. The plaintext is the original word or number, and the coded form is what the rule produces. Because the rule never changes inside one question, your whole job is rule detection: compare the plaintext with its code letter by letter, spot the operation, and replay it. UPSC's coding questions fall into five families: letter coding (letters become other letters), number coding (letters become numbers), direct substitution (each letter stands for a fixed symbol or digit), sentence coding (whole phrases map to code words), and mixed coding (digits stand for whole words).

Start every question by writing the alphabet with positions 1 to 26 under it. Half the battle in letter coding is simply knowing that M is the 13th letter or that the letter three steps before F is C. The classic memory aid EJOTY marks the 5th, 10th, 15th, 20th, and 25th letters (E, J, O, T, Y), giving you anchor points from which any position is at most two steps away. Also memorise reverse positions: A is 26th from the end, B is 25th, and so on, because many codes are built on the reversed alphabet where A becomes Z, B becomes Y, and each pair sums to 27.

The standard coding patterns

Nearly every UPSC coding question uses one of the patterns below, sometimes in combination. Learn to recognise each by its fingerprint, the tell-tale sign in the first pair you are given.

Worked examples

Example 1 (half reversal). A military code writes SYSTEM as SYSMET and NEARER as AENRER. How is FRACTION written? Split SYSTEM into two halves of three letters: SYS and TEM. The first half stays SYS, the second reverses to MET, giving SYSMET. Check with NEARER: NEA stays, RER reverses to RER, giving AENRER, which matches. Apply to FRACTION: halves FRAC and TION reverse to CARF and NOIT, giving CARFNOIT. This is a CSAT 2016 question, and the halves need not be equal only when the word length is odd; here all words are even.

Example 2 (alternating shift). In a code language MATHEMATICS is written as LBSIDNZUHDR. Write CHEMISTRY in the same code. Compare letter by letter: M to L is minus 1, A to B is plus 1, T to S is minus 1, H to I is plus 1. The pattern alternates minus 1, plus 1 down the whole word. Apply to CHEMISTRY: C minus 1 is B, H plus 1 is I, E minus 1 is D, M plus 1 is N, I minus 1 is H, S plus 1 is T, T minus 1 is S, R plus 1 is S, Y minus 1 is X. The answer is BIDNHTSSX, asked in CSAT 2021. Always verify the alternation on at least four letters before applying it.

Example 3 (position arithmetic). The letters A to Z are numbered 1 to 26. If GHI is 1578 and DEF is 912, what is ABC? Write the positions: GHI is 7, 8, 9. Concatenated, that is 789, and 789 times 2 is 1578. Check: DEF is 4, 5, 6, concatenated 456, times 2 is 912, which matches. So ABC is 123 times 2, giving 246 (CSAT 2020). When a code number looks too large for the word, suspect multiplication or concatenation of positions.

Example 4 (sentence coding). '256' means 'red colour chalk', '589' means 'green colour flower', and '254' means 'white colour chalk'. Which digit stands for 'white'? Find the word common to all three phrases: 'colour' appears everywhere, and the digit 5 appears in all three codes, so 5 is 'colour'. 'Chalk' appears in the first and third phrases; the digits common to codes 1 and 3 are 2 and 5, and 5 is already 'colour', so 2 is 'chalk'. In the third code, 254, the remaining digit 4 must be 'white'. Answer: 4 (CSAT 2017). In sentence coding, always match the common words first and eliminate.

Example 5 (reversed alphabet). The order of the alphabet is reversed and each letter stands for the letter in its position, so A is written as Z, B as Y, and so on. Which code represents LUCKNOW? The rule is that the nth letter from the start is replaced by the nth letter from the end, and each such pair sums to 27. L is 12th, so it becomes the 12th from the end, which is O (12 + 15 = 27). Similarly U (21) becomes F, C (3) becomes X, K (11) becomes P, N (14) becomes M, O (15) becomes L, W (23) becomes D. The code is OFXPMLD (CSAT 2022).

Common traps

The costliest mistake is assuming the shift is constant after checking only the first letter. UPSC loves the alternating shift, where the first letter moves minus 1 and the second moves plus 1; checking two letters is not enough, check four. A second trap is the increasing shift, where the steps grow (minus 1, minus 2, minus 3), as in MUMBAI to LSJXVC asked in CSAT 2018. A third trap appears in sentence coding: students match words by position instead of by commonality, but code words need not sit in the same order as the plaintext words. Finally, in number coding, do not assume A is 1 until the pair confirms it; some codes use reversed positions or sums of digits.

Watch the odd-length word in reversal questions. If a word has seven letters and the rule reverses each half, the middle letter usually stays fixed, and options are designed to punish anyone who reverses the whole word. Also, when options are very close (CARFTION versus CARFNOIT in CSAT 2016), write the code out fully instead of choosing by glance.

Speed tips for the exam hall

• Write A to Z with 1 to 26 once on the rough sheet and reuse it for every coding question in the set.

• Compare the first and last letters of the pair first: if the last letter moved the same way as the first, suspect a constant shift; if not, suspect alternation or reversal.

• For reversal patterns, physically split the word into halves with a slash before reversing anything.

• In sentence coding, list each phrase's words and circle the ones that repeat; the repeated words unlock the repeated code words.

• When stuck between two options, test the rule on the second given pair (if any) rather than re-deriving from scratch.

Key Terms

• Code: the transformed form of a word, phrase, or number produced by applying a fixed rule.

• Plaintext: the original, unencoded word or number before the coding rule is applied.

• Letter coding: a question type in which each letter of a word is replaced by another letter according to a positional rule such as a shift or reversal.

• Number coding: a question type in which letters are converted into numbers using their alphabet positions, sums, or products.

• Direct substitution: a code in which every letter always stands for the same fixed digit or symbol across all given examples.

• Sentence coding: a question type in which whole phrases are coded, and the code for each word is found by comparing words that repeat across phrases.

• EJOTY: a memory aid for alphabet positions: E is 5th, J is 10th, O is 15th, T is 20th, and Y is 25th.

• Reverse alphabet position: the position of a letter counted from Z backwards; each letter and its reverse partner sum to 27, so A pairs with Z and B pairs with Y.

• Alternating shift: a coding pattern in which letters at odd positions move one way and letters at even positions move another way.

Practice questions