Core ConceptRead this first — the foundation of the topic
Core Concept
In coding, each letter or word is replaced by another letter, number, or symbol following a fixed rule. In decoding, you reverse this process to find the original word or its coded form
Letter Shift Coding
Each letter moves forward or backward by a fixed number in the alphabet.
2. Number/Position Coding: Letters are replaced by their position numbers in alphabet.
3
Substitution Coding
Specific letters are replaced by other specific letters.
4
Mixed Letter Coding
Combination of different coding methods
Formula for Letter Shift Coding
If A=1, B=2, C=3... Z=26
Forward Shift: New Position = (Original Position + Shift Value)
Backward Shift: New Position = (Original Position - Shift Value)
If result > 26, subtract 26. If result < 1, add 26.
Exam PatternsWhat examiners ask — read before attempting PYQs
Recognition
SSC CGL typically asks 1-2 questions on coding decoding
Common formats include
- If CAT = DOG, then BAT = ?
- In a code language, CHAIR is written as FKDLU. How is TABLE written?
- If MONDAY is coded as 123456, then SUNDAY is coded as?
ShortcutsUse these to save 30–60 seconds per question
#1 - Position Gap Method:
Find the gap between original and coded letters for first 2-3 letters. Apply same gap to remaining letters. This works for 80% of shift coding questions.
Worked ExampleSolve this step-by-step before moving on
1
Step 1
Find the pattern
H → K (H=8, K=11, difference = +3)
O → R (O=15, R=18, difference = +3)
R → U (R=18, U=21, difference = +3)
S → V (S=19, V=22, difference = +3)
E → H (E=5, H=8, difference = +3)
2
Step 2
Pattern identified = Each letter shifts +3 positions
3
Step 3
Apply to TIGER
T → W (T=20, +3=23=W)
I → L (I=9, +3=12=L)
G → J (G=7, +3=10=J)
E → H (E=5, +3=8=H)
R → U (R=18, +3=21=U)
Answer: TIGER = WLJHU
Worked Example 2:
Question: If in a certain code, BRAIN is written as CQBJO, how is SMILE written?
1
Step 1
Analyze each position
B → C (+1)
R → Q (-1)
A → B (+1)
I → J (+1)
N → O (+1)
Apply to SMILE
S → T (+1)
M → L (-1)
I → J (+1)
L → M (+1)
E → F (+1)
Answer: SMILE = TLJMF
Shortcut Trick #2 - Reverse Alphabet Method:
If A→Z, B→Y, C→X pattern appears, use formula: Coded Letter Position = 27 - Original Letter Position.
Shortcut Trick #3 - Number Coding Quick Check:
For number codes, first check if numbers represent alphabetical positions (A=1, B=2...). If not, look for arithmetic progression patterns.
#1 Most Common Trap/Mistake:
Students often assume the same shift applies to all letters without checking each position carefully. Many codes use different shifts for different positions (like Example 2 above). Always verify the pattern for at least 3 letters before applying to the answer.
Additional exam tip: If the pattern seems too complex, try working backwards from answer options - this often saves precious time in competitive exams.
Key Points to Remember
Coding Decoding involves finding patterns in letter/number substitutions
Most common pattern: Each letter shifts by same number of positions in alphabet
Formula: New Position = Original Position + Shift Value (adjust for alphabet limits)
Always check pattern in first 3 letters before applying to entire word
Reverse alphabet coding: A=Z, B=Y, use 27 minus original position
Number codes often represent alphabetical positions (A=1, B=2, C=3)
Mixed patterns have different shifts for different letter positions
Position Gap Method works for 80% of shift coding questions
If pattern is complex, work backwards from answer options to save time
SSC CGL typically includes 1-2 coding decoding questions per paper
Exam-Specific Tips
A=1, B=2, C=3... Z=26 is the standard alphabetical position formula
Forward shift formula: New Position = Original Position + Shift Value
Backward shift formula: New Position = Original Position - Shift Value
Reverse alphabet coding formula: Coded Position = 27 - Original Position
If coded position exceeds 26, subtract 26 to get correct alphabet position
If coded position is less than 1, add 26 to get correct alphabet position
Common shift values in SSC exams are +3, +2, +1, -1, -2, -3
Mixed position patterns typically alternate between +1 and -1 shifts
Practice MCQs
Coding Decoding — Practice Questions
12graded MCQs · easy to hard · full solution & trap analysis
In a code language, the positions of letters in a word are reversed, and then each letter is replaced by the letter 1 position before it in the alphabet (B→A, C→B, D→C, ... Z→Y, A→Z). What is the code for 'JUMP'?
Practice 2easy
In a certain coding system, each letter is replaced by the letter that comes 3 positions ahead of it in the English alphabet. For example, A is coded as D, B is coded as E, and so on. If a letter reaches beyond Z, it wraps around to the beginning (so X→A, Y→B, Z→C). Using this rule, what is the code for the word 'PRINT'?
Practice 3easy
In a certain code, each letter is replaced by the letter that comes 3 positions after it in the English alphabet. For example, A is coded as D, B is coded as E, and so on. If the word 'HELLO' is coded using this rule, what is the code?
Practice 4easy
In a code language, vowels are replaced by the next vowel in sequence (A→E, E→I, I→O, O→U, U→A), and consonants remain unchanged. Using this rule, what is the code for the word 'BRAIN'?
Practice 5easy
In a certain code, each letter's position in the alphabet is replaced by a number (A=1, B=2, C=3, ... Z=26). What is the numeric code for the word 'CAT'?
Practice 6medium
In a certain code, the first and last letters of a word are swapped, then each remaining letter is shifted backward by 1 position in the alphabet. For example, 'CAT' becomes 'TAS' (swap C and T → TAC, then shift A→Z and C→B → TZB... wait, let me recalculate: swap C and T → TAC, then shift A→Z and C→B gives TZB). Using this rule, what is the code for 'SMILE'?
Practice 7medium
In a code, each letter is assigned a value: A=1, B=2, C=3, ..., Z=26. A word is coded by writing the sum of all letter values, followed by the code of the first letter (its position value), followed by the code of the last letter. For example, 'CAT' has values C=3, A=1, T=20, so sum=24, first=3, last=20, and the code is '24320'. What is the code for 'LOGIC'?
Practice 8medium
In a certain code, each letter is replaced by the letter that comes 3 positions after it in the English alphabet. For example, A→D, B→E, C→F, and so on. Z wraps around to C. Using this rule, what is the code for the word 'BANK'?
Practice 9medium
In a code, every alternate letter starting from the first is replaced by the next letter in the alphabet, and every alternate letter starting from the second is replaced by the previous letter. For example, in 'ABCD': A→B (1st, next), B→A (2nd, previous), C→D (3rd, next), D→C (4th, previous). What is the code for 'GARDEN'?
Practice 10hard
In a hybrid code, each letter is assigned a two-character code: consonants use their position in the consonant sequence (1-indexed) followed by a check digit (position mod 3), and vowels use their position in the vowel sequence (1-indexed) followed by a check digit (position mod 5). For example, B (1st consonant) = 11 (1 mod 3 = 1), and A (1st vowel) = 11 (1 mod 5 = 1). What is the code for LOGIC?
Practice 11hard
In a coding system, each letter is replaced by the number representing its position in the alphabet, then the entire sequence is reversed, and finally each number is decreased by 2. If MOTHER is coded as 10 10 16 16 3 8, what is the code for SISTER?
Practice 12hard
In a mirror-image coding system, each letter is first converted to its reverse alphabet position (A↔Z, B↔Y, etc.), then the word is written backwards, and finally vowels are replaced by the digit representing their position among vowels (A=1, E=2, I=3, O=4, U=5). What is the code for EQUATION?
60-Second Revision — Coding Decoding
Remember: Check pattern in first 3 letters before solving completely
Formula: Shift coding = Original Position ± Shift Value
Trap: Don't assume same shift for all positions - verify each letter
Shortcut: Use Position Gap Method for quick pattern recognition
Technique: Work backwards from options if pattern seems complex
Quick check: A=1 to Z=26 for number-based coding questions
Time saver: Reverse alphabet uses 27 minus original position formula