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
17graded MCQs · easy to hard · full solution & trap analysis
In a certain code, consonants are shifted forward by 1 position (B→C, C→D, etc.), and vowels are shifted backward by 1 position (E→A, I→E, O→I, U→O, A→U). What is the code for 'SMILE'?
Practice 2easy
In a code language, each letter is replaced by the letter 2 positions before it in the alphabet (C→A, D→B, E→C, etc.). Letters A and B are coded as Y and Z respectively. What is the code for 'DESK'?
Practice 3easy
In a code, the first and last letters of a word are swapped, and all middle letters remain in their original positions. What is the code for 'GARDEN'?
Practice 4easy
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. Using this rule, what is the code for the word 'HELP'?
Practice 5easy
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. What is the code for 'BRAIN'?
Practice 6easy
In a code, each letter is replaced by a number: A=1, B=2, C=3, ..., Z=26. What is the numerical code for the word 'CAT'?
Practice 7medium
In a certain code, each letter is replaced by the letter that comes 3 positions ahead in the alphabet. For example, A→D, B→E, C→F. Using this rule, what is the code for the word 'HELLO'?
Practice 8medium
In a code, vowels are replaced by the next number (A→1, E→2, I→3, O→4, U→5) and consonants are replaced by the previous letter in the alphabet. For example, B→A, C→B. What is the code for 'BRAIN'?
Practice 9medium
In a certain code, the position of each letter in the word is noted, and the letter is replaced by the letter at the opposite position from the end of the alphabet. For example, A (1st position in alphabet) → Z (26th position), B (2nd) → Y (25th). What is the code for 'LOGIC'?
Practice 10medium
In a code, every consonant is replaced by the consonant that comes 2 positions after it in the alphabet (skipping vowels), and every vowel is replaced by the vowel that comes 2 positions after it (A→E, E→I, I→O, O→U, U→A). What is the code for 'SYSTEM'?
Practice 11medium
In a code, letters are replaced by numbers based on their position in the alphabet, but every 3rd number is doubled. For example, C (3rd letter) → 6 (3×2), F (6th letter) → 12 (6×2). What is the code for 'FACE'?
Practice 12hard
A code reverses the word, then replaces each letter with the letter that comes 5 positions after it in the alphabet (with Z wrapping to E, A wrapping to F, etc.). What is the code for 'SYSTEM'?
Practice 13hard
In a complex code, the word is first split into alternating groups (1st letter alone, 2nd-3rd letters together, 4th letter alone, 5th-6th letters together, etc.), then each group is reversed, and finally all groups are concatenated in reverse order. What is the code for 'PROBLEM'?
Practice 14hard
In a hybrid code, letters in odd positions (1st, 3rd, 5th, etc.) are shifted backward by 2 in the alphabet, and letters in even positions (2nd, 4th, 6th, etc.) are replaced by their position number multiplied by 2. What is the code for 'GARDEN'?
Practice 15hard
In a coding system, each letter is replaced by the number representing its position in the alphabet, then each number is reversed (written backwards). For example, A=1 becomes 1, but when reversed it stays 1; B=2 becomes 2; K=11 becomes 11 reversed to 11. What is the code for the word 'MOTHER'?
Practice 16hard
In a mirror-image coding system, each letter is replaced by its mirror position from the opposite end of the alphabet (A↔Z, B↔Y, C↔X, etc.), and then the entire word is written in reverse order. What is the code for 'BRAIN'?
Practice 17hard
A code uses the rule: consonants are shifted forward by 3 positions in the alphabet, vowels are replaced by the number of vowels that appear before them in the word (starting count from 1). What is the code for 'ELEPHANT'?
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