This page covers Delhi Police Head Constable Coding Decoding with complete concept notes, 45 graded practice MCQs, key points and exam-specific tips. Free to study.
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 Pattern 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?
Shortcut Trick #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 Example 1
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
Pattern identified = Each letter shifts +3 positions
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
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.
Test Coding Decoding under exam conditions
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In a code language, the position of letters in a word is reversed, and then each letter is replaced by the letter 2 positions before it in the alphabet (B→Z, C→A, D→B, etc., wrapping around). What is the code for 'PLAY'?
Practice 2easy
In a certain code language, each letter is replaced by the letter that comes 3 places after it in the English alphabet (A=D, B=E, C=F, ...). Using this rule, what is the code for the word 'COLD'?
Practice 3easy
In a certain code, each letter is replaced by a number: A=1, B=2, C=3, ..., Z=26. If 'CAT' is coded as 3-1-20, what is the code for 'DOG'?
Practice 4easy
In a code language, vowels (A, E, I, O, U) 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 'BRAIN'?
Practice 5easy
In a certain code, the first and last letters of a word are swapped, and all other letters remain in their original positions. For example, 'BOOK' becomes 'KBOO'. What is the code for 'PENCIL'?
Practice 6easy
In a code, each letter is replaced by a number: A=1, B=2, C=3, ..., Z=26. If 'CAT' is coded as 3-1-20, what is the code for 'DOG'?
Practice 7easy
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 the word 'BRAIN'?
Practice 8easy
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 coded word?
Practice 9easy
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, how would the word 'PLAY' be coded?
Practice 10easy
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 11easy
In a certain coding system, 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 system, what will be the code?
Note: After Z, the sequence wraps around to A (i.e., X→A, Y→B, Z→C).
Practice 12easy
In a code, each letter is replaced by the letter that comes 2 positions before it in the English alphabet (B→Z wraps around). For example, C is coded as A. What is the code for 'FRIEND'?
Practice 13easy
In a certain code, 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 'MAKE'?
Practice 14easy
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 15easy
In a code, each letter is replaced by the letter that comes 2 positions before it in the English alphabet. For example, C → A, D → B, and so on. Letters A and B wrap around to Y and Z respectively. Using this rule, what is the code for the word 'FRIEND'?
Practice 16easy
In a code, consonants are coded as the next consonant (B→C, C→D, D→F, F→G, etc., skipping vowels), and vowels are coded as the previous vowel (E→A, I→E, O→I, U→O, A→U). What is the code for 'FRIEND'?
Practice 17easy
In a code language, consonants are replaced by the next consonant (B→C, C→D, D→F, F→G, G→H, H→J, J→K, K→L, L→M, M→N, N→P, P→Q, Q→R, R→S, S→T, T→V, V→W, W→X, X→Y, Y→Z, Z→B), and vowels remain unchanged. What is the code for 'SMILE'?
Practice 18easy
In a code, consonants are numbered by their position in the English alphabet, and vowels are represented by symbols: A=@, E=#, I=$, O=%, U=&. What is the code for 'SYSTEM'?
Practice 19easy
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, and so on. Z wraps around to C. Using this rule, what is the code for the word 'HELLO'?
Practice 20easy
In a code, the word 'BOOK' is written as '2-15-15-11'. Each letter is replaced by its position number in the English alphabet (A=1, B=2, C=3, ... Z=26). Using this rule, how is the word 'CARE' coded?
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