Questions
3 questions per paper
Difficulty
Medium
Importance
High yield for HPCL/NTPC
Overview
Coding-Decoding is a critical reasoning topic that tests an aspirant's ability to identify logical patterns and rules within coded strings of letters, numbers, or symbols. Mastery of this section is essential for clearing the reasoning component in PSU exams, as it helps in scoring quick marks by quickly identifying transformation rules. The core task involves mapping input values to output patterns through observation and deductive logic.
Letter Coding
This involves replacing letters with other letters based on specific logical shifts or arrangements. Aspirants must analyze the alphabetical position and the shift sequence, such as forward, backward, or mixed progressions.
- Use A-Z positional values (A=1, Z=26)
- Check for constant shift rules (+1, -1, +2)
- Identify reverse alphabetical pairings (Sum of opposite letters = 27)
- Analyze circular rotation logic
- Watch for vowel vs. consonant transformation rules
Number Coding
Number coding assigns numeric values to letters or words based on a set pattern. These patterns often correlate to the alphabetical rank, the number of letters, or complex arithmetic operations applied to the positions.
- Sum of positional values in a word
- Positional value multiplied by word length
- Arithmetic operations like squaring or adding constant digits
- Individual digit sum reduction
- Vowel/consonant weighted values
Mixed Coding
In this category, entire sentences are encoded into gibberish or alphanumeric strings where the word order is not necessarily preserved. You must use cross-referencing techniques to isolate specific words and their corresponding codes.
- Use the process of elimination by comparing sentence pairs
- Construct a mapping table for common words
- Focus on common words appearing in multiple lines
- Ignore the sequence order of the original sentence
- Verify isolated words against all given statements
Decoding Messages
Decoding requires reverse engineering the transformation rule applied to a message. This tests your speed in recognizing patterns like substitution, transposition, or substitution-transposition hybrids.
- Look for positional symmetry
- Identify if words are reversed or interleaved
- Check for symbol-to-letter substitution blocks
- Examine vowel/consonant segregation logic
Formula Sheet
Positional Value: A=1, B=2, ... Z=26
Opposite Letter Pair: Letter + Opposite = 27
Reverse Rank = (27 - Original Rank)
Exam Tip
Always write down the A-Z positional values (1-26) at the top of your rough sheet as soon as the test begins to avoid mental math errors.
Common Mistakes
- Assuming the alphabetical shift is linear without checking for reverse or zig-zag patterns
- Spending too much time on a single question in mixed coding by not using the comparison method
- Miscalculating the reverse alphabetical rank by forgetting that the sum of opposites must be 27
More Revision Notes
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