3.3 Coding-Decoding

3.3 Coding-Decoding

Quick Answer: Coding-decoding items give you a word and its coded form, then ask you to code another word using the same rule. The rule may be a letter shift, reverse alphabet, or letter-to-number substitution. Map each letter to its code symbol, then apply the mapping to the target word.

Coding-decoding is a core verbal-intelligence item type on the PAF GD Pilot Initial Test. Each item presents a pair — a plain word and its coded form — and asks you to extend the same rule to a new word.

The Four Main Coding Types

1. Letter Shift Coding

Each letter is shifted by a fixed number of positions. Example: CAT → DBU (each letter +1: C→D, A→B, T→U).

2. Reverse Alphabet Coding

Each letter is replaced by its mirror position: A↔Z, B↔Y, C↔X, … Formula: reverse position = 27 − original position. Example: DOG → WLT (D=4→23=W, O=15→12=L, G=7→20=T).

3. Number Coding of Words

Each letter is replaced by a digit given by the example's mapping. Example: FELLOW = 465531. Here the example itself defines the mapping F=4, E=6, L=5, O=3, W=1; you then apply that same mapping to the target word.

4. Mixed / Substitution Coding

The rule combines multiple operations — e.g., shift then reverse, or code the first half of the word one way and the second half another. These are rarer but appear on harder items.

Step-by-Step Method

1. Write the plain word and the coded form aligned, letter by letter (or letter-group by letter-group).
2. Extract the mapping — for each letter, note what it becomes. If the code is numeric, note which digit(s) correspond to which letter.
3. Check for a positional rule — is the code the same for the same letter wherever it appears? (If both L's in FELLOW map to 5, the rule is consistent letter-to-symbol, not position-dependent.)
4. Apply the mapping to the target word, letter by letter.
5. Verify — re-read the extracted mapping and confirm each letter of the target word has a defined code.

Worked Example 1 — Number Coding

Given: FELLOW = 465531. Question: How is ELLOW coded?

Step 1 — align and extract:

Mapping: F=4, E=6, L=5, O=3, W=1.

Step 2 — apply to ELLOW: E=6, L=5, L=5, O=3, W=1 → 65531.

Worked Example 2 — Letter Shift

Given: CAT = DBU (shift +1). Question: How is DOG coded?

Step 1 — extract the rule: C→D (+1), A→B (+1), T→U (+1). The rule is "shift each letter forward by 1."

Step 2 — apply: D→E, O→P, G→H → EPH.

Worked Example 3 — Reverse Alphabet

Given: A = Z, B = Y (reverse alphabet). Question: How is DOG coded?

• D = 4 → 27 − 4 = 23 = W
• O = 15 → 27 − 15 = 12 = L
• G = 7 → 27 − 7 = 20 = T

Answer: WLT.

Worked Example 4 — Shift by +2

Given: WATER = YCVGT (each letter +2). Question: How is FIRE coded?

• F + 2 = H
• I + 2 = K
• R + 2 = T
• E + 2 = G

Answer: HKTG.

Traps to Avoid

• Positional vs value coding — some codes depend on the letter's position in the word (first letter → 1, second → 2, …), not on the letter's identity. If the same letter maps to different digits in different positions, suspect a positional rule.
• One-to-many mappings — occasionally a single letter maps to a two-digit number (e.g., L = 12 → "12"). When concatenating, the coded string can be ambiguous (is "12" one letter or two?). Use the given example to disambiguate: if FELLOW = 465531 has six digits for six letters, each letter maps to exactly one digit.
• Direction confusion — a "+2 shift" means forward (A→C); a "−2 shift" means backward (C→A). Misreading the direction is the most common single error.
• Inconsistent case — treat upper and lower case identically. 'd' and 'D' both equal 4.
• Mixed rules within one word — some items code the first half with one rule and the second half with another. If the standard shift doesn't fit the whole word, check whether the rule changes at the midpoint.

Speed Tips

• For shift coding, count on your fingers if needed — it's fast and reliable for small shifts.
• For reverse alphabet, use 27 − position rather than counting from Z.
• Always verify the mapping is consistent across every letter of the given example before applying it; one misread letter invalidates the whole answer.