Coding and Decoding
Coding and Decoding is a regular topic of the reasoning paper of ADRE Grade IV. In these questions a word or a group of numbers is written in a secret form called a code, and the candidate has to find the rule by which the code was made and then apply the same rule to a new word. The questions look difficult at first glance, but almost every code is built on a simple and fixed pattern. Once the pattern is discovered, all the questions of that set can be answered quickly and correctly, which makes this a very rewarding topic.
What is a Code
A code is a system in which letters or numbers are replaced by other letters or numbers according to a fixed rule. In coding, a message is written in the normal language, and in decoding, the hidden message is read back into the normal language. The purpose of a code is to keep a message secret, but in reasoning tests the code follows a simple, discoverable logic. The candidate's task is to compare the given word with its code and to write down exactly what change has been made to each letter.
Main Types of Coding
Letter coding replaces each letter of a word by another letter, usually by shifting a fixed number of places in the alphabet. Number coding replaces letters by their position numbers in the alphabet. Substitution coding replaces whole words by other words or symbols. Mixed coding combines letters and numbers. Sometimes a code is based on reversing the word, exchanging the first and last letters, or moving letters in a particular order. Identifying which type is being used is the first and most important step.
How to Solve a Coding Question
- Write the given word and its code one below the other.
- Compare the position of each letter in the word with the corresponding letter in the code.
- Note the change in terms of addition, subtraction or reversal.
- Check whether the same change works for every letter.
- Apply the discovered rule to the new word to get the answer.
Common Coding Patterns
- Shift by a fixed number — each letter moved forward by one or two places.
- Shift by increasing numbers — first letter by one, second by two and so on.
- Reverse the word — the last letter becomes the first.
- Exchange positions — the first and last letters swap places.
- Letter to number — A becomes 1, B becomes 2 and so on.
- Opposite letter — A becomes Z, B becomes Y and so on.
Worked Example
Suppose "CAT" is coded as "DCB". Here C becomes D by moving forward one place, A becomes B by moving forward one place, and T becomes U by moving forward one place. So the rule is that every letter is moved one place forward. Now, to code "DOG", each letter is moved one place forward, giving "EPH". If instead the rule were to move each letter one place backward, "DOG" would become "CNF". This comparison shows why the rule must be tested on every letter before it is used.
How This Topic Appears in the Examination
Coding and decoding usually appears as three to five objective questions of one mark each. Sometimes a single set of two or three codes is given, and two or three questions are asked from the same set, so finding the rule once helps in answering all of them. The questions are usually of a simple letter-shift type, with occasional number codes. A candidate who can quickly compare the letters of the word and its code can answer these questions in a few seconds.
Memory Tips
Write the alphabet and number the letters from 1 to 26 and from 26 down to 1 on rough paper before starting. Compare the first letter of the word and its code first, since the pattern is often visible there. Test the rule on the second and third letters to make sure it is correct. Remember that the shift may increase or decrease, so check the differences one by one. When a code is given for two words, compare them to cross-check the rule.
Common Mistakes to Avoid
- Assuming a rule after comparing only the first letter.
- Confusing a forward shift with a backward shift.
- Forgetting that the shift may change from letter to letter.
- Making an error in the position of letters near the ends of the alphabet.
- Applying the rule to the given word instead of the new word.
How to Prepare This Topic
- Memorise the positions of all letters in the alphabet.
- Practise at least fifteen coding questions every day.
- Work with two given codes to confirm the rule whenever possible.
- Keep a list of the different types of coding patterns.
- Time yourself so that each question takes less than a minute.
Frequently Asked Questions
What is coding and decoding?
Coding means writing a message in a secret form using a fixed rule, and decoding means reading that secret form back into the original message. In reasoning questions you must find the rule and apply it.
What is the most common type of coding question?
Letter coding with a fixed shift is the most common. Each letter of the word is moved forward or backward by a fixed number of places in the alphabet.
How do I find the rule of a code?
Write the word and its code one below the other, compare each letter's position, and note the change. Then check that the same change works for every letter.
Why should I test the rule on more than one letter?
Because a rule that fits the first letter may not fit the others. The shift can increase or decrease, or the letters may be reversed, so testing every letter avoids a wrong answer.
Can coding questions be based on numbers?
Yes. In number coding each letter is replaced by its position number in the alphabet, so A becomes 1, B becomes 2, and so on. Some codes mix letters and numbers.