Input-Output (Machine Input) Test: Types, Methods and Examples
Input-output is a reasoning topic in which a set of numbers or words is passed through a machine that rearranges it step by step according to a hidden rule, and the candidate has to find the next step, a particular step, or the final arrangement. The first line is called the input and each following line is called a step, with the machine changing the order by moving one or two items at a time until the arrangement is complete. The candidate must discover the rule from the given steps and then apply it.
The special feature of this topic is that it is a puzzle of patterns. The rule is never stated directly; it must be read from the way the items move from one step to the next. This article gives a complete deep-study revision of input-output. It explains the basic terms, the types of rearrangement, the method of detecting the rule, the useful shortcuts, the common mistakes and a practical preparation plan so that the topic becomes a dependable source of marks in APSC, ADRE, Assam Police, SSC and Railway examinations.
Basic Terms
- Input: the first line, which is the original arrangement before the machine acts.
- Step: each line after the input, showing the arrangement after one action of the machine.
- Final arrangement: the last step, where no further change is possible.
- Rearrangement: the change in the order of items from one step to the next.
- Rule: the hidden logic by which the machine moves the items.
- Ascending order: arranging numbers from smallest to largest.
- Descending order: arranging numbers from largest to smallest.
- Alphabetical order: arranging words according to the dictionary.
- Shift: moving an item to a new position without changing the others' relative order.
- Machine number: the step number, such as step I or step II.
Core Concepts and Shortcuts
- Compare the input with the first step and note which item moved and where it went.
- Check whether the machine is fixing the smallest or largest number at an end, or arranging in ascending or descending order.
- For word inputs, check whether words are placed alphabetically or by length.
- Notice whether two items change place at a time or only one item is picked and placed.
- Once the rule is found, apply it to the input to write the required step directly.
- Do not try to write every intermediate step if the question asks only for a later step; count the movements instead.
- In "which step" questions, work forward step by step and count carefully.
Types of Rearrangement
The first type arranges numbers in ascending or descending order by fixing the smallest or largest number at one end in each step. The second type arranges a mix of numbers and words, putting numbers in one order and words in another. The third type shifts a particular item by a fixed number of places each time. The fourth type is based on the number of letters in words, and the fifth type changes the input by a rule such as adding or subtracting a fixed value. Recognising the type quickly is the key, because each type has its own short method.
Method of Detecting the Rule
Start with the input and the first step and ask which item has moved and from where to where. Then compare the first step with the second step and see whether the same kind of move is repeated. If the smallest number has come to the first place in step one and the next smallest in step two, the rule is ascending order from the left. If a word has gone to the last place by alphabetical order, the rule is reverse alphabetical fixing. Writing the position of one item in each step helps in seeing the pattern clearly.
Steps to Solve an Input-Output Question
First, read the input and all the given steps carefully. Second, compare consecutive steps and identify the movement of each item. Third, state the rule in one simple sentence, such as "the largest number is fixed at the right end in each step". Fourth, apply the rule to the input and write the step that is asked for. Finally, verify by checking that the following step given in the question matches the rule you have found.
Examination Trends
In competitive examinations, input-output questions usually give the input and four or five steps, and ask for a particular step, the final step, or the step number at which a given arrangement appears. Some papers ask which element is at a particular position in a particular step. Because the rule is hidden, these questions take a little more time than other reasoning questions, but a candidate who practises the common patterns can solve them quickly and accurately.
Common Mistakes to Avoid
- Do not assume the rule from the first step only; confirm it with the second and third steps.
- Do not mix ascending and descending order while applying the rule.
- Do not skip a step while counting; a single missed step changes the answer.
- Do not apply the rule to the wrong end of the arrangement.
- Do not ignore a mixed input where numbers and words follow different rules.
How to Prepare This Topic
Begin by practising the ascending and descending number arrangement, which is the most common pattern. Then study mixed number-word arrangements, followed by letter-count and shifting patterns. For each question, write the rule in one sentence before answering. Solve at least twenty questions so that the common patterns are recognised at first sight. Finally, attempt previous years' papers under timed conditions, since speed in detecting the rule is the main skill this topic demands.
Frequently Asked Questions
What is input-output in reasoning?
Input-output is a reasoning puzzle in which a set of numbers or words is rearranged step by step by a machine according to a hidden rule, and the candidate must find a required step or the final arrangement.
How do I find the hidden rule?
Compare the input with the first step and then the first step with the second, and note which item moves where. If the same kind of move repeats, that is the rule.
How do I solve a "which step" question?
Work forward step by step from the input, applying the rule carefully, and count the steps until the required arrangement appears.
What is a mixed arrangement?
A mixed arrangement has both numbers and words. Usually the numbers are arranged in one order and the words in another, so the candidate must follow two rules at the same time.
Why is this topic slower than other reasoning topics?
Because the rule is not given and must be discovered from the steps. With practice, the common patterns are recognised quickly and the speed improves a lot.