⚙️ Introduction to Input-Output
Understand what Input-Output (Machine Arrangement) questions are, the three main types, exam relevance, and the fundamental approach to solve them quickly
Introduction to Input-Output
Input-Output is one of the highest-scoring topics in the Reasoning section of banking Mains exams. It carries 5-10 marks and appears regularly in Clerk Mains, RRB Mains, IBPS PO Mains, SBI PO Mains, RBI Grade B, SO, and SBI PO Prelims.
The questions test your ability to detect hidden patterns in how a "machine" rearranges elements across multiple steps.
What is Input-Output?
A machine takes a line of input (words, numbers, or both) and rearranges it step by step following a fixed hidden rule. Each step produces a new arrangement until the machine reaches its final output.
Your job is to:
- Detect the rule the machine follows at each step
- Apply it to a new input to determine any intermediate or final step
- Answer questions about element positions, step counts, and relationships
The Three Main Types
| Type | What It Involves | Frequency |
|---|---|---|
| Arrangement | Words and/or numbers are rearranged (sorted, shifted) step by step | Most Common |
| Box / Matrix | Elements placed into a grid or paired boxes, then operated on | Moderate |
| Number Operations | Numbers are transformed by mathematical operations each step | Growing |
Type 1: Arrangement-Based
This is the classic and most frequently asked type. A set of words and numbers is sorted into a specific order (ascending, descending, alphabetical, etc.) one element at a time.
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Introduction to Input-Output
Input-Output is one of the highest-scoring topics in the Reasoning section of banking Mains exams. It carries 5-10 marks and appears regularly in Clerk Mains, RRB Mains, IBPS PO Mains, SBI PO Mains, RBI Grade B, SO, and SBI PO Prelims.
The questions test your ability to detect hidden patterns in how a "machine" rearranges elements across multiple steps.
What is Input-Output?
A machine takes a line of input (words, numbers, or both) and rearranges it step by step following a fixed hidden rule. Each step produces a new arrangement until the machine reaches its final output.
Your job is to:
- Detect the rule the machine follows at each step
- Apply it to a new input to determine any intermediate or final step
- Answer questions about element positions, step counts, and relationships
The Three Main Types
| Type | What It Involves | Frequency |
|---|---|---|
| Arrangement | Words and/or numbers are rearranged (sorted, shifted) step by step | Most Common |
| Box / Matrix | Elements placed into a grid or paired boxes, then operated on | Moderate |
| Number Operations | Numbers are transformed by mathematical operations each step | Growing |
Type 1: Arrangement-Based
This is the classic and most frequently asked type. A set of words and numbers is sorted into a specific order (ascending, descending, alphabetical, etc.) one element at a time.
Sub-types of Arrangement:
| Sub-Type | Description |
|---|---|
| Fixed | At each step, the next element is placed at a fixed position (left end or right end) |
| Shifting (Single) | The smallest/largest element shifts to its correct position, pushing others |
| Double Shifting | Elements shift from both left and right simultaneously |
Type 2: Box / Matrix Based
Elements (usually number pairs or letter pairs) are placed in boxes. At each step, an operation is performed on the pairs (multiplication, addition, subtraction) to reduce them.
These are newer and harder — they appear in SBI PO Mains and RBI Grade B.
Type 3: Number Operation Based
Only numbers are given. At each step, a mathematical operation is performed on all numbers: digit-square, digit-multiply, digit-sum, multiplication by constants, etc.
These require strong mental math and pattern recognition.
The Universal Approach
No matter what type of question you face, follow this 4-step method:
Step 1: Compare Input and Step 1
- What changed? Which element moved? Where did it go?
- Did numbers change value, or only change position?
Step 2: Compare Step 1 and Step 2
- Does the same type of change happen?
- Is the pattern consistent (same direction, same operation)?
Step 3: Confirm the Pattern
- Check Steps 2→3 as well. The pattern should hold for all transitions.
Step 4: Apply to the New Input
- Use the detected rule on the new input to generate each step as needed.
How to Count Steps
A common question type asks: "How many steps are required to complete the arrangement?"
Method: The number of total steps = number of elements that need to be moved to their final position. Count how many elements are already in place and subtract from the total.
Shortcut: In most fixed arrangement problems:
- If n elements need sorting, the total steps = n (worst case) or n - 1 (if the last element is automatically in place).
How to Find Position of an Element
Another common question type asks: "What is the 3rd element from the right end in Step 4?"
Speed trick:
- Write out only the specific step asked about
- Count from the specified end (left or right)
- If the question says "3rd from right end in Step 4", write Step 4 and count 3 from the right
For "between" questions (How many elements are between X and Y):
- Find positions of both X and Y in the asked step
- Count elements between them (not including X and Y themselves)
Position Relationship Questions
The trickiest question type: "Which element is 2nd to the right of the one which is 4th to the left element in Step I?"
Break it into parts:
- Find the left element in Step I (leftmost = position 1)
- Go 4th to the left — this means the element at position 4 from the left
- Then go 2nd to the right of that element — move 2 positions to the right
- That is your answer
Tip: Always resolve the inner part first, then the outer part.
Exam Strategy
| Strategy | Details |
|---|---|
| Time allocation | Spend 1-2 minutes finding the rule, then each question takes 15-30 seconds |
| Write all steps | For arrangement questions, always write out every step on rough paper |
| Number the positions | Mark positions 1, 2, 3... above or below each step for quick counting |
| Don't overthink the rule | 90% of the time, the rule is simple sorting (ascending/descending) with a specific pick order |
| Practice speed | This topic rewards practice more than any other. After 20-30 sets, pattern detection becomes instant |
Quick Worked Example — See the Pattern
Given:
| Elements | |
|---|---|
| Input | grape 41 mango 18 cherry 65 banana 32 |
| Step 1 | banana grape 41 mango 18 cherry 65 32 |
| Step 2 | banana cherry grape 41 mango 18 65 32 |
| Step 3 | banana cherry grape mango 41 18 65 32 |
| Step 4 | banana cherry grape mango 18 32 41 65 |
Detect the pattern (Steps 1-3):
| Step | What moved? | Where? | Rule |
|---|---|---|---|
| Input → Step 1 | "banana" (alphabetically smallest word) | → Left end | Smallest word moves to left |
| Step 1 → Step 2 | "cherry" (next alphabetically) | → 2nd from left | Next smallest word to left |
| Step 2 → Step 3 | "grape" → "mango" sorted | Alphabetical order complete | All words sorted alphabetically on the left |
| Step 3 → Step 4 | Numbers: 18, 32, 41, 65 | Ascending order on right | Numbers sorted ascending on right side |
Pattern: Words sort alphabetically (left side, one per step) → then numbers sort ascending (right side).
Now if asked: "What is the 3rd element from the right in Step 2?" → Step 2: banana cherry grape 41 mango 18 65 32 → Count from right: 32(1st), 65(2nd), 18(3rd) ✓
This is all input-output is — detect which element moves where at each step. Once you see the rule (usually 1-2 minutes), every question takes 10-15 seconds.
Coming Up
In the next lessons, we will cover each type in detail with fully solved examples and practice sets, starting with the most common type: Fixed Arrangement.