Input–output for IBPS PO

Input–output is the one reasoning topic where the whole set can be solved before you read a single question. You find the rule, write out every step, and then each question is a lookup. That makes it one of the most reliable sets in the mains, provided you write neatly and never skip a step.

The errors are almost always clerical: an unmoved element slips out of order, or a step is jumped in the head. This guide shows how to read the rule, how to lay out the steps, and how to deal with the awkward cases.

How the question is built

You are shown a sample: an input line and every step the machine produces until the final arrangement. Then you get a new input and questions about its steps. The rule never changes between the sample and the new input.

Common rule families:

Reading the rule from a sample

Sample input: leaf 36 gate 81 bowl 17 park 54

Finding the rule

1. Compare the input with Step 1. "bowl" moved to the left end; it comes first in the dictionary. "81" moved to the right end; it is the largest number.
2. Check Step 2. "gate" (next in the dictionary) sits just after "bowl". "54" (next largest) sits just before "81".
3. Check what did not move: "leaf 36 … 17 park" keep their old order.

The rule: in each step, the next word in dictionary order joins the arranged words on the left, and the next largest number joins the arranged numbers on the right. The final line has words in dictionary order and numbers in ascending order.

Worked set 1

Using the same rule, input: ruby 42 fern 87 lamp 15 coin 63 bird 29

How each step was written

• Step 1: "bird" goes left and "87" goes right. The rest (ruby 42 fern lamp 15 coin 63 29) keep their order.
• Step 2: "coin" joins after "bird"; "63" joins before "87".
• Step 3: "fern" joins after "coin"; "42" joins before "63".
• Step 4: "lamp" joins after "fern". "29" joins before "42", where it already stands. Now "ruby" and "15" are exactly where they belong, so the arrangement is final.

Questions

1. How many steps complete the arrangement? Four.
2. In Step 2, which element is fifth from the right? lamp. From the right: 87, 63, 29, 15, lamp.
3. In Step 1, how many elements sit between "ruby" and "63"? Five (42, fern, lamp, 15, coin).
4. Which step is "bird coin fern ruby lamp 15 29 42 63 87"? Step 3.
5. In the final step, which element is immediately to the right of "ruby"? 15.

Worked set 2: numbers only

A different machine, whose sample shows this rule: in each step, the largest remaining number joins the arranged numbers on the left, and the smallest remaining number joins the arranged numbers on the right.

Input: 45 12 78 33 91 60 27 58

Questions

1. How many steps are needed? Four. With eight numbers and two placed per step, Step 4 completes the line.
2. In Step 2, which number is fourth from the left? 33.
3. In Step 3, what is the difference between the third number from the left and the third from the right? 60 − 33 = 27.
4. Step 2 of another input under this rule is "88 74 30 52 19 41 15 11". What is Step 4?
   • Step 3: 52 joins the left; 19 joins the right. The unmoved 30 and 41 keep their order: 88 74 52 30 41 19 15 11.
   • Step 4: 41 joins the left; 30 joins the right: 88 74 52 41 30 19 15 11.

A method for every set

1. Read the sample's input and Step 1 only. Note what moved and where.
2. Confirm with Step 2, then check the final step matches your rule.
3. Write the new input. Under it, write each step in full, one line per step.
4. After each line, count the elements. The count must match the input.
5. Answer the questions from your written steps only.

Common mistakes

• Shuffling unmoved elements. Copy them in their old order, every time.
• Writing only the moved elements and filling the rest from memory.
• Assuming the number of steps equals the number of words. The last one or two often fall into place on their own.
• Placing the new element at the extreme end when the rule adds it next to the ones already arranged.
• Confusing dictionary order for words that share first letters: compare the second letter, then the third.

Practice set

Use the rule from worked set 1 (words in dictionary order on the left, numbers from the largest on the right).

Input: tree 56 moon 13 gold 91 sand 38 kite 70 dust 24

1. How many steps complete the arrangement?
2. What is Step 2?
3. In Step 3, which element is fourth from the right?
4. In Step 4, which element is immediately to the left of 13?
5. In which step does "kite" first appear third from the left?
6. In Step 1, how many elements sit between "tree" and "kite"?
7. In the last step, which element is seventh from the left?

Steps

Answers

1. Five. In Step 5, "sand" joins after "moon", and "13" is already next to "24". That leaves "tree" in place, so the line is complete.
2. dust gold tree 56 moon 13 sand 38 kite 24 70 91.
3. 24. From the right: 91, 70, 56, 24.
4. tree.
5. Step 3.
6. Six: 56, moon, 13, gold, sand and 38.
7. 13. The words fill positions 1 to 6.

What to do next

• Rewrite the practice steps from the input alone, then compare with the table line by line.
• Solve one input–output set every other day for two weeks, alternating word-number and number-only rules.
• Revise coding–decoding, where rules are also found by testing, and the mains reasoning guide for where this set fits in the paper.