Input–output for IBPS Clerk mains

Input–output questions show a "machine" that takes a line of words and numbers and rearranges it step by step. You are shown the input and a few steps, then asked questions like "Which step reads…?" or "What is third from the right in Step 4?"

The questions themselves are easy. The whole difficulty is finding the rule. Spend your first minute on that, test it on every given step, and the five questions that follow take a minute between them. Guess the rule and you can lose the whole set.

The common rules

Most bank-exam machines combine two of these, for example alphabetical words with ascending numbers.

Finding the rule: four checks

1. Input to Step 1: what moved, and where did it go? Usually one or two elements move to an end.
2. Why that element? Is it the smallest number, the first word alphabetically, the longest word?
3. Step 1 to Step 2: does the same kind of element move to the next place? If yes, you have the rule.
4. Did the elements that did not move keep their order? In most machines they do. This is what lets you write any step without redoing the ones before it.

Test the rule on every step you are given before you answer a single question.

Worked machine 1: word–number pairs

Input: tree 42 mango 17 apple 65 kite 30

• Step 1: apple 17 tree 42 mango 65 kite 30
• Step 2: apple 17 kite 30 tree 42 mango 65
• Step 3: apple 17 kite 30 mango 42 tree 65

Finding the rule:

1. Input to Step 1: "apple" and "17" moved to the left end. Apple is first alphabetically; 17 is the smallest number.
2. Step 1 to Step 2: "kite" and "30" moved next, after apple 17. Kite is next alphabetically; 30 is next smallest.
3. The unmoved elements (tree 42 mango 65) kept their order.
4. Rule: at each step, the next word in alphabetical order and the next number in ascending order are placed together at the left end, after the ones already arranged.

After Step 3, the last pair (tree, 65) is already in place, so the machine stops. There are four pairs but only three steps.

Questions:

1. How many steps complete the arrangement? Three.
2. Which step reads "apple 17 kite 30 tree 42 mango 65"? Step 2.
3. In Step 2, which element is third from the right? Counting 65, mango, 42.
4. In Step 1, which element is immediately to the left of "kite"? 65.

Worked machine 2: numbers only

Input: 43 18 76 25 91 60

• Step 1: 43 76 25 91 60 18
• Step 2: 43 76 91 60 18 25
• Step 3: 76 91 60 18 25 43
• Step 4: 76 91 18 25 43 60
• Step 5: 91 18 25 43 60 76
• Step 6: 18 25 43 60 76 91

1. Input to Step 1: 18, the smallest, moved to the right end.
2. Step 1 to Step 2: 25, the next smallest, moved to the right end after 18.
3. Rule: at each step, the smallest number not yet arranged moves to the right end. The rest keep their order.
4. Step 5 is not final because 91 is still at the left, so there are six steps.

Applying a rule to a new input

The mains often gives a second input and asks about its steps. Work out only the steps the questions need, using the rule from machine 1.

New input: orange 53 bat 21 lamp 88 desk 47 zoo 12

• Step 1: bat 12 orange 53 21 lamp 88 desk 47 zoo
• Step 2: bat 12 desk 21 orange 53 lamp 88 47 zoo
• Step 3: bat 12 desk 21 lamp 47 orange 53 88 zoo
• Step 4: bat 12 desk 21 lamp 47 orange 53 zoo 88

Notice that the unarranged elements no longer alternate between word and number after Step 1. That is normal. Remove the two elements that moved and leave the rest exactly as they were.

Tips

• Write each step on its own line and keep the elements aligned so you can compare them at a glance.
• Underline the arranged part of each step. The unarranged part is the only part that changes.
• For "which step" questions, look at the arranged part only. Its length tells you the step number.
• If a question gives a new input, write only as many steps as the questions need.

Practice

Questions 1 to 5 use the new input above (machine 1's rule). Questions 6 to 8 use machine 2.

1. How many steps does the new input need?
2. In Step 3 of the new input, which element is 4th from the right?
3. In Step 1 of the new input, how many elements are between "bat" and "lamp"?
4. In Step 4 of the new input, which element is immediately to the right of "desk"?
5. Which step of the new input reads "bat 12 desk 21 orange 53 lamp 88 47 zoo"?
6. In machine 2, in which step is 25 at the right end?
7. In Step 4 of machine 2, which element is 3rd from the left?
8. In Step 3 of machine 2, what is the sum of the first and last elements?

Answers:

1. Four. There are five pairs, and the last one (zoo, 88) falls into place with Step 4.
2. orange. From the right: zoo, 88, 53, orange.
3. Four. bat 12 orange 53 21 lamp: 12, orange, 53 and 21 stand between.
4. 21. The arranged part reads bat 12 desk 21.
5. Step 2. Two pairs are arranged.
6. Step 2. 43 76 91 60 18 25.
7. 18. 76 91 18 25 43 60.
8. 119. Step 3 is 76 91 60 18 25 43, and 76 + 43 = 119.

What to do next

• Solve one input–output set every other day, and write the rule in one sentence before you touch the questions.
• Build your own machine: take a random input, apply the machine 1 rule, and check each step. Making them teaches you to read them.
• Revise alphanumeric series alongside this topic, because "which element is 3rd from the right" is the same counting skill.