Day 2: Fixed point

• Difficulty: Easy
• Programming language: Kotlin

Problem

A fixed point in an array is an element whose value is equal to its index.
Given a sorted array of distinct elements, return a fixed point, if one exists. Otherwise, return false.

For example:

• Given [-6, 0, 2, 40], you should return 2
• Given [1, 5, 7, 8], you should return false

My solution

This is an easy one and I don't want to go to a very complex solution. Let's just iterate the array and find the fixed point 😄

/**
 * Find each fixed point in a sorted array.
 *
 * @param array, sorted array
 * @return an list with each fixed points, the list is empty if there is no fixed points.
 */
fun findFixedPoint(array: IntArray): List<Int>{
    if(array[1] < array[0]) throw IllegalArgumentException("Array not sorted")

    val pointFixedFound = mutableListOf<Int>()
    for ((index, value) in array.withIndex()) {
        if(index == value){
            pointFixedFound.add(index)
        }
    }

    return pointFixedFound.toList()
}

UPDATED VERSION

As

Nick Holmes

Time flies while you're having fun, but then one day your bio says something about being a 30+ year veteran in software engineering. Still, I've not seen it all, let alone done it all (yet).

suggests in the comments, I didn't really follow the question asked. The question is to return A fixed point not all fixed points, so there is the updated version (thanks again to Nick Holmes).

fun findFixedPoint(array: IntArray): Int? {
    val pointFixedFound = mutableListOf<Int>()

    for ((index, value) in array.withIndex()) {
        if(index == value){
            return index
        } else if (value > index) {
            break
        }
    }
    return null
}

The function returns null if there is no fixed point because I didn't find if this is possible to return an int or false in Kotlin, so I return null instead.

Comments

[Nick Holmes]

As a constructive critique of your solution:

A)

if(array[1] < array[0]) throw IllegalArgumentException("Array not sorted")

You are claiming the array is not sorted if the first two elements are not sorted. What about the array [1, 3, 2] or [1, 1, 2]? Also, what if the array is of length 0 or 1?

You weren't asked to validate that the array was sorted, so you could simply remove this line.

B)

return pointFixedFound.toList()

You are asked to return either an integer or a false, but you are returning a list. Although your list contains the answer, if this was an interview question, I would mark you down on this - demonstrating the ability to extract all detail out of a provided specification is important.

C)
Finally, given that this is coding challenge, and that we are told that input list is sorted and distinct, I think a "more optimal" solution is evident;

• as soon as you find a cell value equal to the index, just return the value - no need to go to the end of the list.

• as soon as you find a cell value greater than the index, just return false - there can be no fixed points from here on.

[Xavier Chretien]

First, thank you for your critique! 😃

A) About the first line, you are right I could simply remove that line.

B) In an interview, I would probably follow the problem asked without any modification. In that case, I choose to return every fixed point on the given array but you are right I should just respect the problem.

C) I will update the post with your proposition

Once again thank you I really appreciate your comment 😃