Hello and welcome to our exciting exploration of Scala string manipulation! In today's lesson, you'll discover an intriguing way to select characters from a string using Scala's immutable StringOps and powerful collection methods. We aim to explain this in such a comprehensive and concise way that you'll master it swiftly. Let's dive in!

Imagine this: You are given a string, and you must traverse it to pick its characters in an unusual order. Start with the first character, then move to the last, pick the second character, then the second-to-last, and so on, until no characters are left to select. Sounds fascinating, doesn't it?

Here's what we mean:

You need to write a Scala function solution(inputString: String): String. This function will take inputString as a parameter, a string composed of lowercase English alphabet letters ('a' to 'z') with a length between 1 to 100 characters. The function will return a new string derived from the input string with characters selected in the aforementioned order.

For example, if inputString is "abcdefg", the function should return "agbfced".

Before diving into problem-solving, let's set up our result store. In Scala, we use StringBuilder to construct a mutable string efficiently.

Scala
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1def solution(inputString: String): String = {
2    val result = new StringBuilder

Once initialized, we'll iterate over the inputString. Scala's Range and until offer a simple way to define loop ranges.

How many iterations are required? Since we are taking two characters per iteration — one from the start and one from the end — the loop should run for half the string's length if it's even, or half plus one if it's odd, to include the middle character if needed.

Scala has an elegant solution to this: (length + 1) / 2 handles both even and odd lengths appropriately.

Here's our function so far:

Scala
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1def solution(inputString: String): String = {
2    val result = new StringBuilder
3    val length = inputString.length
4    for (i <- 0 until (length + 1) / 2) {
5        // Loop implementation in next step
6    }

Within our loop, start picking characters and appending them to result.

First, take a character from the beginning of inputString, i.e., inputString(i), and add it to result.

Next, take a character from the end of the string, derived with the index length - 1 - i, but add this only if it's not the same as the one just added — this prevents duplication when dealing with the middle character in an odd-length string.

Adding these, our function now looks like:

Scala
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1def solution(inputString: String): String = {
2    val result = new StringBuilder
3    val length = inputString.length
4    for (i <- 0 until (length + 1) / 2) {
5        result += inputString(i)
6        if ((length - 1 - i) != i) {
7            result += inputString(length - 1 - i)
8        }
9    }
10    result.toString()
11}

And there you have it, our function is complete!

Congratulations! In this lesson, you've mastered a distinctive aspect of string manipulation using Scala. It was not straightforward, but you did it. Now, selecting characters from a string following such an unusual pattern should feel much more natural.

It's now time for you to apply what you've learned. Engage with more problems requiring similar techniques to solidify these concepts. Enjoy coding, and remember, persistence is key!