# Scala algorithm: Compute the steps to transform an anagram only using swaps

Published

## Algorithm goal

An anagram is a rearrangement of letters in a word to produce another word, eg. spar becomes rasp. Find the steps to transform this anagram; eg spar -> spra -> srpa -> rspa -> rsap -> rasp.

There may be multiple solutions to each rearrangement (depending on where you start, for example).

## Test cases in Scala

``````assert(alignPosition("THE EYES", "THEY SEE", 0) == Nil)
assert(
alignPosition("THE EYES", "THEY SEE", 3) ==
List("THE EYES", "THE YEES", "THEY EES")
)
assert(
computeSwaps("spar", "rasp").toList.flatten ==
List("spar", "spra", "srpa", "rspa", "rsap", "rasp")
)
assert(
computeSwaps("THE EYES", "THEY SEE").toList.flatten ==
List("THE EYES", "THE YEES", "THEY EES", "THEY ESE", "THEY SEE")
)
``````

## Algorithm in Scala

33 lines of Scala (compatible versions 2.13 & 3.0), showing how concise Scala can be!

## Explanation

One approach to this is to split this into two functionalities: to align individual positions (eg find the steps to move the r in spar to front), and then to iterate through with this function over the whole string to the end.

Computing the swaps is done using scanLeft, which is more or less Scala's way of doing a while-loop. (this is © from www.scala-algorithms.com)

We collect all these alignments, and this gives us our final result.

## Scala concepts & Hints

1. ### Option Type

The 'Option' type is used to describe a computation that either has a result or does not. In Scala, you can 'chain' Option processing, combine with lists and other data structures. For example, you can also turn a pattern-match into a function that return an Option, and vice-versa!

``````assert(Option(1).flatMap(x => Option(x + 2)) == Option(3))

assert(Option(1).flatMap(x => None) == None)
``````
2. ### Pattern Matching

Pattern matching in Scala lets you quickly identify what you are looking for in a data, and also extract it.

``````assert("Hello World".collect {
case character if Character.isUpperCase(character) => character.toLower
} == "hw")
``````
3. ### Range

The `(1 to n)` syntax produces a "Range" which is a representation of a sequence of numbers.

``````assert((1 to 5).toString == "Range 1 to 5")

assert((1 to 5).reverse.toString() == "Range 5 to 1 by -1")

assert((1 to 5).toList == List(1, 2, 3, 4, 5))
``````
4. ### scanLeft and scanRight

Scala's `scan` functions enable you to do folds like foldLeft and foldRight, while collecting the intermediate results

``````assert(List(1, 2, 3, 4, 5).scanLeft(0)(_ + _) == List(0, 1, 3, 6, 10, 15))
``````
5. ### Stack Safety

Stack safety is present where a function cannot crash due to overflowing the limit of number of recursive calls.

This function will work for n = 5, but will not work for n = 2000 (crash with java.lang.StackOverflowError) - however there is a way to fix it :-)

In Scala Algorithms, we try to write the algorithms in a stack-safe way, where possible, so that when you use the algorithms, they will not crash on large inputs. However, stack-safe implementations are often more complex, and in some cases, overly complex, for the task at hand.

``````def sum(from: Int, until: Int): Int =
if (from == until) until else from + sum(from + 1, until)

def thisWillSucceed: Int = sum(1, 5)

def thisWillFail: Int = sum(1, 300)
``````
6. ### View

The `.view` syntax creates a structure that mirrors another structure, until "forced" by an eager operation like .toList, .foreach, .forall, .count.

# Scala Algorithms: The most comprehensive library of algorithms in standard pure-functional Scala

### Study our 89 Scala Algorithms: 6 fully free, 89 published & 0 upcoming

Fully unit-tested, with explanations and relevant concepts; new algorithms published about once a week.

### Explore the 21 most useful Scala concepts

To save you going through various tutorials, we cherry-picked the most useful Scala concepts in a consistent form.

## Register now (free)

Register with GitHub

This gives you access to free test cases & hints for all our 89 published algorithms as well as our installation-free Scala IDE.

You can then choose to subscribe to "Scala Algorithms Unlimited", which gets you access to every current and upcoming algorithm solution. We use Stripe so your data is safe.

We will e-mail you at most once per week with new algorithms and relevant content.
Your data is protected and unsubscribing is straightforward. 