count char frequency in string with scala - scala

Question

I search a way to count the different chars from a string.
The problem is that's not allowed to use any functions from the scala-api or to use vars (only val).
I want same result like that
val fruit: String = "dasisteintest"
println(fruit.groupBy(identity).mapValues(_.size))
Map(e -> 2, s -> 3, n -> 1, t -> 3, a -> 1, i -> 2, d -> 1)
In every try I made, I have at the end a list[(Char,Int)] where I have to change the Int. But because it's an immutable list I can't change it.
How can I implement that counting char algorithm?

Answer 1

You can use the following snippet:
val fruit: String = "dasisteintest"
val map = scala.collection.mutable.HashMap.empty[Char, Int]
for (symbol <- fruit) {
if (map.contains(symbol))
map(symbol) = map(symbol) + 1
else
map.+=((symbol, 1))
}
println(map)

Answer 2

def countChars(str: String) = {
def loop(chars: List[Char], acc: Map[Char, Int]): Map[Char, Int] = {
chars match {
case Nil => acc
case char :: rest =>
loop(rest, acc + (char -> (acc(char) + 1)))
}
}
loop(str.toList, Map.empty[Char, Int] withDefaultValue 0)
}
Test:
# val fruit: String = "dasisteintest"
fruit: String = "dasisteintest"
# countChars(fruit)
res4: Map[Char, Int] = Map('e' -> 2, 's' -> 3, 'n' -> 1, 't' -> 3, 'a' -> 1, 'i' -> 2, 'd' -> 1)
Whatever you use here is from scala api, a Map.apply, or Map.empty or List.::. It would be difficult not to use any functions from scala api. My guess is that you aren't supposed to be using things like groupBy and you are supposed to do something a bit more low level. Folding is natural solution here, like foldLeft, but if that is considered "using a function from scala api", you can just implement foldLeft yourself, just like I did in my solution.
As for withDefaultValue you can replace it with explicit check if value is present and put a 1 there in that case.
You don't know how to change an in a list or map that is immutable? You just make a new list with that value changed.
For a map, given
val map = Map('a' -> 3)
you can update it doing
# map.updated('a', 4)
res6: Map[Char, Int] = Map('a' -> 4)
or
# map + ('a' -> 4)
res7: Map[Char, Int] = Map('a' -> 4)
Both do exact the same thing - insert or update - and return new map.
Here you can find how to update elements in a list
Replace element in List with scala
although you rarely want to access list by index, you rather just build a new list from old one while iterating over it somehow, e.g. with fold.

Answer 3

What do you mean by "no functions from Scala API"? Does that include no functions from the collections api? If so, then ignore my answer. However if we can't even use reduce methods, I don't see the point of this exercise.
Here's what I came up with:
val fruit: String = "dasisteintest"
fruit.foldLeft[Map[Char,Int]](Map.empty)((map, c) => map + (c -> (map.getOrElse(c, 0) + 1)))
Although can you expand what you mean by ""you have to change the Int"?

Answer 4

Here is the expected code.
First the function which returns the char from the list
def removeFromList(l: List[Char], l2: List[Char], toDelete: Char): List[Char] = {
if (l.isEmpty) l2
else {
if (l.head == toDelete)
removeFromList(l.tail, l2, toDelete)
else
removeFromList(l.tail, l2 :+ l.head, toDelete)
}
}
and then the function which counts the chars and calls removeFromList()
def zaehlZeichen(s: String): List[(Char, Int)] = {
val sl: List[Char] = s.toUpperCase().toList
if (sl.isEmpty) Nil
else {
val l: List[Char] = List()
val tupleList: List[(Char, Int)] = List();
val in: Int = countChar(sl, 0, sl.head)
val t: List[(Char, Int)] = tupleList :+ (sl.head, in)
val cutL: List[Char] = removeFromList(sl, l, sl.head)
t ::: zaehlZeichen(cutL.mkString);
}
}

Answer 5

object CountEachCharInString
{
//Counts the number of a specific character in the Input String
def characterCount(inputChar: Char, str: String): Unit =
{
var num: Int = 0
num = str.count(_ == inputChar);
/*
//Implementation of count method
for(i <- 0 to str.length - 1)
{
if(str(i) == inputChar)
{
num += 1
}
}
*/
println(s"$inputChar appears $num times")
}
def countAllChars(inputStr: String): Unit =
{
//To eliminate duplicates, need one loop inside the other. str(i) == str(j)
for(i <- 0 to inputStr.length - 1)
{
var occurence: Int = 0
for(j <- 0 to i-1)
{
if(inputStr(j) == inputStr(i))
occurence = occurence + 1
}
if(occurence == 0)
{
characterCount(inputStr(i), inputStr)
//var num = inputStr.count(_ == inputStr(i))
//println( inputStr(i) + s" appears $num times")
}
}
}
def main(args: Array[String]): Unit =
{
countAllChars("HelloforHello...^^&&&&")
}
}