Using a predicate to search through a string - Scala - scala

Question

I'm having difficulty figuring out how to search through a string with a given predicate and determining its position in the string.
def find(x: Char => Boolean): Boolean = {
}
Example, if x is (_ == ' ')
String = "hi my name is"
It would add 2 to the counter and return true

Answer 1

I'm guessing that this is what you want...
Since find is a higher-order function (HOF) - that is, it's a function that takes a function as an argument - it likely needs to be applied to a String instance. The predicate (the function argument to find) determines when the character you're looking for is found, and the find method reports the position at which the character was found. So find should return an Option[Int], not a Boolean, that way you don't lose the information about where the character was found. Note that you can still change an Option[Int] result to a Boolean value (with true indicating the search was successful, false not) by applying .isDefined to the result.
Note that I've renamed find to myFind to avoid a clash with the built-in String.find method (which does a similar job).
import scala.annotation.tailrec
// Implicit class cannot be a top-level element, so it's put in an object.
object StringUtils {
// "Decorate" strings with additional functions.
final implicit class MyRichString(val s: String)
extends AnyVal {
// Find a character satisfying predicate p, report position.
def myFind(p: Char => Boolean): Option[Int] = {
// Helper function to keep track of current position.
#tailrec
def currentPos(pos: Int): Option[Int] = {
// If we've passed the end of the string, return None. Didn't find a
// character satisfying predicate.
if(pos >= s.length) None
// Otherwise, if the predicate passes for the current character,
// return position wrapped in Some.
else if(p(s(pos))) Some(pos)
// Otherwise, perform another iteration, looking at the next character.
else currentPos(pos + 1)
}
// Start by looking at the first (0th) character.
currentPos(0)
}
}
}
import StringUtils._
val myString = "hi my name is"
myString.myFind(_ == ' ') // Should report Some(2)
myString.myFind(_ == ' ').isDefined // Should report true
myString.myFind(_ == 'X') // Should report None
myString.myFind(_ == 'X').isDefined // Should report false
If the use of an implicit class is a little too much effort, you could implement this as a single function that takes the String as an argument:
def find(s: String, p: Char => Boolean): Option[Int] = {
// Helper function to keep track of current position.
#tailrec
def currentPos(pos: Int): Option[Int] = {
// If we've passed the end of the string, return None. Didn't find a
// character satisfying predicate.
if(pos >= s.length) None
// Otherwise, if the predicate passes for the current character,
// return position wrapped in Some.
else if(p(s(pos))) Some(pos)
// Otherwise, perform another iteration, looking at the next character.
else currentPos(pos + 1)
}
// Start by looking at the first (0th) character.
currentPos(0)
}
val myString = "hi my name is"
find(myString, _ == ' ') // Should report Some(2)
find(myString, _ == ' ').isDefined // Should report true
find(myString, _ == 'X') // Should report None
find(myString, _ == 'X').isDefined // Should report false

Answer 2

Counter:
"hi my name is".count (_ == 'm')
"hi my name is".toList.filter (_ == 'i').size
Boolean:
"hi my name is".toList.exists (_ == 'i')
"hi my name is".contains ('j')
Position(s):
"hi my name is".zipWithIndex.filter {case (a, b) => a == 'i'}
res8: scala.collection.immutable.IndexedSeq[(Char, Int)] = Vector((i,1), (i,11))
Usage of find:
scala> "hi my name is".find (_ == 'x')
res27: Option[Char] = None
scala> "hi my name is".find (_ == 's')
res28: Option[Char] = Some(s)

Answer 3

I would suggest separating the character search and position into individual methods, each of which leverages built-in functions in String, and wrap in an implicit class:
object MyStringOps {
implicit class CharInString(s: String) {
def charPos(c: Char): Int = s.indexOf(c)
def charFind(p: Char => Boolean): Boolean =
s.find(p) match {
case Some(_) => true
case None => false
}
}
}
import MyStringOps._
"hi my name is".charPos(' ')
// res1: Int = 2
"hi my name is".charFind(_ == ' ')
// res2: Boolean = true