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Scala Assignment: Recursion
package recfun
import scala.collection.mutable.ListBuffer
import common._
/** https://class.coursera.org/progfun-2012-001/assignment/view?assignment_id=4 */
object Main {
def main(args: Array[String]) {
println("Pascal's Triangle")
for (row <- 0 to 10) {
for (col <- 0 to row)
print(pascal(col, row) + " ")
println()
}
}
/**
* Exercise 1: Pascal's Triangle
*/
def pascal(c: Int, r: Int): Int = {
if (c == 0 || c == r) 1
else pascal(c - 1, r - 1) + pascal(c, r - 1)
}
/**
* Exercise 2: Parentheses Balancing
*/
def balance(chars: List[Char]): Boolean = {
def f(chars: List[Char], numOpens: Int): Boolean = {
if (chars.isEmpty) {
numOpens == 0
} else {
val h = chars.head
val n =
if (h == '(') numOpens + 1
else if (h == ')') numOpens - 1
else numOpens
if (n >= 0) f(chars.tail, n)
else false
}
}
f(chars, 0)
}
/**
* Exercise 3: Counting Change
* Write a recursive function that counts how many different ways you can make
* change for an amount, given a list of coin denominations. For example,
* there are 3 ways to give change for 4 if you have coins with denomiation
* 1 and 2: 1+1+1+1, 1+1+2, 2+2.
*/
def countChange(money: Int, coins: List[Int]): Int = {
def f(lastMaxCoin_total_coll: List[(Int, Int)], count: Int): Int = {
if (lastMaxCoin_total_coll.isEmpty) {
count
} else {
val b = ListBuffer[(Int, Int)]()
var newCount = count
for ((lastMaxCoin, total) <- lastMaxCoin_total_coll) {
if (total < money) {
for (c <- coins) {
if (c >= lastMaxCoin) {
val e = (c, total + c)
b += e
}
}
} else if (total == money) {
newCount += 1
}
}
f(b.toList, newCount)
}
}
val b = coins.map { c => (c, c) }
f(b, 0)
}
}
@gsriram7
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gsriram7 commented May 18, 2015

Here is my solution for exercise #2 using recursion and pattern matching..

def parenthesis(input: List[Char], count: Int):Boolean ={
input match {
case Nil if count==0 => true
case Nil if count!=0 => false
case head :: tail if count==(-1) => false
case head :: tail if head == '('=> parenthesis(tail, count + 1)
case head :: tail if head == ')'=> parenthesis(tail, count - 1)
case head :: tail if head!='(' && head!=')' => parenthesis(tail, count)
}
}

@azazi-sa
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azazi-sa commented Jun 2, 2015

How about a tail recursive function?

  /**
   * Exercise 2
   */
  @tailrec
  def balance(chars: List[Char], buffer: Int = 0): Boolean = {

    if (chars.isEmpty || buffer < 0)
      buffer == 0
    else if (chars.head == '(')
      balance(chars.tail, buffer + 1)
    else if (chars.head == ')')
      balance(chars.tail, buffer - 1)
    else
      balance(chars.tail, buffer)
  }

@ganger85
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ganger85 commented Jun 2, 2015

@tailrec
def balance(chars: List[Char], opens: Int = 0): Boolean = {
if (opens < 0) return false
if (chars.length == 0) return opens == 0
chars.head match {
case '(' => balance(chars.tail, opens + 1)
case ')' => balance(chars.tail, opens - 1)
case _ => balance(chars.tail, opens)
}
}

@maximilianmordig
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maximilianmordig commented Jul 3, 2015

  def countChange(money: Int, coins: List[Int]): Int = {
    if ((money < 0) || coins.isEmpty) 0
    else {
      if (money == 0) 1
      else {
        countChange(money - coins.head, coins) + countChange(money, coins.tail)
      }
    }
  }

@rimeh-bennjima
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rimeh-bennjima commented Sep 17, 2015

def balance(chars: List[Char]):Boolean={

val queue= new mutable.Queue[Char]()

def test (chars : List[Char], q: Queue[Char]): Boolean =
{
  chars match {
    case Nil => q.isEmpty
    case '(':: tail => q += '('; test(chars.tail,q)
    case ')':: tail => if (q.isEmpty) false else {q.dequeue  ; test(chars.tail,q)}
    case x::tail => test(chars.tail,q)
  }
}
test(chars,queue)

}

@danielOKeefe
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danielOKeefe commented Sep 20, 2015

another recursive option

def balance(chars: List[Char]): Boolean = {

//true for paren false for other chars
def isParen(aChar : Char) : Boolean = {
  if (aChar == '(' || aChar == ')')
    true
  else
    false
}

def subBalanced(chars: List[Char]) : Boolean = {
  //if we have an empty list we're technically balanced
  if (chars.isEmpty) true
  //if an open paren is at the beginning we aren't balanced
  else if (chars.head == ')') false
  //if we have two of the same type of parentheses we aren't balanced
  else if (chars.head == chars.last)
    false
  else subBalanced(chars.drop(1).dropRight(1))
}

//runs recursive function on filtered list
subBalanced(chars.filter(isParen _))

}

@rohinp
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rohinp commented Apr 20, 2016

  • Exercise 1

`

def pascal(c: Int, r: Int): Int = {

def fact(n:Int):Int =

    if(n == 0) 1

    else n * fact(n - 1)

fact(r) / fact(r-c)

}

`

@haritos30
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haritos30 commented Jun 9, 2016

My solution for Exercise 2:

  def balance(chars: List[Char]): Boolean = {

      def internalCount(chars: List[Char], balance: Int,  needsClosing: Boolean): Boolean = {

      // operation has finished
      if (chars.isEmpty && balance == 0 && needsClosing == false)
        true
      else if(chars.isEmpty)
        false

      else if (chars.head == '(') 
        internalCount(chars.tail, balance+1, true)
      else if (chars.head == ')') 
        internalCount(chars.tail, balance-1, false)
      else 
        internalCount(chars.tail, balance, needsClosing)
    }

    return internalCount(chars, 0, false)
  }

@SangeetaGulia
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SangeetaGulia commented Jun 14, 2016

def factorial(num: Long): Long = {
if(num==0) 1
else num*factorial(num-1)
}

def computation(n: Long, r: Long): Double = {
factorial(n)/ (factorial(r) * factorial(n-r))
}

def pascalTriangle(level: Int)={
for(i <- 0 to level) {
for(j <- 0 to i){
print(computation(i,j) + " ")
}
println("")
}
}

pascalTriangle(5)

@mamalisk
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mamalisk commented Aug 9, 2016

Another solution for the balance could be:

 def balance(chars: List[Char]): Boolean = {

   val onlyBrackets = chars.filter(c => '('.equals(c) || ')'.equals(c))

  @tailrec
  def isBalanced(status : Int, chars : List[Char]) : Boolean = {
     if(chars.isEmpty) status == 0
     else status >= 0 && isBalanced({if(chars.head == ')') status - 1 else status + 1}, chars.tail)
  }

  onlyBrackets.size % 2 == 0 && isBalanced(0,onlyBrackets)
 }

@akshaygunner
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akshaygunner commented Oct 15, 2016

def countChange(money: Int, coins: List[Int]): Int = if(coins.isEmpty) 0 else
{
if(money==0) 1 else if(money < 0) 0
else countChange(money - coins.head,coins) + countChange(money,coins.tail)
}

@wzieba
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wzieba commented Nov 27, 2016

If you want to deeply understand algorithm of the third task (without breaking coursera's honor code) I recommend you post on my blog. I provided text and visual explanation with help of mitpress (thank you @thegovyadina for link!)

https://wzieba.github.io/programming/2016/11/25/count-change-algorithm.html

@tuafeeqahmed
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tuafeeqahmed commented Dec 30, 2016

Can you give solution#3 on erlang please? can anyone?

@ivportilla
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ivportilla commented Jan 22, 2017

def pascal(c: Int, r: Int): Int = {
  if ((c == 0) || (r == c)) 1
  else pascal(c - 1, r - 1) + pascal(c, r - 1)
}

def balance(chars: List[Char]): Boolean = {

  def iterate(chars: List[Char], bal: Int): Boolean = {
    if (chars.isEmpty) bal == 0
    else if (bal < 0) false
    else chars.head match {
      case '(' => iterate(chars.tail, bal + 1)
      case ')' => iterate(chars.tail, bal - 1)
      case _ => iterate(chars.tail, bal)
    }
  }

  iterate(chars, 0)
}

@ipergenitsa
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ipergenitsa commented Feb 28, 2017

exercise 3

def countChange(money: Int, coins: List[Int]): Int = {
  if (coins.isEmpty || money < 0) return 0
  if (money == 0) return 1
  countChange(money - coins.head, coins) + countChange(money, coins.tail)
}

@said026
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said026 commented Mar 2, 2017

For the second exercise 👍

 def balance(chars: List[Char]): Boolean = {

      // Checks if there is a closing parentheses, and the count is still positive
      def isBalanced(chain: List[Char], remaining_parenthesis:Int) : Boolean = {
        if (chain.isEmpty)
          true
        else if (chain.head == '(') {
          val chain_tail =chain.tail
          remaining_parenthesis > 0 && chain_tail.count(_ == ')') > 0 &&
            isBalanced(chain_tail, remaining_parenthesis - 1)
        }
        else
          isBalanced(chain.tail, remaining_parenthesis)
      }

      val opening_parentheses_number = chars.count(_ == '(')
      val closing_parentheses_number = chars.count(_ == ')')

      if (opening_parentheses_number!=  closing_parentheses_number)
        false
      else
        isBalanced(chars, closing_parentheses_number)
}

@horatiuj-telenav
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horatiuj-telenav commented Apr 7, 2017

3rd exercise

class CurrencyCalculator extends ((Int, List[Int]) => Int) {
  override def apply(money: Int, coins: List[Int]): Int = {
    val sortedCoins = coins.sorted.reverse
    getPossibilities(money, sortedCoins)
  }

  private def getPossibilities(money: Int, coins: List[Int]): Int = {
    if (coins.isEmpty) return 0
    val largestCoin = coins.head
    val nbOfLargestCoins = money / largestCoin
    (if (money % largestCoin == 0) 1 else 0) + nbOfLargestCoins * getPossibilities(largestCoin, coins.tail)
  }
}

@zty8023ys
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zty8023ys commented Jul 3, 2017

this is a function program!!!
so
def balance(chars: List[Char]): Boolean = chars.filter(_ == '(').length == chars.filter(_ == ')').length

@deekshaaneja
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deekshaaneja commented Jul 7, 2017

BalanceParenthesis using stack

def balance(chars: List[Char]): Boolean = {if (pfunc(chars)==0) return true else (return false)}

def pfunc(chars: List[Char]): Int ={
var st = mutable.StackChar
if(!chars.contains('(')) return 1
for (char <-chars) {
if (char == '(') st.push(char)
else if (char == ')' && !st.isEmpty)
{if ( st.top=='(') st.pop()}
//System.out.println(st.toString(),st.size)
};return st.size}

@valtih1978
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valtih1978 commented Jul 13, 2017

Balance using fold where int accumulator counts the balance between ( and ) and bool accumulator is used for breaking out of the fold, (https://stackoverflow.com/questions/12892701/abort-early-in-a-fold uses opt for that)

  def balance2(chars: Iterable[Char]): Boolean = {
  	val (bool, int) = chars.foldLeft(true -> 0){
  		case ((bool, int), char) => char match {
  			case '(' => bool -> (int + 1)
  			case ')' => (bool && int != 0) -> (int - 1)
  			case _ => bool -> int
  		}
  	}; bool && (int == 0)
  }                                               //> balance2: (chars: Array[Char])Boolean

or even use return to break out of the fold

  def balance22(chars: Iterable[Char]): Boolean = {
  	chars.foldLeft(0){
  		case (acc, char) => char match {
  			case '(' => acc + 1
  			case ')' => if (acc == 0) return false
  				acc - 1
  			case _ => acc
  		}
  	} == 0
  }                                               //> balance22: (chars: Array[Char])Boolean

  balance22("")                                   //> res11: Boolean = true
  balance22("123")                                //> res12: Boolean = true
  balance22("(df())")                             //> res13: Boolean = true
  balance22("())(")                               //> res14: Boolean = false
  balance22("(((")                                //> res15: Boolean = false

@valtih1978
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valtih1978 commented Jul 13, 2017

Based on https://github.com/hsleep/Coursera/blob/master/parprog1/reductions/src/main/scala/reductions/ParallelParenthesesBalancing.scala

  def balance(chars: Iterable[Char]): Boolean = {
    val code = Map('(' -> 1, ')' -> -1).withDefaultValue(0)

    def loop(chLs: List[Char], acc: Int = 0): Int = chLs match {
      case head::tail if acc >= 0 => loop(tail, acc + code(head))
      case _ => acc
    }
    loop(chars.toList) == 0
  }                                               //> balance3: (chars: Array[Char])Boolean

@saleem-mirza
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saleem-mirza commented Sep 19, 2017

I'm still trying to grasp functional programming concepts and honestly struggling with it. However, I came up with an imperative solution.

  def balance(chars: List[Char]): Boolean = {
    val stack = new mutable.Stack[Char]()
    for (c <- chars) {
      if (c == '(')
        stack.push(c)
      else {
        if (c == ')') {
          if (stack.nonEmpty) {
            stack.pop()
          } else {
            return false
          }
        }
      }
    }
    if (stack.isEmpty) {
      true
    } else
      false
  }

@jeffreyyun
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jeffreyyun commented Dec 28, 2017

My solutions for Exercises 1, 2, and 3:

  def pascal(c: Int, r: Int): Int = {
    if (c == 0 || c == r) 1 else pascal(c - 1, r - 1) + pascal(c, r - 1)
  }
  def balance(chars: List[Char]): Boolean = {
    def balanced(chars: List[Char], count: Int): Boolean =
      if (chars.isEmpty) count == 0
      else if (count < 0) false
      else if (chars.head == '(') balanced(chars.tail, count + 1)
      else if (chars.head == ')') balanced(chars.tail, count - 1)
      else balanced(chars.tail, count)

    balanced(chars, 0)
  }
  def countChange(money: Int, coins: List[Int]): Int = {
    if (money == 0) 1
    else if (money < 0) 0
    else if (coins.isEmpty) 0
    else countChange(money - coins.head, coins) + countChange(money, coins.tail)
  }

@taverasmisael
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taverasmisael commented Oct 14, 2018

I tweeked @vigneshwaranr's solution switching the if/elses to a match. Is the same implementation, don't know if better or not, judge yourself:

def balance(chars: List[Char]): Boolean = {
  def balanced(chars: List[Char], open: Int): Boolean =
    if (chars.isEmpty) open == 0
    else chars.head match {
      case '(' => balanced(chars.tail, open + 1)
      case ')' => open > 0 && balanced(chars.tail, open - 1)
      case _ => balanced(chars.tail, open)
    }

  balanced(chars, 0)
}

@PyAntony
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PyAntony commented Dec 25, 2018

Can anybody please explain how in earth do you get solution for number 1 in just 2 lines of code? I mean, not how it works but how do you notice the pattern in the first place!??? I have a working solution but it takes 10 lines of code. I would have never notice for the life of me that it was that simple. Or is everybody just copying/pasting from the internet?

@BarthesSimpson
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BarthesSimpson commented Jan 9, 2019

@PyAntony it helps to break recursive problems down into a base case and a recursive step. If you find yourself stuck, try figuring out your base case(s) first: the simplest possible input (often 0 or 1) where you can just return a value. Then for the recursive step figure out how you'd get from that to the next case. The base case is usually just a statement (or a couple of statements) and the recursive step is then a (tail) recursive function call.

So in the pascal case, the base case is that you are in the first column or the last column, in which case return 1. (I actually prefer to add another base case of an illegal input where c < 0, r < 0 or c > r, in which case I return 0). Then the recursive step is that assuming you already have everything computed up to the current step, you can get the correct value by adding the adjacent values from the row above, i.e. pascal(c-1, r-1) and pascal(c-1, r).

@BarthesSimpson
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BarthesSimpson commented Jan 9, 2019

@jeffreyyun's solutions above are the best. But here are mine.

  def pascal(c: Int, r: Int): Int = {
    if (r < 0 || c < 0 || c > r) 0
    else if (c == 0 || c == r) 1
    else pascal(c - 1, r - 1) + pascal(c, r - 1)
  }
  def balance(chars: List[Char]): Boolean = {
    val OPEN = '('
    val CLOSE = ')'
    def balanceRec(chars: List[Char], numOpen: Int): Boolean = {
      (chars) match {
        case (Nil) => numOpen == 0
        case (c :: tail) =>
          if (numOpen < 0) false
          else if (c == OPEN) balanceRec(tail, numOpen + 1)
          else if (c == CLOSE) balanceRec(tail, numOpen - 1)
          else balanceRec(tail, numOpen)
      }
    }
    balanceRec(chars, 0)
  }
  def countChange(money: Int, coins: List[Int]): Int = {
    (coins) match {
      case (Nil) => 0
      case (c :: tail) =>
        if (money < 0) 0
        else if (money == 0) 1
        else countChange(money - c, coins) + countChange(money, tail)
    }
  }

@nachteulen
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nachteulen commented Mar 9, 2019

def balance(chars: List[Char]): Boolean = {
  def balanced(chars: List[Char], open: Int): Boolean = {
    if (open < 0) false else
    chars match {
      case Nil => open == 0
      case _ => {
      chars.head match {
        case '(' => balanced(chars.tail, open + 1)
        case ')' => balanced(chars.tail, open - 1)
        case _ => balanced(chars.tail, open)
       }
     }
   }
 }

  balanced(chars, 0)
}

@Knk00
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Knk00 commented Jan 29, 2020

def balance(chars: List[Char]): Boolean = {
  def balanced(chars: List[Char], open: Int): Boolean = {
    if (open < 0) false else
    chars match {
      case Nil => open == 0
      case _ => {
      chars.head match {
        case '(' => balanced(chars.tail, open + 1)
        case ')' => balanced(chars.tail, open - 1)
        case _ => balanced(chars.tail, open)
       }
     }
   }
 }

  balanced(chars, 0)
}

Hi, I have a much more simplified version of the balance code:

def balance(chars : List[Char]) : Boolean = {
val equal : Int = 0
@tailrec
def equality (chars : List[Char], equal : Int) : Boolean = {
if (chars.isEmpty) equal == 0 else {
chars.head match{
case '(' => equality(chars.tail, equal + 1)
case ')' => equality(chars.tail, equal - 1)
case _ => equality(chars.tail, equal)
}
}
}
equality(chars, 0)
}

@dandosi
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dandosi commented Jul 26, 2021

:) The answer is really not smart. Code just like in C but not functional language. Use recursive function.

Indeed... the change function, I'm sure works but it uses two if not three, more advanced elements not lectured in the course yet... that's not the point.
maps,
for
and listbuffer

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