Find 73 palindromes

Question

I am trying to find 73 decimal numbers which binary representations are palindromes. I also need to get the sum of these numbers. I am totally new to Scala, so I've been trying to create this function which defines palindromes and sums them up. The issue is that I do not really know how to combine it into one function and include that there should be 73 numbers.

I already have a simple function which defines palindromes:

def isPalindrome(someNumber: String): Boolean = someNumber.length > 1 && someNumber.reverse.mkString == someNumber

And I have made some kind of a blueprint for my main function. I try to write all the found palindromic numbers into the list (so I could use some filters later):

def findPalindromes(list: List[Int]): Int = {
  for(i <- 0 to 100){
    if(isPalindrome(Integer.toBinaryString(i))) {
      (xs: List[Int]) => i :: xs
    }
    sum(list)
  }
}

I know some collections functions, but I don't have much experience using them. So I will be grateful if you could explain to me which collections and functions can be used in this case. I would be very grateful for any kind of help!

Answer 1

filter and take could be used here instead of looping.

filter will add an element to a new collection that is returned based on the predicate you pass it. In your case it is isPalindrome

take can be used get n elements from a collection. i.e. the first 73 elements from the ones that passed the filter.

So, if you chain these together you get:

def findPalindromes(list: List[Int]): Int = {
  list.filter(x => isPalindrome(Integer.toBinaryString(x))).take(73).sum
}

You could pass in something like Range(1, 10000).toList. Another improvement could be to make this stop when it finds 73 palindromes instead of finding all of them and taking the first 73. A simple counter could be used for that or a Stream

The Stream version is rather elegant:

def findPalindromes(n: Int) = Stream.from(1).filter(x => isPalindrome(Integer.toBinaryString(x))).take(n)

Force the evaluation of the Stream with sum:

scala> findPalindromes(73).sum res10: Int = 35619