Rosetta Code

Kaprekar routine

Apply the Kaprekar routine to a 4-digit number and watch it converge to 6174.

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Sourcerosettacode/popular/kaprekar_routine.vibe
type RunResult = {
  from_1000: array<int>,
  from_1234: array<int>,
  from_3524: array<int>,
  from_9998: array<int>
}

def digits_of(n: int, pad: int) -> array<int>
  d: array<int> = []
  remaining = n
  while remaining > 0
    d.push(remaining % 10)
    remaining = remaining // 10
  end
  while d.length < pad
    d.push(0)
  end
  d
end

def sort_digits(digits: array<int>) -> array<int>
  arr: array<int> = []
  i = 0
  while i < digits.length
    arr.push(digits.fetch(i))
    i = i + 1
  end

  i = 0
  while i < arr.length - 1
    j = i + 1
    while j < arr.length
      if arr.fetch(j) < arr.fetch(i)
        temp = arr.fetch(i)
        arr[i] = arr.fetch(j)
        arr[j] = temp
      end
      j = j + 1
    end
    i = i + 1
  end
  arr
end

def from_digits(digits: array<int>) -> int
  result = 0
  i = 0
  while i < digits.length
    result = result * 10 + digits.fetch(i)
    i = i + 1
  end
  result
end

def reverse_array(arr: array<int>) -> array<int>
  result: array<int> = []
  i = arr.length - 1
  while i >= 0
    result.push(arr.fetch(i))
    i = i - 1
  end
  result
end

def kaprekar_step(n: int) -> int
  d = digits_of(n, 4)
  ascending = sort_digits(d)
  descending = reverse_array(ascending)
  from_digits(descending) - from_digits(ascending)
end

def kaprekar_sequence(start: int) -> array<int>
  steps = [start]
  current = start
  while current != 6174 && steps.length < 10
    current = kaprekar_step(current)
    steps.push(current)
  end
  steps
end

def run -> RunResult
  {
    from_3524: kaprekar_sequence(3524),
    from_1234: kaprekar_sequence(1234),
    from_9998: kaprekar_sequence(9998),
    from_1000: kaprekar_sequence(1000)
  }
end

Output

Click Run example to see the result.

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