Question

Here is the algorithm of insertion sort in C++ (from tutorial):

void insertionSort(int arr[], int length) {
      int i, j, tmp;
      for (i = 1; i < length; i++) {
            j = i;
            while (j > 0 && arr[j - 1] > arr[j]) {
                  tmp = arr[j];
                  arr[j] = arr[j - 1];
                  arr[j - 1] = tmp;
                  j--;
            }
      }
}

and here is what I’m doing in Ruby

a = [12, 1, 18, -3, -2, 66, 31]
puts a

def insertion_sort(source)
    source.to_enum.with_index(1).each do |item, i|
        j = i
        while((j>0) && (source[j-1] > source[j]))
            source[j], source[j-1] = source[j-1], source[j]
            j -= 1  
        end
    end
end

insertion_sort(a)
puts a

it throws an error of comparison of Fixnum with nil failed (ArgumentError). Probably because of overflow.

What did I do wrong?

Answer 1

In the C++ version you have (i = 1; i < length; i++). Which means, it will not run the last round where i = length. That would be out of the array.

In ruby, because you set the offset of the index to 1, the last round, you would have i = length. Hence source[length] is out of source, so returns nil.

1 > nil  # source[j-1] > source[j] when j = length
# ArgumentError: comparison of Fixnum with nil failed