Linq然后无限期地跑

Question

I have a function that is simply meant to print out a dictionary of frequent item sets in an easy-to-understand fashion. The goal is to order first by the size of the dictionary key and then by the lexicographical order of a list of numbers. The issue arises in the ThenBy statement as the commented out "hello" will get printed indefinitely. If I change the ThenBy to not use the comparer and simply use another int or string value, it works fine, so I'm clearly doing something wrong.

public static void printItemSets(Dictionary<List<int>, int> freqItemSet)
{
    List<KeyValuePair<List<int>, int>> printList = freqItemSet.ToList();
    printList = printList.OrderBy(x => x.Key.Count)
                         .ThenBy(x => x.Key, new ListComparer())
                         .ToList();
}

The code for the ListComparer is as follows:

public class ListComparer: IEqualityComparer<List<int>>, IComparer<List<int>>
{
    public int Compare(List<int> a, List<int> b)
    {
        int larger = a.Count > b.Count ? 1: -1;
        for (int i = 0; i < a.Count && i < b.Count; i++)
        {
            if (a[i] < b[i])
            {
                return -1;
            }
            else if (a[i] > b[i])
            {
                return 1;
            }
            else { }
        }
        return larger;
    }
}

VERY simple test case:

int[] a = {1, 3, 5};
int[] b = { 2, 3, 5 };
int[] c = { 1, 2, 3, 5 };
int[] d = { 2, 5 };
int[] e = { 1, 3, 4 };
List<int> aL = a.ToList<int>();
List<int> bL = b.ToList<int>();
List<int> cL = c.ToList<int>();
List<int> dL = d.ToList<int>();
List<int> eL = e.ToList<int>();
Dictionary<List<int>, int> test = new Dictionary<List<int>, int>(new ListComparer());
test.Add(aL, 1);
test.Add(bL, 1);
test.Add(cL, 1);
test.Add(dL, 1);
test.Add(eL, 1);

Answer 1

The issue is that ListComparer is not checking if the arrays are the same. The same array is being passed in twice for both x and y . Checking if x and y are equal will resolve your issue.

Answer 2

Your comparer doesn't handle equal items. If the items are equal the order of the two items is what determines which is considered "larger". The comparer is thus not "reflexive". Being reflexive is a property sorting algorithms rely on.

The first line should be var larger = a.Count.CompareTo(b.Count); instead, so that truly equal lists will return 0 rather than either -1 or 1 .