Foreach Nested Loop No Repetition

Question

This is a very easy problem that I have been stuck on.

I have a IEnumerable IGrouping object with grouped up common key items in it. I now need to compare each of the same common key item in the IEnumerable IGrouping object itself. I used 2 foreach loop to achieve that but the problem is the foreach causes repetition. I need to loop and compare each item without repetition of the one which already went through.

Parallel.ForEach((sameQuickHashGroups, quickHashGroup) =>
{
    foreach (var groupFile in quickHashGroup)
    {
        foreach (var groupFile2 in quickHashGroup)
        {                               
            if (HashTool.ByteToByteCompare(groupFile.FileName, groupFile2.FileName))
            {
                groupFile.FullHash = count.ToString();
                groupFile2.FullHash = count.ToString();
            }                                                           
         }
         count;
     }
});

Can anyone solve this?

Answer 1

Assuming I understand the question now (as apposed to my first answer), You are basically asking how to compare each pair of elements only once.

I would use Linq's ToList() on the IEnumerable, and then use for loops instead of foreach loops.
This way you can compare each pair of elements only once:

var quickHashGroupList = quickHashGroup.ToList();
for(var i = 0; i < quickHashGroupList.Count-1; i++)
{
    var groupFile = quickHashGroupList[i];
    for(var j = i+1; j < quickHashGroupList.Count; j++)
    {
        var groupFile2 = quickHashGroupList[j];
        if (HashTool.ByteToByteCompare(groupFile.FileName, groupFile2.FileName))
        {
            groupFile.FullHash = count.ToString();
            groupFile2.FullHash = count.ToString();
        }     
    }
}

Note that the first loop goes only to count-2, since the second loop starts at i+1.