Question

I have two lists of two different kinds of objects representing data rows from two sql queries. The first list contains data, and the second contains more detailed data. So as an example:

List1:        List2:
 1   Alice     1   15
 2   Bob       1   19
 3   Carol     2    5
 4   Dave      2    7
               2   20
               4   16

I want to insert rows into List2 so that everyone in List1 has at least one row in List2. So when no rows exist in List2 for a certain person, I want to insert a single one with a default value. In the example case I would have to insert one row for Carol, so I would end up with:

List1:        List2:
 1   Alice     1   15
 2   Bob       1   19
 3   Carol     2    5
 4   Dave      2    7
               2   20
               3    0
               4   16

Does anyone have a clever, clean and efficient way of doing this?

I know that to join these tables together into one I would have to use an Outer Join, for example like in this Outer Join Sample. But I don’t want a new result set. I just want those missing rows to be inserted into List2.

Note: Yes, I know the question\title is kind of… blah… but I don’t know how to formulate it better. Someone please fix it if you can.

Note 2: I can not use SQL. I can not insert those rows in the original table. I am reporting on data, which means I do not touch any of the data. I just read it out. The data is to be used in a master-detail report, and my issue is that when no details exist for a certain master row, then you end up with just an empty space. Which is not good. So I want to insert rows with sensible info so that the user can see that there was nothing to show here.

Answer 1

Assuming your lists are sorted by the Key value like in your example (in this case an integer), something like this should work:

int i = 0;

foreach (var item in List1)
{
    // Skip any items in List2 that don't exist in List1
    // (not sure this is needed in your case), or that we've
    // already passed in List1
    while (List2[i].Key < item.Key)
        i++;

    if (List2[i].Key > item.Key)
    {
        // Create new item in List2
        List2.Add(new List2Item(item.Key, 0));
    }
}

// TODO: resort List2

Depending on how many items you expect to be missing, you might want to Insert into List2 instead, eliminating the need for the resorting. If you expect a lot of items to be missing however, this method will be faster. Alternatively, you could use a linked list for List2.

Note that this will fail if there are duplicate Key entries in List1. You’d need to check for that seperately to prevent multiple new items from being created in List2.