Question: Assume I have a dataframe:

```
Col1   Col2   Col3   Val
  a      1      a     2
  a      1      a     3
  a      1      b     5
  b      1      a     10
  b      1      a     20
  b      1      a     25

```

I want to aggregate across all columns and then attach this to my dataframe so that I have this output:

```
Col1   Col2   Col3   Val    Tot
  a      1      a     2      10
  a      1      a     3      10
  a      1      b     5      10
  b      1      a     10     55
  b      1      a     20     55
  b      1      a     25     55

```

Is there something I can do with `tapply` to accomplish this? I'm a little stuck on the most efficient way.

Answer: 
There is no built in way of doing it, You can try something like this

```
var directories = Directory
.GetDirectories(path, "*", SearchOption.AllDirectories)
.Where(x=> Directory.EnumerateFiles(x, "*.jpg").Any() || Directory.EnumerateFiles(x, "*.png").Any())
.ToList();

```
You can use [`Directory.EnumerateFiles`](http://msdn.microsoft.com/en-us/library/system.io.directory.enumeratefiles(v=vs.110).aspx) method to get the file matching criteria and then you can get their Path minus file name using `Path.GetDirectoryName` and add it to the `HashSet`. `HashSet` would only keep the unique entries. 

```
HashSet<string> directories = new HashSet<string>();
foreach(var file in Directory.EnumerateFiles(sourceTextBox.Text, 
                                             "*.png",  
                                             SearchOption.AllDirectories))
{
    directories.Add(Path.GetDirectoryName(file));
}

```

For checking multiple file extensions you have to enumerate all files and then check for extension for each file like:

```
HashSet<string> directories = new HashSet<string>();
string[] allowableExtension = new [] {"jpg", "png"};
foreach(var file in Directory.EnumerateFiles(sourceTextBox.Text,
                                             "*", 
                                             SearchOption.AllDirectories))
{
    string extension = Path.GetExtension(file);
    if (allowableExtension.Contains(extension))
    {
        directories.Add(Path.GetDirectoryName(file));
    }
}

```