Grouping with LINQ in C# - Index out of bounds

Question

I'm trying to group the strings according to their extension (last three characters) to train my LINQ skills (I'm a newbie), but I keep getting an exception:

System.ArgumentOutOfRangeException: 'Index and length must refer to a location within the string.

My code is bellow: Where is my mistake?

string[] files = new string[10] {"OKO.pdf","aaa.frx", "bbb.TXT", "xyz.dbf","abc.pdf", "aaaa.PDF","xyz.frt", "abc.xml", "ccc.txt", "zzz.txt"};

var res = from file in files
    group file by file.Substring(file.IndexOf(".")+1,file.Length-1) into extensions
    select extensions;

var res1 = files.GroupBy(file => file.Substring(file.IndexOf("."), file.Length - 1));

foreach(var group in res)
{
    Console.WriteLine("There are {0} files with the {1} extension.", group.Count(), group.Key);
}

From : file.Substring(file.IndexOf(".")+1,file.Length-1) To : file.Substring(file.IndexOf(".")+1) — jdweng, Jan 14 '20 at 17:58
Thanks, it was real simple. I'm used to substring methods that accept range and not length — NanoTech, Jan 14 '20 at 18:00
https://stackoverflow.com/questions/1886866/how-to-find-the-extension-of-a-file-in-c — Alexei Levenkov, Jan 14 '20 at 18:02
To clarify @jdweng's correct answer - the signature of the two-parameter substring method is `String.Substring(int startIndex, int length)`, so your out of range exception is because you're trying to get a substring with length equal to the length of the string-1, starting from the character after the '.' - the `String.Substring(int startIndex)` method returns the substring from the specified index to the end of the string — Zaelin Goodman, Jan 14 '20 at 18:04

score 3 · Accepted Answer · answered Jan 14 '20 at 18:07

3

As jdweng mentioned on comment section. You just need to use overload of Substring

The substring starts at a specified character position and continues to the end of the string.

string[] files = new string[10] { "OKO.pdf", "aaa.frx", "bbb.TXT", "xyz.dbf", "abc.pdf", "aaaa.PDF", "xyz.frt", "abc.xml", "ccc.txt", "zzz.txt" };

var res = from file in files
          group file by file.Substring(file.IndexOf(".") + 1) into extensions
          select extensions;

foreach (var group in res)
{
    Console.WriteLine("There are {0} files with the {1} extension.", group.Count(), group.Key);
}

Result would be:

There are 2 files with the pdf extension.
There are 1 files with the frx extension.
There are 1 files with the TXT extension.
There are 1 files with the dbf extension.
There are 1 files with the PDF extension.
There are 1 files with the frt extension.
There are 1 files with the xml extension.
There are 2 files with the txt extension

answered Jan 14 '20 at 18:07

Selim Yildiz

5,254
6
18
28

`IndexOf` will return incorrect result if file contains a dot. See my answer. – JohnyL Jan 14 '20 at 19:06
Yes you are right, but question is not about whether use `Substring` or `Split`. Basically It is about implementation of `Substring`. – Selim Yildiz Jan 14 '20 at 19:10
*I'm trying to group the strings according to their extension* Huh? – JohnyL Jan 14 '20 at 19:11
Please, offer improvement over your answer. – JohnyL Jan 14 '20 at 19:11

score 2 · Answer 2 · answered Jan 14 '20 at 19:05

As your filenames can contain dots, the position of dot can be incorrect if you use IndexOf. You can use Split method to solve this problem (note that there's a file with a dot OKO.ZOKO.pdf and see the output):

static void other()
{
    var names = new[] { "OKO.pdf", "OKO.ZOKO.pdf", "aaa.frx", "bbb.TXT", "xyz.dbf", "abc.pdf" };
    var x = names.GroupBy(n => n.Split('.').Last());
    x.ToList().ForEach(g => WriteLine($"There are {g.Count()} files with extension '{g.Key}'"));
}

// Output:
//    There are 3 files with extension 'pdf'
//    There are 1 files with extension 'frx'
//    There are 1 files with extension 'TXT'
//    There are 1 files with extension 'dbf'

Grouping with LINQ in C# - Index out of bounds

2 Answers2