Index Out Of Range Exception: List not working with add range

Question

I am getting the index out of range exception. I debuged it and it is specifically at the part of "s[i].AddRange()" The "GetFiles()" method is just the "Directory.GetFiles()" but it can accept multiple file endings.

static List<List<string>> directories
            {
                get
                {
                    Debug.Log(folders.Length);
                    List<List<string>> s = new List<List<string>>();
                    for (int i = 0; (i < folders.Length); i++)
                    {
                        s[i].AddRange(GetFiles(folders[i], "*.png,*.jpg,*.bmp,*.tif,*.tga,*.psd").ToList());
                        s[i].AddRange(GetFiles(folders[i], "*.prefab").ToList());
                    }
                    return s;
                }
            }

You have created an empty list and are trying to access invalid positions within it — DavidG
– DavidG, Commented Apr 23, 2017 at 0:59

Grant Winney · Accepted Answer · 2017-04-23 01:52:03Z

1

When you do this, you get an empty box with the potential to store boxes of strings, but there's nothing in it yet. Trying to access element i causes it to throw the exception you're seeing.

List<List<string>> s = new List<List<string>>();

Before you can access an item, you need to add an item.

s.Add(new List<string>());

In your case though, I wouldn't bother adding an empty list and then accessing it using the index. Just do this instead:

List<List<string>> s = new List<List<string>>();
for (int i = 0; (i < folders.Length); i++)
{
    var list = GetFiles(folders[i], "*.png,*.jpg,*.bmp,*.tif,*.tga,*.psd").ToList();
    list.AddRange(GetFiles(folders[i], "*.prefab").ToList());
    s.Add(list);
}

edited Apr 23, 2017 at 1:52

answered Apr 23, 2017 at 1:35

Grant Winney

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2 Comments

Bilal Shafi Over a year ago

I'm not sure what's happening. Everyone is suggesting the "s.Add(new List<string>());" but for some reason it's not working. Also, for your second suggestion, the way I'm going to use the list requires it to be done the way I'm showing you.

Bilal Shafi Over a year ago

I'm sorry, I assumed your last code block was what everyone else was suggesting. I looked at it again, pasted the code and voila, the problem was solved. Thank you.

Jarrett Robertson · Accepted Answer · 2017-04-23 01:30:18Z

0

You have newed a List> but not added any new lists.

Here is a very stripped down for loop which should help.

List<List<String>> s = new List<List<string>>();
for (int i = 0; i < 10; ++i) {
    s.Add(new List<String>());
    s[i].AddRange(new String[] { "a", "b", "c" });
}

Updated to fix the problem in my original answer.

edited Apr 23, 2017 at 1:30

answered Apr 23, 2017 at 1:07

Jarrett Robertson

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5 Comments

Scott Chamberlain Over a year ago

Actually he needs a s.Add(new List<string>());.

Bilal Shafi Over a year ago

Sorry, both of these still create an index out of range exception in the "s[i]" part.

Jarrett Robertson Over a year ago

You have to add a new list before you access s[i]. And a new list will need to be added for each iteration of the for loop to make sure that one is available to access.

Bilal Shafi Over a year ago

It is the "s.Add(new List<string>());" is on the line before i use s[i].

Jarrett Robertson Over a year ago

Then is your folders array empty or null? I don't know why s[i] would throw an exception if you explicitly add a new list every iteration of the loop. It doesn't look like your folders should be null but I have no idea at this point.

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