Get index of parameter of element in list

Question

It is hard to explain my problem writing it in "normal" language so I'll just post my code (it would be easier to understand):

I have my class File:

public class File
{
    public string Url { get; set; }
    public string Name { get; set; }
    public string Date { get; set; }
    public Bitmap Bitmap { get; set; }
}

So I created a list of files:

var files = new List<File>();

and, I have populated it with data.

Now I have a string (and only that.) I would like to find the index of the element containing that string in my list.

Something like:

int index = files.IndexOf(new File
        {
            string.Empty,
            myString,
            string.Empty,
            null
        });

Any idea?

Zohar Peled · Accepted Answer · 2018-11-13T09:39:50.143

6

The List class has a method called FindIndex, that takes a Predicate<T> as a parameter.

You can use it like this:

var index = files.FindIndex(f => f.Name == myString);

It will return the index of the first element that matches the predicate or -1 if no such element was found.

edited Nov 13 '18 at 09:39

answered Nov 13 '18 at 09:29

Zohar Peled

79,642
10
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121

This works fine. Thanks. – Oiproks Nov 13 '18 at 09:45
Glad to help :-) – Zohar Peled Nov 13 '18 at 09:46

Dmitry Stepanov · Answer 2 · 2018-11-13T09:37:29.943

0

I guess you want to find the index of the first element with the Name property equals to myString variale:

var index = files.IndexOf(files.Where(x => x.Name == myString).FirstOrDefault());

Or even more simpler way:

var index = files.IndexOf(files.FirstOrDefault(x => x.Name == myString));`

Both return -1 if no such element was found.

edited Nov 13 '18 at 09:37

answered Nov 13 '18 at 09:30

Dmitry Stepanov

2,776
8
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score 0 · Answer 3 · answered Nov 13 '18 at 09:30

Firstly, I find all instances with Name containing the myString, then, use IndexOf to find his index in the list. The result is a collection index of the instances

var indexs = files.Where(f => f.Name.Contains(myString)).Select(ft=> files.IndexOf(ft));

Rahul · Answer 4 · 2018-11-13T09:35:11.123

0

Why not just use a foreach loop like

int pos = -1;
foreach(File file in files)
{
  pos++;
  if(file.Name == "string")
    break;
}

edited Nov 13 '18 at 09:35

answered Nov 13 '18 at 09:31

Rahul

76,197
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because `FindIndex` does the same with just one line of code, with the same efficiency? Also, you should initialize `pos` at -1, otherwise, you'll get a false positive if the file was not found. – Zohar Peled Nov 13 '18 at 09:34
@ZoharPeled setting `pos = -1` is correct .. Thanks – Rahul Nov 13 '18 at 09:35

Get index of parameter of element in list

4 Answers4