Remove lines from List using Linq, if meeting a certain criteria

Question

I've searched around for a solution to this question but can't find an applicable circumstance and can't get my head around it either.

I've got a List<String[]> object (a parsed CSV file) and want to remove any rows if the first value in the row is equal to my criteria.

I've tried the following (with variations) and can't seem to get it to delete the lines, it just passes over them:

rows.RemoveAll(s => s[0].ToString() != "Test");

Which I'm currently reading as, remove s if s[0] (the first value in the row) does not equal "Test".

Can someone point me in the right direction for this?

Thanks, Al.

Edit for wider context / better understanding:

The code is as follows:

  private void CleanUpCSV(string path)
    {

        List<string[]> rows = File.ReadAllLines(path).Select(x => x.Split(',')).ToList();
        rows.RemoveAll(s => s[0] != "Test");

        using (StreamWriter writer = new StreamWriter(path, false))
        {
            foreach (var row in rows)
            {
                writer.WriteLine(row);
            }
        }
    }

So the question is -> Why won't this remove the lines that do not start with "Test" and upon writing, why is it returning System.String[] as all the values?

*'can't seem to get it to work'* - problem description is not clear — Sergey Berezovskiy, Nov 09 '17 at 11:39
That expression should work, except that you shouldn't need `.ToString()`, unless you don't actually have strings. Also, you need to explain what happens when you try that code. Does it remove everything? Nothing? Wrong values, if so which ones? — Lasse V. Karlsen, Nov 09 '17 at 11:39
Try : rows.Where(s => s[0] != "Test").ToList(); Your query should be "==" not "!=". — jdweng, Nov 09 '17 at 11:42
It sounds like you have each csv line as an item in the list. You would need a list of lists if the lines were parsed into tokens. — Crowcoder, Nov 09 '17 at 11:46
@LasseVågsætherKarlsen removed .ToString() and writing to a file, it replaces all with System.String[] as the value — a_user8891, Nov 09 '17 at 11:46
**What exactly is the question?** Are you asking how to remove lines from a list using a predicate or are you asking why the output of your writer loop ends up writing lines containing only "System.String[]"? — Lasse V. Karlsen, Nov 09 '17 at 11:52
Possible duplicate of [Using LINQ to remove elements from a List](https://stackoverflow.com/questions/853526/using-linq-to-remove-elements-from-a-listt) — SᴇM, Nov 09 '17 at 11:55

aloisdg · Accepted Answer · 2017-11-09T11:54:41.433

Did you try with Where? Where is going to filter based on a predicate. You should be able to do something like this:

Demo: Try it online!

    List<string[]> rows = new List<string[]> { new []{"Test"}, new []{ "Foo"} };
    rows = rows.Where(s => s[0] == "Test").ToList();

    foreach(var row in rows)
    {
        Console.WriteLine(string.Join(",", row));   
    }

output

Test

You dont need ToString() because S[0] is already a string
You may want to handle empty case or s[0] could throw
You can use s.First() instead of s[0]
You can learn more about Predicateon msdn

Edit

For your example:

private void CleanUpCSV(string path)
{

    var rows = File.ReadAllLines(path).Select(x => x.Split(','));

    using (StreamWriter writer = new StreamWriter(path, false))
    {
        foreach (var row in rows.Where(s => s[0] == "Test"))
        {
            writer.WriteLine(string.Join(",", row));
        }
    }
}

By the way, you may want to use a library to handle csv parsing. I personally use CsvHelper

Thank you aloisdg and @CSharpie Working as expected now. – a_user8891 Nov 09 '17 at 12:00 — a_user8891, Nov 09 '17 at 12:00

score 1 · Answer 2 · answered Nov 09 '17 at 11:57

The only error in your code is the following:

Since row is string[] this

writer.WriteLine(row);

won't give you the result you were expecting.

Change it like this

writer.WriteLine(String.Join(",", row));

To convert the string[]back into its orginal form.

Any other "optimisation" in all the answers proposed here arent really optimal either.

score 0 · Answer 3 · answered Nov 09 '17 at 11:44

If you're really trying to remove items where the first element isn't "Test", then your code should work, though you don't need to call .ToString() on s[0] since it's already a string. If this doesn't work for you, perhaps your problem lurks elsewhere? If you give an example of your code in a wider context you could get more help

score 0 · Answer 4 · answered Nov 09 '17 at 11:45

0

Filter it like this instead:

var filteredList = rows.Where(s => s[0] == "test").ToArray();

answered Nov 09 '17 at 11:45

Kjartan

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Remove lines from List using Linq, if meeting a certain criteria

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