Remove empty entries from list of list of strings

Question

I'm looking for a more efficient way of removing empty string values from a list of a list of strings.

The code below works but for a very large data set this seems inefficient. Is there a more efficient way to do this?

FYI - The beginning is just to build a Data Set to have a list of a list that contains empty strings

public static void Main()
{       
    //Building the data set
    List<List<string>> list = new List<List<string>>();
    list.Add(new List<string> {"One", "Two", "", "Eight"});
    list.Add(new List<string> {"Three", "Five", "Six"});
    list.Add(new List<string> {"Sixteen", "", ""});
    list.Add(new List<string> {"Twenty-Eight", "Forty", "Nine"});

    //Create an empty List of a List
    List<List<string>> newList = new List<List<string>>();

    //Loop through the original list and purge each list of empty strings
    for(int i = 0; i < list.Count; i++) {
        newList.Add(list[i].Where(x => !string.IsNullOrEmpty(x)).ToList());
    }

    foreach (var s in newList) {
        Console.WriteLine(string.Join(", ", s));    
    }

    /*
    CORRECT OUTPUT:
        "One", "Two", "Eight"
        "Three", "Five", "Six"
        "Sixteen"
        "Twenty-Eight", "Forty", "Nine"         
    */      
}

Since they are lists, you could use List.RemoveAll() instead, that way you're not creating new lists to replace the ones you currently have. — Jeff Mercado
– Jeff Mercado, Commented Oct 22, 2018 at 19:10
Lists are mutable; do you want to mutate the existing lists, or create new ones? Both are reasonable choices. — Eric Lippert
– Eric Lippert, Commented Oct 22, 2018 at 19:16
You say that you are worried about "efficiency" but efficiency is value produced divided by resources consumed; what value are you producing, and what resource are you consuming? — Eric Lippert
– Eric Lippert, Commented Oct 22, 2018 at 19:17
Good questions. I want to simply remove the empty strings in every child list. — bagofmilk
– bagofmilk, Commented Oct 22, 2018 at 19:22
If you want to remove all of a thing, the aptly-named RemoveAll method should be your go-to choice. — Eric Lippert
– Eric Lippert, Commented Oct 22, 2018 at 20:34

Abbas · Accepted Answer · 2018-10-22 19:19:23Z

16

Why not use the List<T>.RemoveAll() method? Definition:

Removes all the elements that match the conditions defined by the specified predicate.

foreach (var l in list)
{
    l.RemoveAll(x => string.IsNullOrEmpty(x));
}

That is all you need. Other answer have Select().Where() and twice ToList(), which is way too much overhead for a simple action like this.

answered Oct 22, 2018 at 19:19

Abbas

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3 Comments

bagofmilk Over a year ago

Interesting.. You believe this is faster than the one-liner LINQ statements above?

juharr Over a year ago

@bagofmilk The speed difference is not going to be that much, but this consumes much less memory in that it doesn't need to create a new list of lists.

Brian Over a year ago

@bagofmilk: Which one is faster really doesn't matter unless this particular task represents a performance bottleneck in your application. What's more important is that Abbas's code is easier to read. Personally, I prefer the use of method groups (l.RemoveAll(string.IsNullOrEmpty);) instead of lambdas (applies to most of the answers to your question). I'll note that if you insist on a one-liner, you still don't need LINQ: Just use (list.ForEach(x=>x.RemoveAll(String.IsNullOrEmpty))). Though personally, I'm not a fan of List<T>.ForEach.

JSteward · Accepted Answer · 2018-10-22 19:10:47Z

2

Since you want a List<List<string>> returned then just Select and filter with the inner list with Where

public List<List<string>> FilterStrings()
{
    List<List<string>> list = new List<List<string>>();
    list.Add(new List<string> { "One", "Two", "", "Eight" });
    list.Add(new List<string> { "Three", "Five", "Six" });
    list.Add(new List<string> { "Sixteen", "", "" });
    list.Add(new List<string> { "Twenty-Eight", "Forty", "Nine" });

    return list.Select(lst => lst.Where(str => !string.IsNullOrEmpty(str)).ToList()).ToList();
}

answered Oct 22, 2018 at 19:10

JSteward

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Comments

Amin Mozhgani · Accepted Answer · 2018-10-22 19:13:26Z

1

newList = list.Select(t=>t.Where(q=>!string.IsNullOrEmpty(q)).ToList()).ToList();

answered Oct 22, 2018 at 19:13

Amin Mozhgani

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Comments

yesenin · Accepted Answer · 2018-10-22 19:14:56Z

0

The main idea: iterate through lists and remove empty strings from each.

list.Select(outerListItem => outerListItem.Where(innerListItem => !string.IsNullOrEmpty(innerListItem).ToList())

answered Oct 22, 2018 at 19:14

yesenin

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