Using Regular Expressions inside Lambda Functions - C#

Question

I've a little test program that builds a List of different strings, all of which contain the same formatted number. I also then declare another list which is supposed to contain the specific numbers of each string in the former list.

My plan is to accomplish this by utilizing a regular expression match inside of a lambda function.

Every time I try and do this I get the following error:

List<string> newList = new List<string>(new string[] { "MyName - v 3.7.5.0 ... CPU:",
                                                           "MyName - v  ... CPU: - 1.5.7.2",
                                                           "4.21.66.2 - v  ... CPU:",
                                                           " - v  ... CPU: 31.522.9.0" });
Regex match = new Regex("(\\d+\\.)+\\d");
List<string> otherList = newList.FindAll(str => match.Match(str).Value);

Is there any way I can use lambda functions to accomplish this?

Well, do you understand the error? FindAll is expecting a predicate - something that returns true or false. match.Match(str).Value returns a string. Perhaps you were looking for ConvertAll? (Or just use Select and LINQ...) — Jon Skeet
– Jon Skeet, Commented Feb 24, 2017 at 21:48

Hung Cao · Accepted Answer · 2017-02-24 21:44:31Z

7

You can try this:

List<string> otherList = newList.Select(str => match.Match(str).Value).ToList();

Btw, your code is failing because the predicate is expecting bool.

answered Feb 24, 2017 at 21:44

Hung Cao

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3 Comments

Delfino Over a year ago

What do you mean? I thought the predicate was supposed to "essentially" assert the regex against each index in the first list, and assign that match to a spot in the new list.

Hung Cao Over a year ago

Nope, check this msdn.microsoft.com/en-us/library/bfcke1bz(v=vs.110).aspx It will explain about Predicate class. FindAll is more like Where but they are slightly different in term of execution and performance. If you want to "transform" your collection then select.

Delfino Over a year ago

Ah, I see. I'm still familiarizing myself with the System.Linq library for .NET, so thank you for the pointers!

huserben · Accepted Answer · 2017-02-24 21:50:01Z

1

you can try the following: var otherList = newList.Select(str => match.Match(str).Value);

FindAll expects a Predicate, so you would need to do: newList.FindAll(str => match.IsMatch(str)); But then you would have an IEnumerable that would contain the full strings and not just the numbers you are looking for.

answered Feb 24, 2017 at 21:50

huserben

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Tim.Tang · Accepted Answer · 2017-02-24 21:50:59Z

0

    List<string> newList = new List<string>(new string[] { "MyName - v 3.7.5.0 ... CPU:",
                                                   "MyName - v  ... CPU: - 1.5.7.2",
                                                   "4.21.66.2 - v  ... CPU:",
                                                   " - v  ... CPU: 31.522.9.0" });
    Regex match = new Regex("(\\d+\\.)+\\d"); 
    var result = match.Matches(string.Join(" ", newList)).Cast<Match>().Select(m => m.Value);

answered Feb 24, 2017 at 21:50

Tim.Tang

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