C# How to know which elements of a list are substrings of a string?

Question

If I have a list of string like

var MyList = new List<string>
{
    "substring1", "substring2", "substring3", "substring4", "substring5"
};

is there any efficient way to determine which elements of that list are contained in the following string

"substring1 is the substring2 document that was processed electronically"

In this case the result should be

var MySubList = new List<string>
{
    "substring1", "substring2"
};

Are you matching whole words, or should hello asubstring1z world match too? — Caius Jard
– Caius Jard, Commented May 24, 2022 at 6:12

Caius Jard · Accepted Answer · 2022-05-24 06:16:29Z

1

We can use LINQ Where to query, for every substring, whether the large string Contains the substring:

var MyList = new List<string>
{
    "substring1", "substring2", "substring3", "substring4", "substring5"
};

var Text = "substring1 is the substring2 document that was processed electronically";

var output = MyList.Where(x => Text.Contains(x)).ToList();

edited May 24, 2022 at 6:16

Caius Jard

75k6 gold badges61 silver badges95 bronze badges

answered May 23, 2022 at 23:40

Enigmativity

117k12 gold badges101 silver badges184 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Peter Csala · Accepted Answer · 2022-05-24 07:25:08Z

0

Split the Text by whitespaces
Sort the words alphabetically
Create a unique list from that

var words = Text.Split(" ").OrderBy(word => word).Distinct().ToList();

Create an accumulator collection for the matches
Create two index variables (one for the words, one for the patterns)

List<string> matches = new();
int patternIdx = 0, wordIdx = 0;

Iterate through the lists until you reach one of the collections' end

while(patternIdx < patterns.Count && wordIdx < words.Count)
{

}

Perform a string comparison
Advance index variable(s) based on the comparison result

int comparison = string.Compare(patterns[patternIdx],words[wordIdx]);
switch(comparison)
{
    case > 0: wordIdx++; break;
    case < 0: patternIdx++; break;
    default: 
    {
        matches.Add(patterns[patternIdx]); 
        wordIdx++;
        patternIdx++;
        break;
    }
}

Here I've used C# 9 new feature switch + pattern matching.
If you can't use C# 9 then a if ... else if .. else block would be fine as well.

For the sake of completeness here is the whole code

var Text = "substring1 is the substring2 document that was processed electronically";
var words = Text.Split(" ").OrderBy(x => x).Distinct().ToList();
var patterns = new List<string> {  "substring1", "substring2", "substring3", "substring4", "substring5" };

List<string> matches = new();
int patternIdx = 0, wordIdx = 0;
while(patternIdx < patterns.Count && wordIdx < words.Count)
{
    int comparison = string.Compare(patterns[patternIdx], words[wordIdx]);
    switch(comparison)
    {
        case > 0: wordIdx++; break;
        case < 0: patternIdx++; break;
        default: 
        {
            matches.Add(patterns[patternIdx]); 
            wordIdx++;
            patternIdx++;
            break;
        }
    }
}

Dotnetfiddle link

answered May 24, 2022 at 7:25

Peter Csala

23.7k16 gold badges51 silver badges96 bronze badges

6 Comments

Enigmativity Over a year ago

If I change the text to "substring1 is the substring2, document that was processed electronically" then this code doesn't work anymore.

Peter Csala Over a year ago

@Enigmativity Did you just add a comma after substring2?

Peter Csala Over a year ago

@Enigmativity In that case all you need to do is to replace Split("") call to Split(' ', ',')

Enigmativity Over a year ago

I could have added any character after the substring.

Enigmativity Over a year ago

It was a poor specification from the OP. I took it as literally a substring.

|

Collectives™ on Stack Overflow

C# How to know which elements of a list are substrings of a string?

2 Answers 2

Comments

6 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

6 Comments

Your Answer

Sign up or log in

Post as a guest

Related